FET

3. R.M.K COLLEGE OF ENGINEERING AND
TECHNOLOGY
DEPARTMENT OF ECE
EC8252-ELECTRONIC DEVICES
SECOND SEMESTER-I YEAR- (2020-2024 BATCH): SECTION A & B
Mrs.P.Sivalakshmi M.E
AP/ECE
SESSION:18
DATE: 27.05.2021
UNIT 3 FIELD EFFECT TRANSISTORS

4. 2017- Regulation syllabus
IN THIS
SESSION
ONWARDS

5. The ordinary or bipolar transistor has two principal disadvantages.
• First, it has a low input impedance because of forward biased emitter junction.
• Secondly, it has considerable noise level.
• The field effect transistor(FET) has, by virtue of its construction and biasing,
large input impedance which may be more than 100megaohms.
• The FET is generally much less noisy than the ordinary or bipolar transistor.
• In addition temperature stable, as well small in size compared to BJT.

6. 6 IMPORTANT
FEATURES
BJT & FET-3 TERMINAL
DEVICES
BJT- CURRENT CONTROLLED DEVICE
FET-VOLTAGE CONTROLLED DEVICE
FET-UNIPOLAR DEVICE
BJT-BIPOLAR DEVICE
BJT & FET does both
amplification and switching
FET
JFET
N CHANNEL
P CHANNEL
MOSFET
N- CHANNEL
P- CHANNEL

7. JULIUS EDGER LILIE
FIELD-1926
OSKAR HEIL-1934
D.KAHNG-1959
MOSFET
WILLIAM SCHOKLEY-
1947-BJT
HISTORY OF FET

8. Why field effect?
An effect of field is created by
charges present in the material,
which in turn controls the flow of
current in the conduction path of
the output circuit.

9. Constructional details:
A JFET consists of a p-type or n-type
silicon bar containing two pn
junctions at the sides as shown in Fig
The two PN junctions forming diodes
are connected internally and a
common terminal called gate is taken
out. Other terminals are source and
drain taken out from the bar as
shown. Thus a JFET has essentially
three terminals via., gate (G),
source(S) and drain(D).

10. SPIGOT/TAP ANALOGY
The Ohmic contact is a low resistance junction (non-rectifying) provides current conduction from metal to
semiconductor and vice versa. Theoretically speaking the current should increase/ decrease linearly with the
applied voltage.

11. Thus JFET operates on the principle that width and resistance of the
conducting channel can be varied by changing the reverse voltage VGS.
In other words, the magnitude of drain current (ID) can be changed by
altering VGS.
Principle of JFET

12. Working of JFET Principle of JFET

14. Working of JFET

15. Pinch-off Voltage:

16. Working of JFET
Case: iv
Negative voltage of VGS & VDD’ (<VDD)

17. Output or Drain characteristics curve of JFET

18. • As the reverse gate-source voltage is increased, the cross-sectional area of the channel
decreases. This in turn decreases the drain current.
• At some reverse gate-source voltage, the depletion layers extend completely across the channel.
In this condition, the channel is cut off and the drain current reduces to zero.
• The gate voltage at which the channel is cutoff (i.e. channel becomes non-conducting) is called
gate-source cut off voltage VGS (off).

19. Circuit diagram of N-Channel JFET & P- Channel JFET