Development of Improved I-V Characteristics Models for Nanometer Size MESFETs

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Development of Improved I-V Characteristics Models for Nanometer Size<br /> MESFETs

Development of
Improved I-V Characteristics Models for Nanometer Size MESFETs

CHAPTER
1

INTRODUCTION
TO MESFET

In this chapter, the MESFET is
being introduced. This chapter includes what is MESFET, what it stands for. The
history of the MESFET is also discussed briefly here.

 

1.1   Introduction

            MESFET stands for Metal
Semiconductor Field Effect Transistor. The metal-semiconductor field effect
transistor was first introduced by Mead in 1966 and subsequently fabricated by
Hooper and Lehrer using a GaAs epitaxial layer on semi insulating GaAs
substrate. MESFET is a unipolar device, a converse of bipolar transistor. The
difference of these two is apparent that, in bipolar device the current
conducting results for both minority carrier electron in p region and minority
carrier hole in n region on  p-n junction
while the in a MESFET the current conduction results only from the majority
carrier. Like in a n type MESFET, electrons are the particles that give rise
current and in p type MESFET same operation is done by majority carrier holes
in the conduction channel. MESFET offers certain processing and performance
advantages, such as low temperature formation of the metal-semiconductor
barrier (as compared to a p-n junction made by diffusion or grown process), low
resistance and low IR drop along the channel width and good heat dissipation
for power devices (the rectifying contact also serve as an efficient thermal
contact to heat sink). The operation of Junction Field Effect Transistor (JFET)
and MESFET is apparently same. MESFET offers superior performance in some
processing realm than JFET, especially in RF microwave operation that makes RF
amplifier more efficient.

 

There are various
types of MESFET fabricated by different semiconductors. The family tree of
MESFET is given below—

 

 





















 

Figure
1.1:

Family tree of MESFET from ref [2]

 

The main concern of
this publication is proposing an I-V Characteristics for a typical MESFET.
Before we move to the main topic, we must look for the history of MESFET.

 

1.2 History

            It
was early 1925 when FET model was proposed. Since 1925, advent of advanced
technology led to more sophisticated devices. At one stage at 1966 MESFET model
was proposed. A brief timeline of the history of MESFET has been given.

·        
1925 Austrian- Hungarian physicist Julius Edgar
Lilienfeld
has
first proposed
field-effect transistor principle. This principle was
patented by Canada.

 

·        
1934 German physicist Dr. Oskar Heil
patented another field-effect transistor

·        
Lilienfeld
and Heil did not construct any devices.

 

·        
1939 William Shockley and a co-worker at Bell Labs, Gerald Pearson, had
built operational versions from Lilienfeld’s patents

 

·        
1943 Ralph Bray, a fresh graduate
student of PURDUE University had observed the minority carrier injection but
this phenomenon was not being recognized then and it was taken as anomaly

 

·        
1948 effect of minority carrier
injection realized by Bell Labs and they Constructed first transistor

 

·        
1949 Bipolar junction transistor by
Brattain, Bardeen and Shockley of Bell Laboratories

 

·        
1950 spacistor type transistor was
developed

 

·        
1951 alloy junction transistor, Grown
junction transistor had been developed

 

·        
1952 William Shockley along with his
co-workers proposed the principle of Junction Field Effect Transistor.

 

·        
1954 Morris Tanenbaum et al. at Bell
Laboratories were the first to develop a working silicon transistor

 

·        
1955 A superior method was developed
by Morris Tanenbaum and Calvin S. Fuller at Bell Laboratories by the gaseous
diffusion of donor and acceptor impurities into single crystal silicon chips.

 

·        
1958 First Integrated Circuit by Jack
Kilby of Texas Instrument

 

·        
1959 Si planar process: Procedure
resembles integrated circuits of today by Robert Noyce of Fairchild

 

·        
1960 p-channel Si MOSFET was
described by Dawon Kahng, a member of Atallai group

 

·         
1966 The first Gallium-Arsenide
Schottky-gate field-effect transistor (
MESFET)
was made by
Carver Mead and reported in 1966

 

A brief timeline of
the above discussion is given below—



 

Figure
1.2
:
Timeline of history of MESFET

 

 

1.3
Application of the MESFET

The
MESFET is used in many RF amplifier applications. The MESFET semiconductor
technology provides for higher electron mobility and in addition to this the
semi-insulating substrate there are lower levels of stray capacitance. This combination
makes the MESFET ideal as an RF amplifier. In this role MESFETs may be used as
microwave power amplifiers, high frequency low noise RF amplifiers,
oscillators, and within mixers. MESFET semiconductor technology has enabled
amplifiers using these devices that can operate up to 50 GHz and more, and some
to frequencies of 100 GHz.

The GaAs MESFET has a
number of differences and advantages when compared to bipolar transistors. The
MESFET has a very much higher input as a result of the non-conducting diode
junction. In addition to this it also has a negative temperature co-efficient
which inhibits some of the thermal problems experienced with other transistors.

When compared to the
more common silicon MOSFET, the GaAs FET or MESFET does not have the problems
associated with oxide traps. Also a MESFET has better channel length control
than a JFET. The reason for this is that the JFET requires a diffusion process
to create the gate and this process is far from well defined. The more exact
geometries of the GaAs MESFET provide a much better and more repeatable
product, and this enables very small geometries suited to RF microwave
frequencies to cater for.

In many respects GaAs
technology is less well developed than silicon. The huge ongoing investment in
silicon technology means that silicon technology is much cheaper. However GaAs
technology is able to benefit from many of the developments and it is easy to
use in integrated circuit fabrication processes.

The GaAs MESFET is
widely used as an RF amplifier device. The small geometries and other aspects
of the device make it ideal in this application. Typically a supply voltage of
around 10 volts will be used. However care must be taken when designing the
bias arrangements because if current flows in the gate junction, it will
destroy the GaAs FET. Similarly care must be taken when handling the devices as
they are static sensitive. In addition to this, when used as an RF amplifier
connected to an antenna, the device must be protected against static received during
electrical storms.

If these precautions are
observed, the GaAs MESFET will perform exceedingly well. The MESFET is an
electronics component that is relatively cheap, and will perform well.

Numerous MESFET
fabrication possibilities have been explored for a wide variety of
semiconductor systems. Application of MESFET can be summarized as below—

·        
RF
Amplifiers

·        
Microwave
Power Amplifiers

·        
Oscillators

·        
Mixers

·        
Military
communications

·        
Military
radar
devices

·        
Commercial
optoelectronics

·        
Satellite
communications

In this study both SiC and GaAs
MESFETs has been discussed.

Specific application of the SiC
and GaAs MESFET is discussed in the next section.                    

 

 

            1.3.1 Application of SiC MESFET

            SiC
MESFET has some advantages. SiC MESFET device is wide energy band gap device.
It exhibits high breakdown electric device. It has also high electrical and
thermal conductivity. It also exhibits high saturation electron velocity. Its
melting point is high and it is chemically inert.

The major applications include

         
Wireless
Communication

         
Microwave
Circuits

         
High
Power

         
High
Frequency

         
Power
Amplifiers

            1.3.2 Application of GaAs MESFET

            GaAs
MESFET has some advantages over Si MESFET. The room temperature mobility of the
GaAs MESFET is five times than that of Si. As a result the saturation velocity
of the GaAs MESFET is about twice of the Si MESFET. Moreover semi insulating
(SI) GaAs MESFET can be fabricated which eliminates the problem of absorbing microwave
power in the substrate due to free carrier absorption.

