Development of
Improved IV 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 metalsemiconductor 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 pn 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 metalsemiconductor
barrier (as compared to a pn 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 IV 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 fieldeffect transistor principle. This principle was
patented by Canada.
·
1934 German physicist Dr. Oskar Heil
patented another fieldeffect transistor
·
Lilienfeld
and Heil did not construct any devices.
·
1939 William Shockley and a coworker 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
coworkers 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 pchannel Si MOSFET was
described by Dawon Kahng, a member of Atallai group
^{·
}1966 The first GalliumArsenide
Schottkygate fieldeffect 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
semiinsulating 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 nonconducting diode
junction. In addition to this it also has a negative temperature coefficient
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
radar
devices
·
Commercial
optoelectronics
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 IV characteristics are
also discussed here. For predicting IV 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:
nMESFET 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
pn junction. So the depletion width is along the lightly doped n implant i.e.
the active region according to the following equation.
the whole depletion width t, then built in potential can be found as—
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
Here we have assumed that
metalsemiconductor 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.
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—
On the semiconductor side,
in potential barrier. This barrier, similar to the case of pn 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—
This makes the potential
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.
V_{GS}

V_{DS}

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
dropped across the depletion region when it just cover the whole channel
thickness (
calculated as below—
So the threshold
voltage
The energy band
diagram of the junction of gatechannel under reverse bias is shown below—
Figure
2.4:
MESFET band diagram under reverse bias V_{R}: 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 IV characteristics, the velocity saturation
phenomenon should be described briefly to support the IV 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 nMESFET this electric field is in negative
Xaxis direction. From the curve, it has been seen for low electric field the
velocity increases with the field linearly. If
of particle (here electron) and E is the electric field then velocity can be
written as
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—
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
Figure 2.6
V_{GS}

V_{DS}

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,
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
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 (
of the channel. This voltage varies from 0 V at the source to the
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
of
written as
voltage distribution is
The current in the
channel can written as
(m/sec)
electron passing (m^{2})
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
Here
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
Here
of the light n region as depicted earlier and
of the depletion region. So effective undepleted region along which electron
can pass through is
Let the doping
concentration at the light n region is
(2.10) and (2.11) can be substituted in equation (2.9) and it looks like below—
In this equation
(2.12) negative sign actually means that direction of current
of drift velocity of electron is opposite.
From the definition
of potential it can be written as
So equation (2.12)
becomes
Now the expression
of d and
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 pn junction
reverse bias by applying a voltage
region and the part of the depletion region at the n region is given as
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
(2.15) can be written as
Now let
required to reverse bias our MESFET bar such that depletion region covers the
whole thickness
as pinch off voltage, so from equation (2.16)
Assuming,
as
Now the threshold
voltage of the device will be the voltage required to fully deplete the light n
region and that will be
For the expression
of
that the effective reverse bias voltage along the interface between gate and
channel is
equation (2.17) we can write
Now recalling
equation (2.14) and putting the value of
From equation
(2.18), using the
integratin on both sides of equation (2.22)
Assuming,
Then
for
for
Equation (2.23) can
be written as
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—
For
As we know,
The equation is
modified as below
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 IV modeling there are different approaches of
deducing drain current equation. Here, our main goal is to establish a drain
current equation and compare its IV characteristics with other models.
2.1.5 IV
characteristics of MESFET
An IV characteristic of
a typical MESFET is similar to that of a MOSFET and JFET. An IV characteristic
of a typical MESFET is shown in Figure 2.7. In this Figure the drain current is
plotted against drainsource 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 IV 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:
IV 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 gatechannel 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 I_{D}
is very low for lower
the channel will stop. It can be said from intuition that the threshold voltage
of the Figure 2.7 is near the lowest V_{GS} curve. So for current
conducting region or modes are valid for
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
fig 2.7, it is seen that for low value of
to source depends almost linearly on V_{DS} for a particular value of
so the region is called linear region.
The region is also called a triode region. This region is applicable for
the value of
The
for all value of
locus.
With the decrease
of
decreases as shown in the Figure 2.7
It was noted
earlier that the drain current can be written as below—
Where
(m/sec)
electron passing (m^{2})
From Figure 2.7 one
can see that with the increase in
section
it is expected to decrease in current
increase in
not the case. Because of the presence of drift velocity
for low E_{X}, with the increase of E_{X} drift velocity of v
increases linearly and it overpowers the decrease in cross section A. So with
the increase in voltage
linear region.
2.1.5.2 Saturation Region (
In linear region, with
increase of
But after a certain value of
change considerably. Normally this saturation of
reached at
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
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,
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
depletion on drain side increases. At a certain value of
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
greater than
Pinch off

