1) MESFET stands for Metal-Semiconductor Field Effect Transistor and consists of a conducting channel between a source and drain contact that is controlled by a Schottky metal gate. 2) The I-V characteristics of a MESFET can be modeled as a voltage-controlled current source where the drain current is varied by small changes in the gate potential. 3) MESFETs have advantages over other transistors for RF applications due to their high electron mobility, low stray capacitance from the Schottky gate, and negative temperature coefficient.

1. MESFET
Course coordinator: Arpan Deyasi
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2. MESFET
= MEtal Semiconductor Field Effect Transistor
= Schottky gate FET
Schottky gate
p
n - Source n -Drain
Back contact
semi-insulating substrate
Depletion region
channel
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3. MESFET consists of a conducting channel positioned
between a source and drain contact region
Structure
carrier flow from source to drain is controlled by a
Schottky metal gate
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base material on which the transistor is fabricated is
semi-insulting substrate (at present GaAs is used)

4. buffer layer is epitaxially grown over the substrate to isolate
defects in the substrate from the transistor
Structure
lightly doped (n) conducting layer of semiconducting
material is developed for channel formation
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two Ohmic contacts, the source and drain, are fabricated on
the highly doped layer to provide access to the external
circuit
Schottky contact is fabricated as gate contact

5. depletion region is wide enough to pinch off the channel
without applied voltage, so the enhancement-mode
MESFET is naturally "OFF“
When a positive voltage is applied between the gate and
source, the depletion region shrinks, and the channel
becomes conductive
Enhancement-mode MESFET
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becomes conductive
a positive gate-to-source voltage puts the Schottky diode
in forward bias, where a large current can flow

6. If the depletion region does not extend all the way to
the p-type substrate, the MESFET is a depletion-mode
MESFET
A depletion-mode MESFET is conductive or "ON" when
VGS is not applied and is turned "OFF" upon the
application of a -V , which increases the width of the
Depletion mode MESFET
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application of a -VGS, which increases the width of the
depletion region such that it "pinches off" the channel

7. Symbol of MESFET
Depletion
p-type
Enhancement
p-type
Depletion
n-type
Enhancement
n-type
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p-type p-type n-type n-type

8. I-V characteristics
Resistance of the channel
e D
L L
R
A q N A
ρ
µ
= =
L
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( )
e D dep
L
R
q N W a X x
µ
=
 
−
 
Xdep(x): width of depletion layer of Schottky
barrier at x along the channel

9. I-V characteristics
( )
D
D
e D dep
I dx
dV I dR
q N W a X x
µ
= =
 
−
 
voltage drop across elemental section ‘dx’ of the channel
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( )
D e D dep
I dx q N W a X x dV
µ  
= −
 

10. I-V characteristics
where
2 ( )
( )
s G bi
dep
D
V x V V
X x
qN
ε  + + 
 
=
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( ) ( )
D
dep dep
s
qN
dV X x dX x
ε
=

11. I-V characteristics
2 2
1 1
( )
X X
D e D dep
X X
I dx q N W a X x dV
µ  
= −
 
∫ ∫
Integrating, we get
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1 1
X1
X2
S
D

12. I-V characteristics
2
1
1
( )
( ) ( )
X
D e D dep
X
D
I q N W a X x
L
qN
X x dX x
µ  
= −
 
×
∫
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( ) ( )
D
dep dep
s
qN
X x dX x
ε
×

13. I-V characteristics
2
2
2 2 3 3
2 1 2 1
2
( ) ( )
2 3
D e
D
s
q N W
I a X X X X
L
µ
ε
 
= − − −
 
 
X : minimum depletion layer width of channel
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X1: minimum depletion layer width of channel
X2: maximum depletion layer width of channel

14. 2 ( )
( )
s G bi
dep
D
V x V V
X x
qN
ε  + + 
 
=
I-V characteristics
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2
2
2 ( )
( )
s G bi
dep
D
a V x V V
X x
qN a
ε  + + 
 
=

15. I-V characteristics
2
2 ( )
( )
s G bi
dep
D
V x V V
X x a
qN a
ε  + + 
 
=
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( )
( )
G bi
dep
P
V x V V
X x a
V
 + + 
 
=

16. I-V characteristics
1 ( ) 0
( )
G bi
dep V x
P
V V
X X x a
V
=
 + 
 
= =
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2 ( )
( )
D
G bi D
dep V x V
P
V V V
X X x a
V
=
 + + 
 
= =

17. I-V characteristics
2
2
2 2 3 3
2 1 2 1
2
( ) ( )
2 3
D e
D
s
q N W
I a X X X X
L
µ
ε
 
= − − −
 
 
3/2
2 G bi
D
V V
V
 
 
 + 
 
 
+  
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3/2
2
3
2
3
G bi
D
P P
D P
G bi D
P
V V
V
V V
I I
V V V
V
 + 
 
 
+  
 
 
 
 
=
 
 
 + + 
 
 
−  
 
 
 
 

18. I-V characteristics
where
2
2
3
2
D e
P
s
q N W
I a
L
µ
ε
=
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IP: pinch-off current

19. ID
VGS1
VGS2
VGS3
I-V characteristics
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VD

20. Features of MESFET
As the drain current can be varied by introducing small
variations in the gate potential, so the MESFET can be
modelled as a voltage-controlled-current-source (VCCS)
It may be used to increase the power level of a
microwave signal
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microwave signal

21. Cut-off Frequency
2 4
m s
co
GS
g v
f
C L
π π
= =
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Cut-off frequency depends on
[i] gate length
[ii] saturated drift velocity

22. Maximum Frequency of Oscillation
max
2
m s
o
GS
g v
f
C L
π π
= =
It is half-of-the cut-off frequency
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It is half-of-the cut-off frequency

23. Maximum Frequency of Oscillation
When input and output ports are matched
and power gain is maximum
max 0.5 D
o co
R
f f
R R R
 
=  
+ +
 
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maxo co
G i S
R R R
 
+ +
 

24. Advantages of GaAs MESFETs over other Transistors
high level of electron mobility which is required for
high performance RF applications
Schottky diode gate structure results in very low stray
capacitance levels which lend themselves to excellent
RF and microwave performance
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RF and microwave performance
MESFET has a very much higher input when compared
to bipolar transistors as a result of the non-conducting
diode junction

25. Advantages of GaAs MESFETs over other Transistors
MESFET has a negative temperature co-efficient which
inhibits some of the thermal problems experienced
with other transistors
Compared to MOSFET, MESFET does not have the
problems associated with oxide traps
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Low parasitic capacitances
Very low noise figure
problems associated with oxide traps
MESFET has better channel length control than a JFET

26. Application
RF amplifier
Satellite communications
RADAR
Cell Phones
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Switching circuit in microwave region