1. Junction Field Effect Transistor
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 EMOSFET
n-Channel DMOSFET
p-Channel DMOSFET
p-Channel EMOSFET

6. Introduction.. (Advantages of FET over BJT)
High input impedance (M) (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. 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. SYMBOLS Gate Drain Source Gate Drain Source n-channel JFET
p-channel JFET

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. N-channel JFET..

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. P-channel JFET..

14. Water analogy for the JFET control mechanism

16. 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)

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

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

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

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

21. 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

23. p-Channel JFET

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

25. Characteristics for n-channel JFET

26. Characteristics for p-channel JFET+ + + P

28. 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).

29. 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)

30. 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

31. Transfer Characteristics
JFET Transfer Characteristic Curve
JFET Characteristic Curve

32. Exercise 1 Sketch the transfer defined by
IDSS = 12 mA dan VGS(off) = - 6
VGS ID
IDSS
0.3Vp
IDSS/2
0.5Vp
IDSS/4 Vp
0 mA

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

34. Answer 1

35. Exercise 2 Sketch the transfer defined by
IDSS = 4 mA dan VGS(off) = 3 V
VGS ID
IDSS
0.3Vp
IDSS/2
0.5Vp
IDSS/4 Vp
0 mA

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

37. Answer 2
Answer 2