The major applications include:

         
Radar

         
Cellular
phone

         
Satellite
Receivers

         
Microwave
Devices

CHAPTER 2

MESFET OPERATION

The
Chapter is introduced by the operation principle of MESFET under equilibrium
and non equilibrium state. The modes of operation and I-V characteristics are
also discussed here. For predicting I-V characteristics the drain current
equation is derived and above all some processing descriptions are given as well.

 

2.1 Operation of MESFET

 

                        2.1.1. Equilibrium State          

 

Earlier
it has been said that MESFET is a unipolar device, which means only the
majority carrier particle is responsible for the conduction of the MESFET.
MESFET is operation is very similar to JFET & MOSFET (Metal Oxide
Semiconductor Field Effect Transistor). In this section operation of the MESFET
is main concern. A schematic diagram of a MESFET is shown in Figure 2.1. MESFET
is fabricated on a semi insulating substrate. On the top of the semi insulating
layer there is light n (n MESFET) or light p (p MESFET) implant. Then source
and drain are of highly doped n region (for n MESFET) or highly doped p region
(for p MESFET). These source and drain terminal are indicated as S and D
respectively. The gate terminal is metal deposited. This region acts as a
Schottky diode.





t









Gate









Figure
2.1:

n-MESFET with no voltage at gate electrode

 

The active region
where current conduction takes place is between source and drain and type of
particle flow or the direction of current depends on what type of MESFET is
used, whether n or p type. Let it’s a n MESFET, light n implant is diffused in
the semi insulating substrate and then high n implant is given to form the
source and drain region. On the top of the highly doped source and drain, metal
is given. This is ohmic contact. There is a junction between gate metal and the
light n region under the gate. This junction controls the current in the active
region. When no voltage source is given at any terminal then there exist a
depletion barrier between light n region and gate metal due to intrinsic
potential barrier. This interface between gate and light n region under it,
behave almost like a p+-n interface. It can be thought as a single sided abrupt
p-n junction. So the depletion width is along the lightly doped n implant i.e.
the active region according to the following equation.



 …………………………………………….……….………………..(2.1)



 is the almost
the whole depletion width t, then built in potential can be found as—



……………………………………………………………………………(2.2)

Here we are tactically assuming
that thickness of channel length is greater than the depletion region, which is
actually the case for a depletion type MESFET. However extensive study is going
on this depletion type MESFET due to having some advantages over other type
MESFET.

The band diagram of the junction
of gate and channel is like below at equilibrium condition—



(a)



(b)

 

Figure
2.2:

(a) ideal metal and semiconductor before making any contact (b) metal-
semiconductor band diagram after contact is made and voltage is not given at
gate electrode

 

In Figure 2.2 (a), the vacuum
level is used as reference level. The parameter



: Lower conduction band energy of semiconductor



: Upper valance band energy of semiconductor



: Fermi level of semiconductor



: Intrinsic Fermi Energy,

 



: Metal work function



: Semiconductor work function



: Electron affinity



: Ideal barrier height of semiconductor contact



 :

 

Here we have assumed that

, the ideal thermal equilibrium
metal-semiconductor energy band diagram, for this situation, is shown in Figure
2.2 (b). Before the contact, the Fermi level in the semiconductor was above
that in the metal. In order for the Fermi level to become a constant through
the system in thermal equilibrium, electrons from the semiconductor flow into
the lower energy states in the metal. Positively charged donor atoms remain in
the semiconductor, creating a space charge region.



 potential
barrier is seen by the electrons in the metal trying to move into the
semiconductor. This barrier is known as Schottky barrier and is given by,
ideally—

           

                                                                       (2.3)

On the semiconductor side,

  is the built
in potential barrier. This barrier, similar to the case of p-n junction, is
barrier seen by electrons in the semiconductor in the conduction band trying to
move into the metal. The built in potential barrier is given by—



                                                                                (2.4)

This makes the potential

 be a function
of doping concentration.

2.1.2
Non equilibrium state





Gate


            From Figure 2.1 it’s obvious that
the channel length can be varied by varying the depletion region. Now to
control the depletion region we can reverse bias the p+-n junction, i.e. the
gate and light n region, under gate, Interface. This is illustrated in the
schematic of Figure 2.3.







VGS





VDS






d









Figure
2.3:

Depletion type MESFET under negative voltage at gate electrode

 

 

The source is
connected to ground and to reverse bias the Schottky diode a negative voltage
is given with respect to source at gate. As its being reverse biased, so the
depletion width will increase. By varying this depletion region one can control
the effective channel length to flow the current conducting particle. To flow
the electron here (n type MESFET), positive voltage is given with respect to
the drain. As drain is at positive voltage, drain part of Schottky diode is
more reversed bias than the source part, so depletion region extends as moving
from the source to drain. Due to drain is in more positive potential than the
source, the electron will flow from the source to drain along the channel
length. Now if more negative voltage with respect to gate, then the depletion
width will increase and eventually stops the channel. So it’s apparent that the
threshold voltage for depletion type MESFET is negative. Assuming that


 voltage is
dropped across the depletion region when it just cover the whole channel
thickness (


),

 can be
calculated as below—



                                                                                       (2.5)

 

So the threshold
voltage


 is



                                                                                     (2.6)

The energy band
diagram of the junction of gate-channel under reverse bias is shown below—



Figure
2.4
:
MESFET band diagram under reverse bias VR: Reverse bias voltage
applied at gate electrode

 

As negative voltage
is given at the gate metal, so the Fermi of metal moves upward and that of
semiconductor moves downward. The difference between these two Fermi energies
is the amount of reverse bias voltage applied. As reverse bias voltage is
given, it effectively increases the barrier height seen by electrons tend to flow
from metal to semiconductor.

 

2.1.3
Velocity Saturation

                        Before moving to the
derivation of drain current and I-V characteristics, the velocity saturation
phenomenon should be described briefly to support the I-V characteristics
section. Most famous MESFET in recent time is GaAs MESFET. Now just scheme
through the velocity versus electric field curve below for some semiconductor
materials—

 



 

Figure
2.5:

Drift Velocity vs. Electric Field curve of some semiconductors

 

This electric field
is along the channel length. For a n-MESFET this electric field is in negative
X-axis direction. From the curve, it has been seen for low electric field the
velocity increases with the field linearly. If


 is velocity
of particle (here electron) and E is the electric field then velocity can be
written as-



                                                                                               (2.7)

Here µ is mobility
and a proportional constant. For GaAs this mobility is higher than Si, as the
slope curve is steeper for lower electric field.  As one increases the voltage at drain
terminal and it thus increase the electric field E and after some time the
velocity of the particle will be saturated. In this region constant mobility
term is not met. Often mobility has been assumed constant in the operation of
JFET, MOSFET etc. But for MESFET channel length is less than others, which
results higher electric field for relatively smaller voltage, as E depends on—



                                                                                                             (2.8)

 

So in short channel
GaAs MESFET saturation in current is achieved when velocity is saturated
resulting from application of certain voltage at drain terminal.

 

                        2.1.4 Drain Current
Equation Derivation

                        In this section, the
current flowing between drain and source while giving enough voltage on gate (greater
than threshold voltage) will be deduced. For the derivation, the cross section
of MESFET, It is assumed that a MESFET bar is taken of width W. after initial
fabrication process of producing light n region over semi insulating substrate,
high n region drain and source region are formed and gate is formed by
depositing and etching the metal. The length of the channel is the length of
the gate is L. It is also assumed that in initial fabrication process the depth
of implanting light n region is


. The whole diagrammatic structure is shown in
Figure 2.6

 




VGS




VDS




A










Depletion region




n+ Source




n- active layer




Substrate




n+ Drain

te




Gate






















Figure
2.6:

MESFET Current Conducting Situation

 

 Now positive bias is given on drain side,

and negative voltage is given to the gate side,

 as it makes
the junction between gate and channel a reversed bias. So a depletion region is
formed. Proper voltage application on the gate ensures that the channel is not
fully depleted. As a voltage is applied to the drain so there will be an
electric field


 in the
direction of the drain to source. So the electron will be attracted to the
drain and will flow in a direction from source to drain through the undepleted
region and this makes the conducting current to give rise. As the depletion
region is dependent on the Schottky metal gate, so the current through the
channel also depends on it.

