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ECX 5239

Physical ElectronicsPresentation 3

Answers for Assignment #3

Name: A.H.T.T.N.S. Thotahewa

Reg # : 60664829

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Different types of

Ga As MESFET models

Characteristics, Uses & Limitations

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What is FET ?

A field-effect transistor (FET) is a three-terminal device in

which current flows through a narrow conducting channel

between two electrodes called source and drain. The current ismodulated by the electric field caused by voltage applied at the

third electrode called gate. Current flow along the channel is

almost entirely due to the motion of majority carriers. So, the

FET is a unipolar device and there are two types of FETs:

n-channel devices and p-channel devices.

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FETs

Junction FETs

(JFET)IG FETs,MISFETs,

MOSFETs

Ga As FETs,

MESFETs

DE-MOSFETs E-MOSFETs

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Ga As MESFET =

Gallium Ar senide Metal Semiconductor FieldEffect Transistor

Gallium Arsenide is a compound semiconductor madefrom Gallium(Group III) & Arsenic(Group V) elements.

The MESFET consists of a conducting channel positioned between a source and drain contact region.

The charge carriers(electrons) flow from source to drain iscontrolled by a Schottky barrier gate.

The control of the channel is obtained by varying thedepletion layer width underneath the metal contact whichmodulates the thickness of the conducting channel andthereby the current.

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Fig 1 : Basic structure of a MESFET

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Gallium Arsenide Vs Silicon

The saturated electron drift velocity of GaAs is ~2 times Si.

(That means GaAs devices require less voltage to enter

saturation)

The mobility of electrons in GaAs is 6-7 times that of Si.(i.e. very fast electron transit time)

Intrinsic bulk resistivities are higher, which minimizes

parasitic capacitances and allows easy isolation of multiple

devices in a single substrate. (GaAs=108 & Si=2.2x105 cm)

GaAs has wider operating temperature(±2000C), due to wider

band gap.

GaAs substrate is more brittle than Si and therefore thicker.

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MESFET Operation

The channel in a MESFET is formed by doping GaAs substrate.

A depletion region extends only part away through the channelfor a depletion device (highly doped thick channel) and all the waythrough for an enhancement device (lightly doped thin channel).

MESFETs are similar in operation to MOSFETs except for aschottky diode formed at the gate junction; the capacitance of ehich is used to control the effective charge in the channel.

The threshold voltage is given by:

(barrier volatge pinch off voltage )

The pinch off voltage is simply the total voltage, both built in potential voltage and externally applied voltage necessary tocompletely deplete the channel of mobile charge carriers.

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The drain current in a MESFET is controlled by VGS & VDS and

the device has three regions of operation.

Cutt-off ; where the channel is completely cut off by the

depletion region, which extends into the channel. This occurs

when the external bias voltage applied to the schottky diode is

less than the threshold voltage, 0eTH GS

V V

Linear or Ohmic ; where there is a voltage applied to V GS

above the threshold voltage , and V DS is positive

and less than the drain-source saturation voltage, V DS sat . The

drain current is linear with V DS in this region so the channel act

as a resister.

0" TH GS V V

Saturation ; if V GS ± V TH > 0 and V DS > V DS sat , the depletion

region becomes wedge shaped and the channel becomes pinched

off at the drain end limiting the flow of current.

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VGS

S G D

0e

TH GS V V

Off Cut

VDS

S G D

S G D

sat DS DS

TH GS

V V

V V

Linear

" 0

sat DS DS

TH GS

V V

V V

S aturation

u

" 0

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Approximations used in

MESFET circuit simulation models

1. Uniform charge distribution ; The MESFET channel is assumed to

consist of a uniformly doped n-region, which ends abruptly at a specific

depth.

2. Gradual channel approximation ; The potential in the channel at the

gate junction is assumed to be a slowly varying function of the position in

the x direction.

3. Abrupt depletion layer ; The depletion layer that forms in the channel

under the MESFET gate is assumed to end abruptly.

4. Piecewise-linear approximation of the electron velocity as a function of

the electric field in the channel.

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The Curtice Model

This is the modified version (by Curtice) of the model which is proposed by V.Tuyl in 1974.

This model is also known as Hyperbolic Tangent Model.

This model describes I DS

(Drain to Source current) as a function

of V DS and V GS as ;

DS DS

Exp

TH GS DS V V V V I EP F tanh1 v!

I DS

± drain to source current

V GS ± gate to source voltage

V DS ± drain to source voltage

± Transconductance coefficient

V TH

± Threshold voltage

± tanh constant

± channel length modification coefficient

Exp ± variable exponent ( Exp = 2 for curtice model)

F

E

P

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Here is an example fit of this model¶s VI characteristic;

Figure 1 : VI characteristic of hyperbolic tangent model (original curtice model)

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The main disadvantage of this model is , it takes long time to

evaluate tanh function in the equation. This leads to slow down

the simulation process. Also by varying the value of Exp, we can

get better approximation to measured VI curve. Usually Exp

values smaller than 2 gives best results.

As an example consider the following graph, obtained for

Exp = 1.51.

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Figure 2 : VI characteristic of hyperbolic tangent model (Exp = 1.51)

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The Statz Model

In 1987, Statz proposed a FET model to simulate IDS(VDS, VGS)characteristics by using the following expressions called Statz

Model (also known as Raytheon model)

Linear region :

E

E

H

FP

30 for

311

11

32

¼¼½

»

¬¬-

«¹ º

¸©ª

¨v

¼¼½

»

¬¬-

«

! DS

DS

TH GS

TH GS DS DS V

V

V V

V V V I

Saturation region :

EH

FP

3 for

11

2

u¼¼½

»

¬¬-

«

!

DS

TH GS

TH GS

DS DS V

V V

V V V I

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The term in linear region equation is a polynomial

approximation to the tanh function that saves evaluation time of the

simulation process. So the simulation process takes less time than that of in

Curtice model.

¼¼½

»

¬¬-

«¹ º

¸©ª

¨

3

311

DS V E

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The following graph shows example fit of Statz model VI characteristic

curves.

Figure 3 : VI characteristic of Statz model

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GaAs MESFET Modeling for Digital I ntegrated Circuit

Simulation by Mikael Anderson, Department of Electrical En

gineering, Helsinki University of Technology, 1991

References

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T hank You

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