MESFET
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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.
![Cut-off Frequency
2 4
m s
co
GS
g v
f
C L
π π
= =
2/19/2021 Arpan Deyasi, India 21
Cut-off frequency depends on
[i] gate length
[ii] saturated drift velocity](https://image.slidesharecdn.com/mesfet-210218192904/85/MESFET-21-320.jpg)
MESFET
- 1. MESFET Course coordinator: Arpan Deyasi 2/19/2021 1 Arpan Deyasi, India
- 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 2/19/2021 2 Arpan Deyasi, India
- 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 2/19/2021 3 Arpan Deyasi, India 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 2/19/2021 4 Arpan Deyasi, India 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 2/19/2021 5 Arpan Deyasi, India 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 2/19/2021 6 Arpan Deyasi, India 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 2/19/2021 Arpan Deyasi, India 7 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 2/19/2021 8 Arpan Deyasi, India ( ) 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 2/19/2021 9 Arpan Deyasi, India ( ) 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 + + = 2/19/2021 10 Arpan Deyasi, India ( ) ( ) 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 2/19/2021 Arpan Deyasi, India 11 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 = − × ∫ 2/19/2021 12 Arpan Deyasi, India ( ) ( ) 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 2/19/2021 13 Arpan Deyasi, India 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 2/19/2021 14 Arpan Deyasi, India 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 + + = 2/19/2021 15 Arpan Deyasi, India ( ) ( ) 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 = + = = 2/19/2021 16 Arpan Deyasi, India 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 + + 2/19/2021 17 Arpan Deyasi, India 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 = 2/19/2021 18 Arpan Deyasi, India IP: pinch-off current
- 19. ID VGS1 VGS2 VGS3 I-V characteristics 2/19/2021 19 Arpan Deyasi, RCCIIT 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 2/19/2021 20 Arpan Deyasi, India microwave signal
- 21. Cut-off Frequency 2 4 m s co GS g v f C L π π = = 2/19/2021 Arpan Deyasi, India 21 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 2/19/2021 Arpan Deyasi, India 22 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 = + + 2/19/2021 Arpan Deyasi, India 23 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 2/19/2021 24 Arpan Deyasi, India 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 2/19/2021 Arpan Deyasi, India 25 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 2/19/2021 26 Arpan Deyasi, India Switching circuit in microwave region