It should be
mentioned here that the application of the voltage on the drain side (


), it is uniformly distributed throughout the length
of the channel. This voltage varies from 0 V at the source to the


 at the drain.
For this distribution of voltage, the depletion region created under biasing
tends to be more at the drain side. Assuming the length of channel is along +ve
x direction, voltage is distributed to the x axis uniformly. So the thickness
of the depletion region


 is a function
of


 and it can be
written as


 the uniform
voltage distribution is


The current in the
channel can written as

 



                                                                                           (2.9)

 



: drain current (A)



: Electron concentration (for n-MESFET) (per m3)



: Electron charge (1.602×

C)



: drift velocity of electron through the channel
(m/sec)



: Cross section of the channel through which
electron passing (m2)

 

From the section
2.1.3 it has been seen that for low electric field, the drift velocity of the
electron is proportional to the electric field applied it can be written as

 



                                                                                         (2.10)

 

Here

 is
the electron mobility of the device which depends on material of the device.

And the area
through which current is flowing can be written as

 



                                                                             (2.11)

 

Here

 is the depth
of the light n region as depicted earlier and


 is the depth
of the depletion region. So effective undepleted region along which electron
can pass through is


. These are illustrated in Figure 2.6.

 

Let the doping
concentration at the light n region is


 then equation
(2.10) and (2.11) can be substituted in equation (2.9) and it looks like below—

 



                                                           (2.12)

 

In this equation
(2.12) negative sign actually means that direction of current


 and direction
of drift velocity of electron is opposite.

 

 

From the definition
of potential it can be written as



                                                                                (2.13)

So equation (2.12)
becomes

 



                                                       (2.14)

 

Now the expression
of d and


 should be
derived. The derivation is beyond this publication and this derivation can be
seen from any solid state device book and it is as follows

 

For a p-n junction
reverse bias by applying a voltage


, the depletion region extends both on p and n
region and the part of the depletion region at the n region is given as

 



                             (2.15)

 



: Permittivity of semiconductor



: built in potential (V)



: reverse bias voltage (V)



: Hole concentration (1/m3)



: Electron concentration (1/m3)



: Electron charge (1.602×

C)

 

The derivation of
equation (2.15) can be found in ref [13]

In our study, we
are dealing with an abrupt one sided interface and it can be thought as a p+ –
n interface between gate and the channel. So


 , so equation
(2.15) can be written as



                                                           (2.16)

 

Now let

 voltage is
required to reverse bias our MESFET bar such that depletion region covers the
whole thickness


, this voltage

 is also known
as pinch off voltage, so from equation (2.16)



                                                                (2.17)

Assuming,

, and equation (2.17) can alternatively be written
as

 



                                                                                  (2.18)

 

Now the threshold
voltage of the device will be the voltage required to fully deplete the light n
region and that will be

 



                                                                                   (2.19)

 

For the expression
of


 we can see
that the effective reverse bias voltage along the interface between gate and
channel is


 and from
equation (2.17) we can write

 



                                                        (2.20)

 

Now recalling
equation (2.14) and putting the value of


 from (2.20)

 



                                                (2.21)

 

  


                                   (2.22)

From equation
(2.18), using the


 terms and
integratin on both sides of equation (2.22)-

 

  

           (2.23)

 

Assuming,

Then         

            and             
for


,

                                                                    for

,

 

Equation (2.23) can
be written as

 

               

                                                         (2.24)



       (2.25)

For


This equation
(2.25) found for the Typical MESFET device is valid for the region,



For the grater
drain voltage the current saturates, the drain voltage at which drain current
saturates—



Drain saturation
current will be given as—

 



                 (2.26)

                                                                                                                        For


As we know,



                                                                                                          (2.27)

 

 

 

The equation is
modified as below



                                       (2.28)

                                                                          For


 

This is the drain
current equation of a MESFET. It’s derived on some assumption

·        
It
assumed that electron mobility is constant

·        
Electric
field along the channel doesn’t increase further than saturated electric field

This equation holds good for
these constraints imposed but unsatisfactory result may occur if these criteria
are not met. So, for proper I-V modeling there are different approaches of
deducing drain current equation. Here, our main goal is to establish a drain
current equation and compare its I-V characteristics with other models.

 

                        2.1.5 I-V
characteristics of MESFET

 

                        An I-V characteristic of
a typical MESFET is similar to that of a MOSFET and JFET. An I-V characteristic
of a typical MESFET is shown in Figure 2.7. In this Figure the drain current is
plotted against drain-source voltage. Drain current is also a function of gate
source voltage. So the individual curve in the Figure represents the dependency
of drain current on drain- source voltage for a particular value of gate-
source voltage. In this Figure the I-V characteristics of a depletion type
MESFET has been shown. Form the Figure 2.7, it is seen there are two modes of
operation of MESFET. There is another mode which is breakdown mode due to
excessive application of drain- source voltage.



 

Figure
2.7:

I-V characteristics of MESFET

 

            As in discussion of operation of a
MESFET in section 2.1 it has been mentioned that necessary voltage should be
given at the gate electrode. By applying more negative voltage at the gate
electrode makes junction of gate-channel more reversed biased. So the depletion
width increases. At one stage of increasing of negative voltage at gate
electrode, makes the channel fully depleted, then no current flows. So the
voltage at the gate electrode should be such that it is greater than the
minimum value of gate voltage that makes the phenomenon of full depletion,
namely threshold voltage. In the Figure it is seen that the current ID
is very low for lower


. If we further increase the negative voltage on gate
the channel will stop. It can be said from intuition that the threshold voltage
of the Figure 2.7 is near the lowest VGS curve. So for current
conducting region or modes are valid for


,

 be threshold
voltage. Now the individual modes of current conduction are explained below.

 

                        2.1.5.1 Linear Region    (

)

                        This region is valid for
low value of


. From
fig 2.7, it is seen that for low value of


, drain current ID which flows from drain
to source depends almost linearly on VDS for a particular value of


. As the dependency of

  on

  is linear,
so the region is called linear region. 
The region is also called a triode region. This region is applicable for
the value of


 less than

 where



                                                                           (2.29)

The

 is not same
for all value of


. This change has been shown by the dotted
locus. 

With the decrease
of


, the value of

 also
decreases as shown in the Figure 2.7

It was noted
earlier that the drain current can be written as below—



                                                                                        (2.30)

Where



: Drain current (A)



: Electron concentration (for n-MESFET) (per m3)



: Electron charge (1.602×

C)



: Drift velocity of electron through the channel
(m/sec)



: Cross section of the channel through which
electron passing (m2)

 

From Figure 2.7 one
can see that with the increase in


 the cross
section


 decreases, so
it is expected to decrease in current


   with the
increase in


 but such is
not the case. Because of the presence of drift velocity


. With the increase of voltage

, electric field EX also increases and
for low EX, with the increase of EX drift velocity of v
increases linearly and it overpowers the decrease in cross section A. So with
the increase in voltage


,

 increases in
linear region.