Gate

V_{DS}

V_{GS}

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
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
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
For pinching off
the voltage required can be written as—
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
Typical values of
these parameters may be—
ϵ=13
So pinch off
voltage,
So it has been seen
before pinch off occurs, the drift velocity of carrier will saturate and thus
it saturates the drain current
Gate

V_{DS}

V_{GS}

Figure 2.9: Current Saturation in a MESFET
due to velocity saturation
2.1.5.3
Breakdown
If one keeps increasing
the
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
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 
IV Curve 
Linear 



Saturation (due to velocity 



Saturation 



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 nMESFET. Each element is intended to represent
the electrical character of a particular region of the device.
·
C_{gs}:
Capacitance represents the charge storage in the depletion region at source
gate region
·
C_{gd}:
Capacitance represents the charge storage in the depletion region at gate drain
region
·
R_{s}:
Resistance represents the bulk resistance of Nlayer in the source gate region
together with any contact resistance at the source ohmic metallization
·
R_{d}:
Resistance represents the bulk resistance of Nlayer in the gate drain region
together with any contact resistance at the drain ohmic metallization
·
R_{g}:
parasitic resistance at the gate
·
R_{i}:
internal sourcegate resistance
·
g_{m0}v:
represents the drain current flow along the channel from drain to source
·
r_{0}:
substrate resistance
·
C_{0}:
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 
R_{g} 
0.53 
C_{gs} 
0.150.4 
R_{i} 
210 
R_{s} 
15 
R_{d} 
15 
C_{gd} 
0.010.03 
gm_{0} 
2040 
C_{0} 
0.050.1 
r_{0} 
250500 
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.
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 IV curve
of Depletion type MESFET—
Figure
2.16:
IV characteristics of Depletion type MESFET
It is seen that
here
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 gatechannel junction due to built in potential is
greater than the channel width i.e.
fully covers the channel region, so the device is normally OFF device. Assume
that this is an nMESFET 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
through the channel. So here the threshold voltage is positive and giving more
positive voltage at the gate electrode the drain current
This is depicted in
the following Figure—
Figure
2.17:
enhancement type MESFET
Typical IV
characteristics of an enhancement type MESFET has been shown below. It is clear
that the IV characteristics for both depletion and enhancement type are same
i.e. with the increase in
Figure
2.18:
IV 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 Nonself 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 IV 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 IV 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 IV characteristics of devices. So a reasonable
and practical IV model is often needed by a designer. Some of the importances
of IV model are mentioned below
·
Significant
aids to integrated circuit design
·
Eliminates
the delay of cut and try design approaches
·
IV
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 IV characteristics, he has to remind some factors. The IV
model should be such that it nearly follows the practical values. Some of the
specifications of IV 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 IV 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,
SchichmanHodges proposed another model which is stated below. Along with
Curtice model there are KacprzakMaterka 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 

