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MESFET

Electrical characteristics of MESFET

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MESFET

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  1. Course: Electron Device Arpan Deyasi Electron MESFET Device Course coordinator: Arpan Deyasi 7/16/2021 Arpan Deyasi, India 1

  2. MESFET = MEtal Semiconductor Field Effect Transistor = Schottky gate FET Arpan Deyasi Electron Schottky gate n - Source Device n -Drain p Depletion region channel semi-insulating substrate Back contact Arpan Deyasi, India 7/16/2021 2

  3. Structure MESFET consists of a conducting channel positioned between a source and drain contact region Arpan Deyasi carrier flow from source to drain is controlled by a Schottky metal gate Electron base material on which the transistor is fabricated is semi-insulting substrate (at present GaAs is used) Device 7/16/2021 Arpan Deyasi, India 3

  4. Structure buffer layer is epitaxially grown over the substrate to isolate defects in the substrate from the transistor Arpan Deyasi lightly doped (n) conducting layer of semiconducting material is developed for channel formation Electron two Ohmiccontacts, the source and drain, are fabricated on the highly doped layer to provide access to the external circuit Device Schottky contact is fabricated as gate contact 7/16/2021 Arpan Deyasi, India 4

  5. Enhancement-mode MESFET depletion region is wide enough to pinch off the channel without applied voltage, so the enhancement-mode MESFET is naturally "OFF“ Arpan Deyasi When a positive voltage is applied between the gate and source, the depletion region shrinks, and the channel becomes conductive Electron a positive gate-to-source voltage puts the Schottky diode in forward bias, where a large current can flow Device 7/16/2021 Arpan Deyasi, India 5

  6. Depletion mode MESFET If the depletion region does not extend all the way to the p-type substrate, the MESFET is a depletion-mode MESFET Arpan Deyasi Electron A depletion-mode MESFET is conductive or "ON" when VGS is not applied and is turned "OFF" upon the application of a -VGS, which increases the width of the depletion region such that it "pinches off" the channel Device 7/16/2021 Arpan Deyasi, India 6

  7. Symbol of MESFET Arpan Deyasi Electron Depletion p-type Enhancement p-type Depletion n-type Enhancement n-type Device 7/16/2021 Arpan Deyasi, India 7

  8. I-V characteristics Resistance of the channel Arpan Deyasi L A L N A  =  = R Electron q e D Device L = R      − ( ) x q N W a X e D dep Xdep(x): width of depletion layer of Schottky barrier at x along the channel 7/16/2021 Arpan Deyasi, India 8

  9. I-V characteristics voltage drop across elemental section ‘dx’ of the channel Arpan Deyasi I dx   Electron = = dV I dR D D    − ( ) x q N W a X e D dep Device     =  − ( ) x I dx q N W a X dV D e D dep 7/16/2021 Arpan Deyasi, India 9

  10. I-V characteristics where Arpan Deyasi   + +   2 ( ) V x  V V s G bi = ( ) x X Electron dep qN D Device qN  = ( ) x dX ( ) x dV X D dep dep s 7/16/2021 Arpan Deyasi, India 10

  11. I-V characteristics Integrating, we get Arpan Deyasi X  X  2 2 I dx Electron     =  − ( ) x q N W a X dV D e D dep X X 1 1 Device S X1 D X2 7/16/2021 Arpan Deyasi, India 11

  12. I-V characteristics Arpan Deyasi X  1 L 2     =  − ( ) x I q N W a X D e D dep Electron X 1 qN   ( ) x dX ( ) x X D Device dep dep s 7/16/2021 Arpan Deyasi, India 12

  13. I-V characteristics 2 Arpan Deyasi 2        2 3 q N Electron W 2 2 3 3 = − − − ( ) ( ) I a X X X X D  e 2 1 2 1 D 2 L s X1: minimum depletion layer width of channel Device X2: maximum depletion layer width of channel 7/16/2021 Arpan Deyasi, India 13

  14. I-V characteristics dep Arpan Deyasi   + +   2 ( ) V x  V V s G bi = ( ) x X Electron qN D dep Device  V x   + +   2 2 ( ) qN a a V V s G bi = ( ) x X 2 D 7/16/2021 Arpan Deyasi, India 14

  15. I-V characteristics dep Arpan Deyasi   + +   2 ( ) qN a V x  V V s G bi = ( ) x X a 2 Electron D Device V x   + +   ( ) V V G bi = ( ) x X a dep V P 7/16/2021 Arpan Deyasi, India 15

  16. I-V characteristics Arpan Deyasi   +   V V G bi = = ( ) x X X a 1 Electron dep V = ( ) 0 V x P Device   + +   V V V V G bi D = = ( ) x X X a 2 dep = ( ) V x V D P 7/16/2021 Arpan Deyasi, India 16

  17. I-V characteristics 2  2 Arpan Deyasi       2 3 q N W 2 2 3 3 = − − − ( ) ( ) I a X X X X D  e 2 1 2 1 D 2 L s Electron                 3/2           +   V V 2 3 V V G bi + Device D V P P = I I D P 3/2          + +   V V V V 2 3 G bi D −  P 7/16/2021 Arpan Deyasi, India 17

  18. I-V characteristics where Arpan Deyasi 2  2 q N W Electron = 3 I a D  e P 2 L s Device IP: pinch-off current 7/16/2021 Arpan Deyasi, India 18

  19. I-V characteristics ID VGS3 Arpan Deyasi VGS2 Electron VGS1 Device VD 7/16/2021 Arpan Deyasi, India 19

  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) Arpan Deyasi Electron It may be used to increase the power level of a microwave signal Device 7/16/2021 Arpan Deyasi, India 20

  21. Cut-off Frequency Arpan Deyasi g v  = = f m s Electron co  2 4 C L GS Device Cut-off frequency depends on [i] gate length [ii] saturated drift velocity 7/16/2021 Arpan Deyasi, India 21

  22. Maximum Frequency of Oscillation Arpan Deyasi g C v  = = f m s max o  2 L It is half-of-the cut-off frequency Electron GS Device 7/16/2021 Arpan Deyasi, India 22

  23. Maximum Frequency of Oscillation Arpan Deyasi When input and output ports are matched and power gain is maximum Electron       R R = 0.5 f f D max o co + + R R Device G i S 7/16/2021 Arpan Deyasi, India 23

  24. Advantages of GaAs MESFETs over other Transistors Arpan Deyasi high level of electron mobility which is required for high performance RF applications Electron Schottky diode gate structure results in very low stray capacitance levels which lend themselves to excellent RF and microwave performance Device MESFET has a very much higher input when compared to bipolar transistors as a result of the non-conducting diode junction 7/16/2021 Arpan Deyasi, India 24

  25. Advantages of GaAs MESFETs over other Transistors Arpan Deyasi MESFET has a negative temperature co-efficient which inhibits some of the thermal problems experienced with other transistors Electron Compared to MOSFET, MESFET does not have the problems associated with oxide traps Device MESFET has better channel length control than a JFET Low parasitic capacitances Very low noise figure 7/16/2021 Arpan Deyasi, India 25

  26. Application RF amplifier Arpan Deyasi Satellite communications Electron RADAR Cell Phones Device Switching circuit in microwave region 7/16/2021 Arpan Deyasi, India 26

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