 

                        2.1.5.2 Saturation Region (

)

                        In linear region, with
increase of


,

 increases.
But after a certain value of


, with the increase in

, the value of

 does not
change considerably. Normally this saturation of


 current is
reached at


, which is shown in Figure 2.7. Though saturation is
achieved, drain current still tends to increase. This is due to the fact that
the pinch off point shifts toward the source from the drain; this effectively
decreases the channel length. As the channel length is decreased, so current


 increases.
Saturation may occur in two different ways for a MESFET depending upon the
material used. They are discussed below—

 

 

                        2.1.5.2.1 Saturation by pinch off

                        In the MESFET device
fabricated by the material like Si which has reasonable high saturation
electric field value,


 saturates by
pinching off the channel between source and drain. This is illustrated in
Figure 2.8. High saturation electric field value means, the drift velocity of
carrier saturates at a higher value of electric field along the channel. The
case is such that, the mobility may be considered as constant throughout the
process and with the increase in


 voltage,
depletion on drain side increases. At a certain value of


 this
depletion on drain side pinches off the channel, so at this stage this part of
channel posses a high impedance and carrier cannot be passed through. But with
the high electric field some of the carriers are swept off to the drain side
and thus saturates the current. In the following Figure (Figure 2.8) the case
of pinch off is shown while applying


 voltage
greater than





Pinch off








Gate








VDS





VGS






Figure
2.8:

Current Saturation in a MESFET due to pinch off

                       

2.1.5.2.2
Saturation by velocity saturation

                        We have discussed about
the velocity saturation of some semiconductors in section 2.1.3. From the
section of 2.1.3 it has been seen that the drift velocity of the carrier
saturates comparably at a lower electric field for the material like GaAs. As
beyond the saturation a greater value of


 is applied,
so the electric field is high. For this high electric field the velocity of the
carriers through the channel saturates. The phenomenon is also likely to happen
if the device has short channel as it ensures greater strength of the electric
field. In this type of device the saturation in drain current


 occurs due to
the velocity saturation of the carrier.

                        For an example, it can
be approximated that the saturated electric field for GaAs is about 4 kVcm-1
(estimated from Figure 2.5) and let the channel length of a MESFET in
consideration is 1 µm then the voltage required to saturate the carrier drift
velocity is



=4kVcm-1×1µm=0.4 V                                                               (2.31)

For pinching off
the voltage required can be written as—



                                                                          (2.32)

Note that this
pinch off voltage alternatively can be written as threshold voltage, this is
the voltage required to fully deplete the channel region. Where



: Electron charge (1.602×10-19C)



: Doping concentration for n MESFET



: Total width of the active channel



: Permittivity of the semiconductor used



: built in potential

Typical values of
these parameters may be—



=3×1023 m-3



= 0.15µm

ϵ=13



=0.8 V

So pinch off
voltage,



                            (2.33)



So it has been seen
before pinch off occurs, the drift velocity of carrier will saturate and thus
it saturates the drain current


. Figure 2.9 illustrates the phenomenon—




Gate





VDS





VGS









Figure 2.9: Current Saturation in a MESFET
due to velocity saturation

 

                        2.1.5.3
Breakdown

                        If one keeps increasing
the


 voltage,
expecting a regulatory response of drain current the breakdown may occur. If
this occurs, device fails to operate and act as short circuit medium. With a
small change of


, large current of

 flows. This
mode is not expected obviously and each device is provided by its breakdown
voltage by manufacturer.

 

                        2.1.5.4 Summary

Table
2.1
:
Overview of different regions of operation of a MESFET

 

Region

Range

Device Cross section

I-V Curve

Linear







Saturation (due to velocity
saturation)







Saturation
(due to pinch off)







           

                        2.1.6 Equivalent Circuit
of a MESFET

From
the discussion of the operation and modes of operation of a MESFET, it is
possible to predict an equivalent circuit of the MESFET. The component of the
circuit are superimposed on the cross sectional view of a MESFET in the
following diagram

 



Figure
2.10:

Equivalent circuit component imposed on sectional view of MESFET

 

Here we have
assumed that the MESFET be an n-MESFET. Each element is intended to represent
the electrical character of a particular region of the device.

·        
Cgs:
Capacitance represents the charge storage in the depletion region at source
gate region

·        
Cgd:
Capacitance represents the charge storage in the depletion region at gate drain
region

·        
Rs:
Resistance represents the bulk resistance of N-layer in the source gate region
together with any contact resistance at the source ohmic metallization

·        
Rd:
Resistance represents the bulk resistance of N-layer in the gate drain region
together with any contact resistance at the drain ohmic metallization

·        
Rg:
parasitic resistance at the gate

·        
Ri:
internal source-gate resistance

·        
gm0v:
represents the drain current flow along the channel from drain to source

·        
r0:
substrate resistance

·        
C0:
substrate capacitance

·        
v:
the direction of voltage is along gate to source

 

For the circuit
behavior analysis of the above modeling of components the following circuit can
be considered–



Figure
2.11
:
Equivalent Circuit of MESFET

 

Typical range of the values of
the component is shown below

Table
2.2
:
Typical values of the circuit components of MESFET

Component

Value

Rg

0.5-3
Ω

Cgs

0.15-0.4
pF

Ri

2-10
Ω

Rs

1-5
Ω

Rd

1-5
Ω

Cgd

0.01-0.03
pF

gm0

20-40
mS

C0

0.05-0.1
pF

r0

250-500
Ω

 

2.2
Processing of MESFET

There
are various ways to fabricated MESFET but two most popular methods are

·        
Ion
Implantation

·        
Epitaxial
Growth

As fabrication
technique of MESFET is not the main concern of this paper. But one should have
a basic knowledge of fabricating the device. So here a brief description of
fabrication of MESFET is given. For further study see ref

 

                        2.2.1 Ion Implantation

                        In
this fabrication process the MESFET is produced from a single wafer of a semi
insulating semiconductor substrate like GaAs. The whole process is shown
schematically from top to bottom. At the top a semi insulating Substrate is
taken. In this substrate lightly n doped layer is given by ion diffusion. Then
the highly doped n region is given by ion implantation. And finally metal
contact is given to the gate, source and drain.

 






 

 

 

Figure
2.12:

(top to bottom): semi insulating substrate, diffusion of n implant,
n+ ion implantation, contact given

 

 

                        2.2.2 Epitaxial Growth:

In
this process a lightly doped n region is fabricated epitaxial on the semi
insulating substrate. It is also described schematically—





Figure
2.13
(left to
right): semi insulating sub, n region epitaxial grown, n+
ion implantation, etching and contact given

 

From left to right,
at most left corner, a semi insulating substrate is taken. Then lightly doped n
region is epitaxial formed over the semi insulating surface. Then highly doped
region is given by ion implantation. After that except the source and drain
region, its being etched and finally gate contact as well as drain, source
contacts are given.

 

            2.2.3 Summary of Fabrication

The
fabrication of a GaAs MESFET can be summarized by the following procedures—



Figure
2.14:

MESFET fabrication

 

 

 

 

2.3 Types of MESFET

There
are 2 types of MESFET depending upon the fabrication. They are stated below—

·        
Depletion
MESFET

·        
Enhancement
MESFET

Each type is
discussed below—

 

 

                        2.3.1 Depletion MESFET

                        In Depletion type MESFET
the depletion width of the gate- channel junction due to the built in potential
is less than the channel depth i.e.


