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)
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. Introduction.. (Advantages of FET
over 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. 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
Drain Drain
Gate Gate
Source 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. 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=β IB
which β 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:
2
 VGS 
ID = IDSS 1 - ÷ VP=VGS (OFF)
 VP 
The relationship is more complicated (and not
linear)
As a result, FET’s are often referred to a
square law devices

30. 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

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  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. 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 ÷
 

34. Answer 1

35. Exercise 2
Sketch the transfer defined by
IDSS = 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

36. 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

37. Answer 2
Answer 2