Pinchoff Voltage 

Drain to Source Voltage 

Modulation factor 

Transconductance parameter 

Simulates the dependency of 

Measure of doping profile 

Saturation Current ( 

Simulated the dependency of 
3.2.1 SchichmanHodges 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 IV
characteristics analysis. The earliest IV model for MESFET device may be
SchichmanHodges 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 IV model is used is SchichmanHodges
Model. This model implies the following drain current equation—
This model is separated by
boundary region. The equation of drain current consists of 3 parts. The device
is not turned on if the
the threshold voltage. When
sufficiently larger than the threshold voltage V_{T0} then the drain
current will conduct if a voltage is applied between drain and source terminal,
it be
part when the
than 0 but lower than the
described in section 2.1.5. In this region drain current increases almost
linearly with the increase of
sufficiently large, nominally larger than
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
region. In modern modeling more factors has been considered like there are
dependency factor of I_{D} on
region which is not included here. Dependency of V_{T0} on V_{DS}
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 IV model of MESFET in the year of 1980, a decade
after SchichmanHodges.
The drain current equation for
the Curtice model is as below—
Obviously
the
this state, sufficient voltage is not applied to overcome the underlying
depletion region. As the
than threshold voltage
voltage, V_{DS} 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
linear. That means with low value of
approaches unity. This is shown below graphically.
Figure
3.1:
plot of
So it can be summarized—
In Curtice model for lower value
of
low value of
follows
approximately. When the
enough that the function
unity, then the current
fixed at the value
α, which determines at which value of
saturate.
Curtice is simplest one but its
accuracy is poor overall and deteriorates considerably with reduced I_{D}.
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 –
For V_{GS}> V_{T0}
In this model it’s seen that an
extra parameter is being included which is
law with the
is negligible. Statz also stated that for higher value of
root law. But while simulating, it’s found that it is more fits with the
practical values if the dependency of
with
than unity and a linear relation between
He also has included the parameter
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
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
profile with pinch off voltage V_{P}=
Apart from Curtice, Statz has
introduced another term instead of tan hyperbolic function which is
approximated polynomial instead of tan hyperbolic function. This considerably
reduces the computational time for CPU simulation. In the equation the part for
which
saturation region and
region.
3.2.4 KacprzakMaterka Model
The
error produced in both linear and saturation region is considerable in the
previous models. To improve the response of IV characteristics to fit with the
actual values, Kacprzak and Materka had proposed a model in 1983, which is
known as KacprzakMaterka model and as follows—
For V_{GS}>V_{T0}
The model proposes a more complex
term of tan hyperbolic function and another parameter
included here. Actually the inclusion of
follows the practical values of
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
drain voltage,
Here Kacprzak Materka has used the Taki formula [20] which is
Kacprzak Materka has modified
the pinch off voltage by
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—
For V_{GS}>V_{T0}
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
investigated.
parasitic resistances in the source and drain region. It was seen that
depend on the bias. The most dependency factor on
actually taken consider in all of the previous models. But
himself to obtain the most accurate and less taking CPU time model by
considering this dependency of
reached to a solution by including a parameter
represents the dependency of
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 IV model suitable for nonlinear smallsignal circuit
design in 1997. KacprzakMaterka 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—
For V_{GS}>V_{T0}
Mansoor M. Ahmed started with
analysis of KacprzakMaterka model. He has seen the KacprzakMaterka 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 V_{T0} due to submicron geometry of the device
·
The
poor control in simulating the output conductance, especially at V_{GS}=0
In this model threshold voltage
is redefined as —
geometrical shift in the threshold voltage and it’s given as
Finally Mansoor M. Ahmed has
reached to the above DC model where
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 IV 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 
Poor but 40% 
Poorest 
Most accurate, 
Moderate 
High Accuracy 
Accuracy at 
poorest 
Poor but better 
More accurate 
Accuracy is 
Most accurate 
Overall Accuracy 
poor 
Poor 
Moderate 
Good 
Best 
CPU Execution 
next best to 
smallest 
highest 
Moderate 
Moderate 
Continues model 
YES 
No 
YES 
YES 
YES 
Error increase 
Highest, but 
Highest 
Higher 
Moderate 
Less 
Error in the 
Highest 
Highest 
Less 
Less but not less 
Less 
CV model accuracy 
worse 
best 
moderate 
good 
good 
Error at Pinch 
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: IV Error as function of
pinch off voltage, number of fingers and gate width (normalized at VDS=4V, VGS=0V) from ref [11]
MODEL 
Pinchoff voltage 
Error^{2} 
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 pinchoff 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
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 