. This is depicted in the following Figure—





Figure
2.15:

Depletion type MESFET

 

In depletion type
of MESFET, the device is normally ON. The depletion width is varied by varying
the voltage at the gate electrode. Suppose it is an n- MESFET then negative
voltage is needed at the gate electrode to increase the depletion width. At one
moment of increase of the negative voltage at gate electrode the depletion
width fully covers the channel region. This specifies the threshold voltage and
thus the threshold voltage for depletion MESFET is negative.

Typical I-V curve
of Depletion type MESFET—

 



Figure
2.16:

I-V characteristics of Depletion type MESFET

 

It is seen that
here


 is negative
and also the threshold voltage is negative and curves shift upward with the
increase of


 

                        2.3.2 Enhancement MESFET

                        In Enhancement MESFET
the depletion width of the gate-channel junction due to built in potential is
greater than the channel width i.e.


. In this type of MESFET, as the depletion width
fully covers the channel region, so the device is normally OFF device. Assume
that this is an n-MESFET then if positive voltage is given at gate electrode,
it makes the junction between gate and channel be forward biased. Due to
forward biasing, the depletion width along the channel will be reduced and at
certain value of


 the current

 will conduct
through the channel. So here the threshold voltage is positive and giving more
positive voltage at the gate electrode the drain current


 increases.

This is depicted in
the following Figure—





Figure
2.17:

enhancement type MESFET

 

 

Typical I-V
characteristics of an enhancement type MESFET has been shown below. It is clear
that the I-V characteristics for both depletion and enhancement type are same
i.e. with the increase in


 current

 increases.



Figure
2.18:

I-V characteristics of enhancement type MESFET

 

 

2.4 Structure of MESFET

There
are 2 types of structure of MESFET, they are—

·        
Self
aligned source and drain MESFET

·        
Non
Self aligned source and drain MESFET

 

                        2.4.1 Self aligned
source and drain

                        This form of structure
reduces the length of the channel and the gate contact covers the whole length.
This can be done because the gate is formed first, but in order that the
annealing process required after the formation of the source and drain areas by
ion implantation, the gate contact must be able to withstand the high
temperatures and this results in the use of a limited number of materials being
suitable.

The diagram below
shows this type of structure—








Gate


 





 

Figure 2.19: Self aligned MESFET

                        2.4.2 Non-self aligned
source and drain

                        For this form of MESFET,
the gate is placed on a section of the channel. The gate contact does not cover
the whole of the length of the channel. This arises because the source and
drain contacts are normally formed before the gate.

Following Figure
illustrates such structure—

 








Gate


 

 


        



 

Figure 2.20: Non self aligned MESFET

 

 

 

CHAPTER 3

LITERATURE REVIEW

Since
1966 of the proposal of first MESFET device by Mead, there are different models
are introduced to establish the I-V characteristics of MESFET which suits best
with the practical values. Among many of the models, some of them are
highlighted. The comparison of these models is also given later in the chapter.

 

3.1 Importance and Specification
of I-V model

 

                        3.1.1 Importance

In
any integrated circuit design one usually starts with a computer simulation of
the circuit to be built. In order to construct the device, one has to design
the circuit by considering the I-V characteristics of devices. So a reasonable
and practical I-V model is often needed by a designer. Some of the importances
of I-V model are mentioned below

·        
Significant
aids to integrated circuit design

·        
Eliminates
the delay of cut and try design approaches

·        
I-V
model opens the door to PC analysis and simulation of the device

·        
Due
to the analysis and simulation in PC is possible, manual gross approximation
can be eliminated which often leads to invalidity

 

      3.1.2 Specification

When
one wants to model I-V characteristics, he has to remind some factors. The I-V
model should be such that it nearly follows the practical values. Some of the
specifications of I-V model are given below—

·        
It
should be simple

·        
It
should be compact

·        
It
should have minimum number of variables

·        
It
should be such that it takes less CPU execution time

·        
It
should predict the device characteristics regardless of size

·        
It
may be changed during fabrication

·        
Simulated
result should match the actual measurement

 

3.2 Existing I-V Models for MESFET

There are different models
proposed by different scholars. Among them, Curtice model which is very
preliminary but still is widely used in educational purpose. Before Curtice,
Schichman-Hodges proposed another model which is stated below. Along with
Curtice model there are Kacprzak-Materka model, Statz model , Rodriguez model and
most recent Mansoor model. There may be other models, but here we are
discussing about these models and will compare these models in the next
section.

Before we move to the description
of various models, the notation of the symbols should be stated in table 3.1

Table
3.1
:
Notation of the symbols are used in the following discussion

Symbol

Notation



Drain Current



Gate to Source Voltage



Pinch-off Voltage



Drain to Source Voltage



Modulation factor



Transconductance parameter



Simulates the dependency of

 on



Measure of doping profile
extending to the insulating part



Saturation Current (

)



Simulated the dependency of

 on

 

            3.2.1 Schichman-Hodges Model

            From
the very beginning of the production of MESFET, it was very vital to establish
an equation for the drain current of the device, and thus enabling the I-V
characteristics analysis. The earliest I-V model for MESFET device may be
Schichman-Hodges Model. This model was established in the year of 1968. As we
know the introduction of MESFET in modern technology has been arrived on 1966.
So in early days of MESFET technology the I-V model is used is Schichman-Hodges
Model. This model implies the following drain current equation—



                                                                                            (3.1)



                  (3.2)



                                          (3.3)

This model is separated by
boundary region. The equation of drain current consists of 3 parts. The device
is not turned on if the


 is lower than
the threshold voltage. When


 is
sufficiently larger than the threshold voltage VT0 then the drain
current will conduct if a voltage is applied between drain and source terminal,
it be


. This drain current is divided in two parts. First
part when the


 is greater
than 0 but lower than the


, in this case the region is in linear region as
described in section 2.1.5. In this region drain current increases almost
linearly with the increase of


. When

 is
sufficiently large, nominally larger than


 then the device
is in saturation region. Current will increase with a small slope (almost
constant) with the increase of


.

This model of MESFET drain
current is almost obsolete, because further advancement of modeling has made
the equation unique for all regions. Further this model has only considered the
modulation factor, the factor by which drain current depends on


 in saturation
region. In modern modeling more factors has been considered like there are
dependency factor of ID on


 in the linear
region which is not included here. Dependency of VT0 on VDS
is also neglected.

            3.2.2 Curtice Model

            The
most simplified model and probably the most famous one is Curtice model.
Curtice has proposed his I-V model of MESFET in the year of 1980, a decade
after Schichman-Hodges.

The drain current equation for
the Curtice model is as below—



                                                                                            (3.4)



           (3.5)

Obviously

 lower than
the


, threshold voltage; the device is in OFF state. In
this state, sufficient voltage is not applied to overcome the underlying
depletion region. As the


 gets higher
than threshold voltage


, the device is ON. In this stage, if drain-source
voltage, VDS is applied the current will flow. The interesting part
of this model is that the both linear and saturation region is modeled in the
same equation.

Composing of linear region and
saturation region in the same equation has been possible for the introduction
of the tan hyperbolic function. Curtice has tactically included the tan
hyperbolic function. If we look at the tan hyperbolic function of


, it will be seen that with the small value of

,

 is approximately
linear. That means with low value of


, tanh(x) follows

. with the greater value of

,

 function
approaches unity. This is shown below graphically.



Figure
3.1:

plot of


 vs.

 

So it can be summarized—



                                                           (3.6)



                                              (3.7)

In Curtice model for lower value
of


,

the drain current follows

. As the function t

 becomes

 with
low value of


. In this case the device is in linear region and ID
follows


 linearly
approximately. When the


 is high
enough that the function


 becomes
unity, then the current


 saturates and
fixed at the value


. In this model Curtice has included a new parameter
α, which determines at which value of


,

 will
saturate.

