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JFET

JFET. Junction Field Effect Transistor. 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?. BJT is Current-controlled. FET is Voltage-controlled.

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JFET

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  1. JFET Junction Field Effect Transistor

  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. BJT is Current-controlled

  4. FET is Voltage-controlled

  5. Types of Field Effect Transistors (The Classification) • JFET MOSFET(IGFET) n-Channel JFET p-Channel JFET FET Enhancement MOSFET Depletion MOSFET n-Channel DMOSFET p-Channel DMOSFET n-Channel EMOSFET p-Channel EMOSFET

  6. Introduction.. (Advantages of FET over BJT) • Temperaturestable than BJT • Smaller than BJT • Can be fabricated with fewer processing • BJT is bipolar – conduction is done by hole and electron both. • FET is unipolar – uses only one type of current carrier • Less noise compare to BJT • Usually used as an Amplifier and logic switch

  7. Disadvantages of FET • Easy to damage compare to BJT

  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. Drain Drain Gate Gate Source Source SYMBOLS n-channel JFET p-channel JFET

  10. N-channel JFET • N channel JFET: • Major structure is n-type material (channel) between embedded p-type materialto 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. N-channel JFET..

  12. P-channel JFET • P channel JFET: • Major structure is p-type material (channel) between embedded n-type materialto 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. P-channel JFET..

  14. Water analogy for the JFET control mechanism

  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 IDbecomes nearly constant (called as IDmaximum, IDSS-Drain to Source current with Gate Shorted)

  16. JFET for VGS = 0 V and 0<VDS<|Vp| Channel becomes narrower as VDS is increased

  17. Pinch-off (VGS = 0 V, VDS= VP).

  18. ID versus VDS for VGS = 0 V and 0<VDS<|Vp| JFET Characteristic Curve

  19. JFET for (Application of a negative voltage to the gate of a JFET)

  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. p-Channel JFET

  22. p-Channel JFET characteristics with IDSS= 6 mA and VP = +6 V.

  23. Transfer Characteristics • The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT. In BJT: • IC= IB • which is defined as the relationship between IB (input current) and IC (output current).

  24. Transfer Characteristics.. In JFET, the relationship between VGS (input voltage) and ID (output current) is used to define the transfer characteristics. It is called as Shockley’s Equation: The relationship is more complicated (and not linear) As a result, FET’s are often referred to a square law devices VP=VGS (OFF)

  25. Transfer Characteristics… • Defined by Shockley’s equation: • 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

  26. Transfer Characteristics JFET Transfer Characteristic Curve JFET Characteristic Curve

  27. Exercise 1 Sketch the transfer defined by IDSS = 12 mA and VGS(off) = - 6

  28. Exercise 1 Sketch the transfer defined by IDSS = 12 mA and VGS(off) = Vp= - 6 IDSS VGS =0.3VP IDSS/2 VGS =0.5VP IDSS/4

  29. Answer 1

  30. Exercise 2 Sketch the transfer defined by IDSS = 4 mA and VGS(off) = 3 V

  31. Exercise 2 Sketch the transfer defined by IDSS = 4 mA and VGS(off) = 3V IDSS IDSS/2 IDSS/4 VP VGS =0.3VP VGS =0.5VP

  32. Answer 2 Answer 2

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