JFET

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JFET

  1. 1. JFETJunction Field Effect Transistor
  2. 2. Introduction (FET) Field-effect transistor (FET) are important devices such as BJTs Also used as amplifier and logic switches What is the difference between JFET and BJT?
  3. 3. BJT is Current-controlled
  4. 4. FET is Voltage-controlled
  5. 5. Types of Field Effect Transistors(The Classification) n-Channel JFET FET  JFET p-Channel JFET MOSFET (IGFET) Enhancement Depletion MOSFET MOSFET n-Channel p-Channel n-Channel p-Channel EMOSFET EMOSFET DMOSFET DMOSFET
  6. 6. Introduction.. (Advantages of FETover BJT) High input impedance (MΩ) (Linear AC amplifier system) Temperature stable than BJT Smaller than BJT Can be fabricated with fewer processing BJT is bipolar – conduction both hole and electron FET is unipolar – uses only one type of current carrier Less noise compare to BJT Usually use as an Amplifier and logic switch
  7. 7. Disadvantages of FET Easy to damage compare to BJT
  8. 8. Junction field-effect transistor..  There are 2 types of JFET  n-channel JFET  p-channel JFET  Three Terminal  Drain – D  Gate -G  Source – S
  9. 9. SYMBOLS Drain DrainGate Gate Source Source n-channel JFET p-channel JFET
  10. 10. N-channel JFET N channel JFET:  Major structure is n-type material (channel) between embedded p-type material to form 2 p- n junction.  In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source (S). Current flows into the Drain (D), through the channel, and out of the Source (S)  Because the resistance of the channel depends on the gate-to-source voltage (VGS), the drain current (ID) is controlled by that voltage
  11. 11. N-channel JFET..
  12. 12. P-channel JFET P channel JFET:  Major structure is p-type material (channel) between embedded n-type material to form 2 p-n junction.  Current flow : from Source (S) to Drain (D)  Holes injected to Source (S) through p- type channel and flowed to Drain (D)
  13. 13. P-channel JFET..
  14. 14. Water analogy for the JFET controlmechanism
  15. 15. JFET Characteristic for VGS = 0 V and 0<VDS<|Vp| To start, suppose VGS=0 Then, when VDS is increased, ID increases. Therefore, ID is proportional to VDS for small values of VDS For larger value of VDS, as VDS increases, the depletion layer become wider, causing the resistance of channel increases. After the pinch-off voltage (Vp) is reached, the ID becomes nearly constant (called as ID maximum, IDSS-Drain to Source current with Gate Shorted)
  16. 16. JFET for VGS = 0 V and 0<VDS<|Vp| Channel becomes narrower as VDS is increased
  17. 17. Pinch-off (VGS = 0 V, VDS = VP).
  18. 18. ID versus VDSfor VGS = 0 V and 0<VDS<|Vp| JFET Characteristic Curve
  19. 19. JFET for(Application of a negative voltage to the gate of a JFET )
  20. 20. JFET Characteristic Curve.. For negative values of VGS, the gate-to-channel junction is reverse biased even with VDS=0 Thus, the initial channel resistance of channel is higher. The resistance value is under the control of VGS If VGS = pinch-off voltage(VP) The device is in cutoff (VGS=VGS(off) = VP) The region where ID constant – The saturation/pinch- off region The region where ID depends on VDS is called the linear/ohmic region
  21. 21. p-Channel JFET
  22. 22. p-Channel JFET characteristics with IDSS = 6mA and VP = +6 V.
  23. 23. Characteristics for n-channelJFET
  24. 24. Characteristics for p-channelJFET + + + P
  25. 25. Transfer CharacteristicsThe input-output transfer characteristic ofthe JFET is not as straight forward as it isfor the BJT. In BJT: IC=β IBwhich β is defined as the relationshipbetween IB (input current) and IC (outputcurrent).
  26. 26. Transfer Characteristics..In JFET, the relationship between VGS (inputvoltage) and ID (output current) is used todefine the transfer characteristics. It is calledas Shockley’s Equation: 2  VGS  ID = IDSS 1 - ÷ VP=VGS (OFF)  VP The relationship is more complicated (and notlinear)As a result, FET’s are often referred to asquare law devices
  27. 27. Transfer Characteristics… Defined by Shockley’s equation: 2  VGS  I D = I DSS 1 −  VP = VGS ( off )  VGS   ( off )  Relationship between ID and VGS. Obtaining transfer characteristic curve axis point from Shockley:  When VGS = 0 V, ID = IDSS  When VGS = VGS(off) or Vp, ID = 0 mA
  28. 28. Transfer CharacteristicsJFET Transfer Characteristic Curve JFET Characteristic Curve
  29. 29. Exercise 1Sketch the transfer defined byIDSS = 12 mA dan VGS(off) = - 6 VGS ID  ID  VGS = VP 1 -  ÷ 0 IDSS  IDSS ÷  2 0.3Vp IDSS/2  VGS  ID = IDSS  1 - ÷ 0.5Vp IDSS/4  VP  Vp 0 mA
  30. 30. Exercise 1 Sketch the transfer defined by IDSS = 12 mA dan VGS(off) = Vp= - 6 IDSS 2  VGS  ID = IDSS  1 - ÷  VP  VGS =0.3VP IDSS/2  ID VGS =0.5VP IDSS/4 VGS = VP 1 - ÷  IDSS ÷  
  31. 31. Answer 1
  32. 32. Exercise 2Sketch the transfer defined byIDSS = 4 mA dan VGS(off) = 3 V VGS ID  ID  VGS = VP 1 -  ÷ 0 IDSS  IDSS ÷  2 0.3Vp IDSS/2  VGS  ID = IDSS  1 - ÷ 0.5Vp IDSS/4  VP  Vp 0 mA
  33. 33. Sketch the transfer defined by Exercise 2 IDSS = 4 mA dan VGS(off) = 3V IDSS 2  VGS  ID = IDSS  1 - ÷  VP  IDSS/2 IDSS/4 VP  ID  VGS = VP 1 -  ÷  IDSS ÷ VGS =0.3VP VGS =0.5VP
  34. 34. Answer 2 Answer 2

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