Curtice is simplest one but its
accuracy is poor overall and deteriorates considerably with reduced ID.
Further specification will be seen in the next section where a brief comparison
is done.

            3.2.3 Statz Model

            After
Curtice, Hermann Statz has realized that the hyperbolic function inclusion
makes CPU to take more time for calculation. So Hermann Statz has proposed a
model in 1987 and he removed the hyperbolic function and deduced equation
separately for different regions.

The drain current equation stated
in Statz model is –



                                                                                (3.8)

For VGS> VT0



                   (3.9)



                                                  (3.10)

In this model it’s seen that an
extra parameter is being included which is


. Statz has fooled thoroughly the

 vs

curve, where he had seen for low

, the drain current approximately follows the square
law with the


. It’s being clear from equation as for low

 denominator
is negligible. Statz also stated that for higher value of


 the curve of

 vs

 is a square
root law. But while simulating, it’s found that it is more fits with the
practical values if the dependency of


 is linear
with


. So for higher value of

, denominator term

 is greater
than unity and a linear relation between


 and

 established.
He also has included the parameter


. This parameter actually depends on the fabrication
process. Statz has mentioned about the gradual change of doping concentration
from the channel to the insulating part. For the gradual change of doping
profile due to diffusion the


 vs

 curve of
other models do not fit well. With that empirical equation including b, it’s
possible for track the practical value more. Furthermore b actually represents
the uncertainty of source and drain parasitic resistors in a bare transistor.
As a whole b is a measure of doping profile extending to the insulating part
and depends on fabrication .Typical value of


 for an abrupt
profile with pinch off voltage VP=

0.5 and 1×20µm gate.

Apart from Curtice, Statz has
introduced another term instead of tan hyperbolic function which is


 . This is
approximated polynomial instead of tan hyperbolic function. This considerably
reduces the computational time for CPU simulation. In the equation the part for
which


 is for
saturation region and


 is for linear
region.

 

            3.2.4 Kacprzak-Materka Model

            The
error produced in both linear and saturation region is considerable in the
previous models. To improve the response of I-V characteristics to fit with the
actual values, Kacprzak and Materka had proposed a model in 1983, which is
known as Kacprzak-Materka model and as follows—



                                                                     (3.11)

For VGS>VT0



           (3.12)

The model proposes a more complex
term of tan hyperbolic function and another parameter


 has been
included here. Actually the inclusion of


 makes

 to more
follows the practical values of


. In previous models the dependency of

 on bias
voltage has not been considered. In this model it is being considered and gives
rise to more accuracy.  Kacprzak- Materka
had stated that the gross assumptions of abrupt transition to substrate and
zero substrate conductivity are not valid for a non ideal device. Typically the
MESFET is produced epitaxial on the semi insulating substrate. This substrate
posses some conductivity, so some injection of electron in substrate takes
place. If the channel is thin then the number of electron injected is not
negligible compared to the number of electrons in residual channel. Moreover,
large drain voltage beyond saturation gives rise to holes generation by impact
ionization within the high field domain and injection of these carriers into
substrate. The injected electrons and holes are the leakage current flowing in
the substrate from source to drain and shunting the main path of drain current
in the channel. So the increase of drain source voltage beyond saturation
increases leakage and effectively decreasing the channel current which can be
modeled by shifting the effective pinch off potential


 which is
drain voltage,


 dependent.
Here Kacprzak- Materka has used the Taki formula [20] which is



                                                                             (3.13)

 

Kacprzak- Materka has modified
the pinch off voltage by


 . The pinch
off voltage is the almost equal to threshold voltage.

 

            3.2.5 Rodriguez Model

            J.Rodriguez-
Tllez has proposed a model by simplifying the complex Materka model. Form the
point of view of accuracy, Materka model and Rodriguez model is almost same. In
fact about 12% improvement in Rodriguez model has been seen from the Materka
model. Rodriguez model proposes the drain equation as below—



                                                                                         (3.14)

For VGS>VT0



                             (3.15)

Objective of the Rodriguez model
was to achieve a model which imposes less error than the practical one and
simultaneously take less CPU processing time. For the better accuracy,
initially the dependency of


 are
investigated.


 and

 are the
parasitic resistances in the source and drain region. It was seen that


 and

 does not
depend on the bias. The most dependency factor on


 is

 which is
actually taken consider in all of the previous models. But


,

has small dependency on bias voltage. Rodriguez put
himself to obtain the most accurate and less taking CPU time model by
considering this dependency of


,

 on


 and he
reached to a solution by including a parameter 


 which
represents the dependency of


 on

. For CPU to take less time Rodriguez did not make
the tan hyperbolic function a complex term and the function of tan hyperbolic
function is same as depicted in section 3.2.2.

 

            3.2.6 Mansoor Model

            Mansoor
M. Ahmed has proposed an I-V model suitable for nonlinear small-signal circuit
design in 1997. Kacprzak-Materka who started the prediction of behavior of
submicron devices, Mansoor model takes this a one step further. Mansoor
proposed a model by including a new concept of shift in threshold voltage.
Drain current equation of this model is as follows—



                                                                                         (3.16)

For VGS>VT0



                      (3.17)

Mansoor M. Ahmed started with
analysis of Kacprzak-Materka model. He has seen the Kacprzak-Materka model is
not suitable for submicron devices and well for large signal analysis. He
started to modify the model of Materka to make suitable for small signal
analysis. He found two factors that may be the reason of discrepancy of Materka
model between the observed and simulated characteristics. Two factors are—

·        
The
shift in VT0 due to submicron geometry of the device

·        
The
poor control in simulating the output conductance, especially at VGS=0

In this model threshold voltage
is redefined as —



                                                                                             (3.18)



 is the
geometrical shift in the threshold voltage and it’s given as



                                                                            (3.19)

 



: Gate length



: Active channel thickness

Finally Mansoor M. Ahmed has
reached to the above DC model where


,

,

 can be
determined readily by knowing the doping density and geometry of the device and
others are empirical constants.

 

3.3
Comparison of Models

Form the above section, a summary
of all models are tabulated as below

Table
3.2
:
Summary of the different models proposed

Model

Expressions

Curtice [6]



Statz [7]



 



Materka [8]



Rodriguez [22]



Mansoor [1]



 

The nonlinear I-V models which
are discussed above, each has its own merits demerits. With the advent of new
technology, it is necessary to establishing a new model which best fits for the
current situation. One of the earliest models is Curtice model which is not
accurate with low channel length device. This obsolescence of Curtice model is
due to the advance of modern technology. Obviously Curtice model and other
earlier models are very important because these models have formed the base to
the further analysis of MESFET modeling.