V_{GS }(V) 

0 





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
in table 3.4 and for different values of
the error between the simulated and actual I_{D} 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.
Error^{2} is plotted with
varying
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 IV CHARACTERISTIC MODEL FOR
NANOMETER RANGE GaAs MESFETS
An improved nonlinear five
parameter IV 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 lightemitting diodes, laser diodes and solar cells.
Key advantages of GaAs
are:
·
Higher saturated
electron velocity
·
Higher electron mobility (8500 cm^{2}/Vs)
·
GaAs devices
generate less noise
·
Breakdown voltage
is higher (4·10^{5} V/cm)
·
It is a direct
band gap material
·
Higher mobility
leads to a higher current, transconductance and transit frequency of the device.
Gallium arsenide is used in the
manufacture of lightemitting 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
semiinsulating (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 (N_{D}=10^{17} 
4000 
cm^{2}V^{1}s^{1} 
Electron 
1.4 x 10^{7} 
cm^{2}s^{1} 
Hole mobility (N_{D}=10^{17} 
250 
cm^{2}V^{1}s^{1} 
Dielectric 
12.6 

Intrinsic 
10^{9} 
Ohm cm 
Energy gap 
1.43 (Direct) 
eV 
Schottky 
0.70.8 
V 
Thermal 
0.9 
W cm^{1} 
4.3 Model development
To simulate the IV characteristics_{ }as a
function of
uniformly doped GaAs MESFET, the following relationship has been reported in
Ahmed et al. model [1]:
Here,
saturation current at
0V.
where the drain current characteristic saturates.
that simulates the effective threshold voltage displacement as a function of
voltage and
The
by
And
Respectively in
Where,
doping density,
charge,
permittivity of GaAs,
barrier height,
In the subthreshold region
(lower values of
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
for the development of IV characteristics of MESFETs. Hence the term
(4.1) has been modified to
which determines the gate voltage where the drain current characteristic
saturates.
In equation (4.1), the effect of
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
this change of output conductance occurs both before and after saturation for
nanometer GaAs MESFETs.
Figure
4.1:
The effect of
more negative bias,
Therefore, the output conductance
not only depends on
dependencies of output conductance on
also reported in. The value of output conductance (
characteristics and is considered one of the major variables which generate
discrepancy in simulated and the observed characteristics, specially in
submicron devices [2225]. In order to add the above mentioned effects on
output conductance, the term
(4.1) has been modified to
introduced. The following expression is proposed for the simulation of IV
characteristics in nanometer GaAs MESFETs [16]:
The IV characteristics of MESFETs are modeled for
experiment, i.e. for depletion mode MESFETs (DMESFETs). Therefore,
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 IV 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
Where
source current found from the simulation
source current found from experimental analysis
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 (
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 (
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
IV characteristics are also different. The nominal gate lengths of the devices
are 200230 nm.
For simulation
purposes, data is generated on a PC by employing an algorithm. The values of
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
al. model (equation 4.1). Figure 4.2 shows both the simulated and the observed IV 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. 
Proposed Model 

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
Transconductance (
represents the change of draincurrent
Equation of (
found by differentiating the equation of
Transconductance
derived from Mansoor et al. model [1]:
Transconductance
derived from proposed model [16] (differentiating equation 4.4 with respect to
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
I_{DS} = V_{T}+ΔV_{T}

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 (
respect to draintosource voltage VDS. Equation of (
with respect to
In equation (4.4), the value of
finite output conductance of nanometer MESFETs in the saturation region of
operation. The magnitudes of output conductance (
follows:
Output conductance
derived from Mansoor et al. model [1].
Output conductance
derived from proposed model [16].
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

V_{GS} (Volts) 
Ahmed et al. Model [1] 
Proposed Model [16] 

RMS Error 
Avg. RMS Error 
RMS Error 
Avg. RMS Error 

Device 
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 
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 
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 
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 (