Comparison of different models
described in previous section is given in a tabular format. It has to remind
that the comparison of the following table has made with respect to the 5
models discussed in section 3.2

Table 3.3: Comparison of the different
models discussed in section 3.2

Specification

Model

Curtice

Statz

Materka

Rodriguez

Mansoor

Accuracy at
linear region with low ID

Poor but 40%
better than Statz

Poorest

Most accurate,
62% better than Statz

Moderate
accurate, 50% better than Statz

High Accuracy

Accuracy at
saturation region under high current

poorest

Poor but better
than Curtice

More accurate
than Curtice and Statz but not good enough

Accuracy is
higher than the previous three models

Most accurate

Overall Accuracy

poor

Poor

Moderate

Good

Best

CPU Execution
time

next best to
Statz

smallest

highest

Moderate

Moderate

Continues model

YES

No

YES

YES

YES

Error increase
with device size decrease

Highest, but
lesser than Statz

Highest

Higher

Moderate

Less

Error in the
large devices

Highest

Highest

Less

Less but not less
than Materka

Less

CV model accuracy

worse

best

moderate

good

good

Error at Pinch
off

worse

worse

worse

good

best

 

Before Mansoor proposed his
model, Rodriguez had compared the remaining four models. He investigated the
four models Curtice, Statz, Materka and Rodriguez by varying the finger and
gate width by keeping channel length at 0.5µm. the table has shown below—

 

Table 3.4: I-V Error as function of
pinch off voltage, number of fingers and gate width (normalized at   VDS=4V, VGS=0V) from ref [11]

MODEL

Pinch-off voltage

Error2

CPU seconds

1×25µm

1×100µm

2×25µm

2×100µm

4×75µm

6×150µm

Curtice

-2

0.46

2.11

1.05

4.75

8.30

24.52

2.578125

-1

0.52

1.23

0.43

1.79

4.29

5.50

Statz

-2

0.47

2.29

1.03

4.78

9.21

28.30

1.152344

-1

0.46

0.65

0.46

1.65

4.05

7.10

Rodriguez

-2

0.21

0.93

0.56

2.38

4.10

15.66

2.628906

-1

0.18

1.17

0.22

0.92

1.38

2.59

Materka

-2

0.40

1.49

0.48

1.68

2.12

7.97

2.640625

-1

0.81

2.00

1.18

4.50

7.16

9.27

 

The pinch off voltage has a
strong bearing on the accuracy of all models with the lower (-2 V) pinch off
device providing the large model error as seen in the table 3.3. Possible
reasons for this may be due to thermal effects which become more important with
the lower pinch-off devices (-2 V) because of the increased current density.
This may also be the reason why the error of each model increases with
increasing device size. The number of finger does not disturb this pattern.

We have also investigated four models
(Curtice, Materka, Rodriguez and Mansoor model) for 4×25µm wide and 0.2µm long
GaAs with 192 mA


.. As it is seen that the channel length is very
low, so from above discussion it’s expected that the error due to Curtice,
Materka, and Rodriguez model should be high. Mansoor model is more consistent
in the reduced channel length. From computer simulation following result has
been achieved—

 

Table
3.5
: Simulation
result of different models proposed, RMS error=


Model

RMS error

Avg. RMS error

VGS (V)

0



0.5



1



1.5



2

Curtice

122.6431

116.6551

77.8200

5.4965

103.2513

72.2841

Materka

49.0615

9.5671

13.1202

19.9934

14.8648

18.1546

Rodriguez

47.6307

12.7764

7.4384

14.0555

9.3331

15.6136

Mansoor

0.6376

2.1696

4.9546

8.5496

10.0398

3.9901

 

The table 3.5 is a gross result.
It is often needed to see which part of the model exhibit more errors. So with
the values of


 as depicted
in table 3.4 and for different values of


 the square of
the error between the simulated and actual ID has been plotted for
the four devices. This shows that Mansoor model is best accurate in all regions
than others. It was expected because we have chosen 0.2µm channel length
device. For such low submicron devices other models show greater error. 



                                                  (3.20)

 

 

 

 

 

 

 

Error2 is plotted with
varying


 and



Figure
3.2:

Comparison of error in drain current from simulated and actual characteristics
for individual VGS and VDS voltage (a) Curtice Model (b) Materka Model (c)
Rodriguez Model         (d) Mansoor Model
for 4×25µm wide and 0.2µm long GaAs

 

CHAPTER 4

DC I-V CHARACTERISTIC MODEL FOR
NANOMETER RANGE GaAs MESFETS

An improved non-linear five
parameter I-V characteristics model is proposed here for nanometer range GaAs
MESFETs. The effect of bias voltage on drain current as well as on conductance
is also analyzed. The developed model was also compared to the existing models
employing the root mean square (RMS) error technique.

 

 

 

4.1 Introduction

Gallium arsenide
(GaAs) is a
compound of two elements, gallium and arsenic. Its band gap is 1.42
eV. It is an important
semiconductor and is used to
make devices such as
microwave
frequency
integrated circuits, infrared light-emitting diodes, laser diodes and solar cells.

Key advantages of GaAs
are:

·        
Higher saturated
electron velocity

·        
Higher electron mobility (8500 cm2/V-s)

·        
GaAs devices
generate less noise

·        
Breakdown voltage
is higher (4·105 V/cm)

·        
It is a direct
band gap material

·        
Higher mobility
leads to a higher current, trans-conductance and transit frequency of the device.

Gallium arsenide is used in the
manufacture of
light-emitting diode s (LEDs), which are found in
optical communications and control systems. Gallium arsenide can replace
silicon in the manufacture of linear ICs and
digital
ICs. Linear (also called
analog
) devices include
oscillators
and
amplifier
s. Digital devices are used for electronic switching, and also in computer
systems.  It is possible to fabricate
semi-insulating (SI) GaAs substrates which eliminate the problem of absorbing
microwave power in the substrate due to free carrier absorption.

 

4.2
Properties of GaAs

Some electronic and physical
properties of GaAs are listed below in table 4.1

Table
4.1:
  properties of GaAs [28]

Property

Value

Units

Electron
Mobility

(ND=1017
cm-3)

4000

cm2V-1s-1

Electron
saturation velocity

1.4 x 107

cm2s-1

Hole mobility

(ND=1017
cm-3)

250

cm2V-1s-1

Dielectric
Constant

12.6

 

Intrinsic
resistivity

109

Ohm cm

Energy gap

1.43 (Direct)

eV

Schottky
barrier height

0.7-0.8

V

Thermal
conductivity

0.9

W cm-1
K-1

 

 

4.3 Model development

To simulate the I-V characteristics as a
function of


 and


 of a
uniformly doped GaAs MESFET, the following relationship has been reported in
Ahmed et al. model [1]:



                           (4.1)

 

Here,



 =
saturation current at


 =
0V.



 = parameter that determines the drain voltage
where the drain current characteristic saturates.



 = parameter
that simulates the effective threshold voltage displacement as a function of


.



 = threshold
voltage and



= the geometric shift in threshold voltage.

The

 and

 are defined
by



                                                                                     (4.2)

And



                                                                                               (4.3)

 Respectively in

Where,



 = channel
doping density,



 = electronic
charge,



 =
permittivity of GaAs,



 = Schottky
barrier height,



= the active channel thickness and



= the gate length.

In the sub-threshold region
(lower values of


), the I-V curves are not linear rather it has some
nonlinear behavior. Furthermore the curve transitions from the triode to
saturation region are not sharp. The experimental data shows that the drain
current characteristics saturation depends not only on


, but also on

. These facts should be considered
for the development of I-V characteristics of MESFETs. Hence the term


 in equation
(4.1) has been modified to


. Another parameter

 is introduced
which determines the gate voltage where the drain current characteristic
saturates.

In equation (4.1), the effect of

 on
output conductance in the saturation region has been excluded. When the gate is
more negatively biased, the depletion layer under the gate expands as shown in
figure 4.1. Therefore, more positive charge is stored under the gate in
response to the increase in gate bias, i.e. the depletion layer exhibits charge
storage properties which can be described in electrical terms by a capacitance.
The depleted electron due to positive charge storage is transported to the
drain. The gate shaded region of figure 4.1 can be treated as the combined
properties of depletion layer capacitance and channel resistance [4]. So the
output conductance also varies with the variation of


 and
this change of output conductance occurs both before and after saturation for
nanometer GaAs MESFETs.



Figure
4.1:

The effect of


 on depletion layer for constant

. (Solid line for
more negative bias,


)

 

Therefore, the output conductance
not only depends on


 but also on

. For submicron MESFETs, the
dependencies of output conductance on


 and

 are
also reported in. The value of output conductance (


), is the most unpredictable parameter in the device
characteristics and is considered one of the major variables which generate
discrepancy in simulated and the observed characteristics, specially in
submicron devices [22-25]. In order to add the above mentioned effects on
output conductance, the term


 in equation
(4.1) has been modified to


. Another parameter

 is
introduced. The following expression is proposed for the simulation of I-V
characteristics in nanometer GaAs MESFETs [16]:



        (4.4)

 

The I-V characteristics of MESFETs are modeled for

 in out
experiment, i.e. for depletion mode MESFETs (DMESFETs). Therefore,


 dependence in


, using equation (4.4) has been discussed for
DMESFETs. However, equations (4.1) and (4.4) can also be utilized for the
modeling of enhancement mode MESFETs (EMESFETs).

4.4
The Algorithm

A
simulator was designed using MATLAB to employ expressions given in previous
equations to simulate the I-V characteristics. The optimum values of the
parameters were found out by comparing the simulated results with the
experimented results. The set of parameter values were used for which the root
mean square error (RMSE) is minimized. The RMSE can be expressed by the formula
4.5



                                                          (4.5)

Where

 



 = drain to
source current found from the simulation



 = drain to
source current found from experimental analysis



 = total
number of entities

 

The algorithm that
calculates the optimum values for the parameters is shown in figure 4.2



 

Figure
4.2:
Flowchart
for the optimization of proposed model [16] parameters

 

 

The algorithm is
designed as to choose the best possible set of values for the parameters
resulting minimum RMSE. At first an approximate initial value is set to each
parameter. Then the simulator calculates the drain current for all possible
combinations of these parameters within the defined range. This process is
repeated for different set of data for different values of gate to source
voltage (


). The simulated values of the drain current are
then compared with the experimented results. The deviation is measured in terms
of RMSE and the minimum value of RMSE searches out the best possible
combination of parameters (


). This calculates the optimal output
characteristics

 

4.5 Measured and Modeled
Characteristics

In order to demonstrate the
validity of the proposed model a wide range of GaAs MESFETs with different aspect
ratios were selected. Since these devices have different aspect ratios, their
I-V characteristics are also different. The nominal gate lengths of the devices
are 200-230 nm.

For simulation
purposes, data is generated on a PC by employing an algorithm. The values of


,

 and

 are attained
from the terminal measurements of the device, and empirical constants were
estimated by computing Root Mean Square Errors (RMSE) from the observed and the
simulated characteristics. The same algorithm has also been used for the
optimization of model parameters


 of Ahmed et
al. model (equation 4.1). Figure 4.2 shows both the simulated and the observed I-V characteristics of four different
GaAs MESFETs.

 



Figure
4.3:

Observed and simulated output characteristics of GaAs MESFETs. (The solid
circles represent the experimental data (Figs. 4.3 (a), 4.3 (b), 4.3 (c) from
[27], & 4.3 (d) from [26]), dotted lines show the simulated characteristics
using Ahmed et al.[1] model and solid lines show the simulated characteristics
using the proposed model)

 

Table
4.1:

the optimum values of the parameters used in the Mansoor et al. model and
proposed model

 

Ahmed et al.
model[1]

Proposed Model
[16]

α

λ

γ

α

β

γ

λ

η

Device of

Fig. 4.3 (a)

1.8

0.08

-0.08

1.77

0.38

-0.36

0.09

-1.31

Device of

Fig. 4.3 (b)

1.4

0.11

-0.26

1.351

-0.005

-0.246

0.11

-0.52

Device of

Fig. 4.3 (c)

1.27

0.07

-0.31

1.14

0.09

-0.44

0.077

-0.6

Device of

Fig. 4.3 (d)

1.2870

 

0.0210

-0.3580

1.2

-0.2

-0.151

0.015

-0.19

 

 

4.6 Transconductance and Output
Conductance

 

            4.6.1 Transconductance

            Trans-conductance (

)
represents the change of drain-current


 with respect to gate-to-source voltage

.
Equation of (


) is
found by differentiating the equation of


 with respect to

 

Transconductance
derived from Mansoor et al. model [1]:



                 (4.6)

 

Transconductance
derived from proposed model [16] (differentiating equation 4.4 with respect to


)







                                         (4.7)

 

The comparison in performance to
calculate the transconductance between the existing Mansoor et al. model and
the proposed model is shown below with the help of figure 4.3







IDS =
192mA/mm

VT+ΔVT
= -2.0 V




Figure
4.4:

Observed and simulated transconductance characteristics of GaAs MESFETs. (The
solid circles represent the experimental data (Figs. 4.3 (a) from [27]), dotted
lines show the simulated characteristics using Ahmed et al.[1] model and solid
lines show the simulated characteristics using the proposed model)

 

            4.6.2 Output
conductance

            Output conductance (

) represents the change of drain-current ID with
respect to drain-to-source voltage VDS. Equation of (


) is found by differentiating the equation of ID
with respect to


.

In equation (4.4), the value of

 simulates the
finite output conductance of nanometer MESFETs in the saturation region of
operation. The magnitudes of output conductance (


), can be evaluated from (4.4) and is expressed as
follows:

Output conductance
derived from Mansoor et al. model [1].







                                                  (4.8)

Output conductance
derived from proposed model [16].









                                                  (4.9)

The output
conductance was measured for all the test devices and the comparison is showed
in figure 4.4. It is obvious from the figure that the prediction of the
experimental result using the proposed model is more accurate.



Figure
4.5:

Observed and simulated output conductance characteristics of GaAs MESFETs. (The
solid circles represent the experimental data (Figs. 4.3 (a), 4.3 (b), 4.3 (c)
from [27], & 4.3 (d) from [26]), dotted lines show the simulated
characteristics using Ahmed et al.[1] model and solid lines show the simulated
characteristics using the proposed model)

 

4.7 Result Analysis

 

The RMS error for
the four devices for different bias conditions as well as the average error for
the overall analysis is compared between the proposed model and Mansoor et al.
model. The comparison is shown in table 4.2

Table 4.2: a comparative list of RMS
errors for four devices

 

VGS

(Volts)

Ahmed et al. Model [1]

Proposed Model [16]

RMS Error

Avg. RMS Error

RMS Error

Avg. RMS Error

Device
of Fig. 4. 3(a)

0.0

0.6376

3.9901

5.0816

1.6522

-0.5

2.1696

0.4371

-01

4.9546

0.0543

-1.5

8.5496

0.7323

-02

10.0398

0.7109

Device
of Fig.4. 3(b)

 

0.0

0.2017

4.7426

0.1547

3.3827

-.75

1.5221

2.5223

-1.5

6.5649

4.9784

-2.25

.2780

1.0830

Device
of Fig.4. 3(c)

0.0

2.1088

7.8314

4.7644

4.4013

-0.87

3.4199

1.1636

-1.75

6.4474

4.0356

-2.63

0.6360

1.3365

-3.5

12.8857

4.4781

Device
of Fig.4. 3(d)

0.0

12.9505

11.4001

6.4343

4.8620

 

-0.7

0.4333

6.1966

-1.4

0.9191

0.2497

-2.1

7.2179

6.3223

-2.8

24.3297

6.6529

 

The accuracy of
predicting the transconductance and the output conductance with the proposed
model is also revealed here. And for better perception of the model the results
are also compared with the existing model.

Table
4.3:

RMSE in predicting transconductance (


)