11. INVESTIGATION OF A JUNCTION FIELD-EFFECT TRANSISTOR

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45


11. INVESTIGATION

OF A JUNCTION FIELD
-
EFFECT
TRANSISTOR


11.1.

Objective of the test

Knowledge of field
-
effect transistors (FET), their structures and
classification, principles of operation, characteristics and parameters;
learning to measure static volt
-
amper
e characteristics (VACs) and
some other major parameters of FETs.


11.2.

Important notes

A FET is a three
-
terminal device in which current flows through a
narrow conduction channel between two electrodes (source and drain)
and is modulated by an electric field c
aused by a voltage applied at the
third electrode (gate).

FETs were introduced after BJTs. The first FET was proposed by
Shockley in 1952. It was a
junction field
-
effect transistor

(JFET).
Later another very similar family of
insulated
-
gate

FETs (IGFETs)
was developed. More widely they are referred as
metal
-
oxide
-
semiconductor

transistors
(MOSTs),
metal
-
oxide
-
semiconductor

FETs (MOSFETs) or
metal
-
insulator
-
semiconductor

FETs
(MISFETs). Extremely high frequency (microwave) amplifiers are
being designed usin
g
gallium arsenide
-
based FETs (GASFETs) that
are
metal
-
semiconductor
FETs (MESFETs).


BJTs are current
-
controlled devices while FETs are voltage
-
controlled devices. FETs have extremely high input impedance, are
more temperature stable than BJTs, can tolera
te a much higher level
of radiation, are simpler to fabricate and are attractive for use in
integrated circuits.

Generally, the drain current of a FET is dependent on the input and
output voltages:


.



46

According to this, there are two

major parameters of a FET


transconductance
g
m

and output resistance
R
o

that are given by



when

U
DS

is constant;



when
U
GS

is constant.



Here
I
D

is drain current,
U
GS

and
U
DS

are gate
-
source and drain
-

source voltages correspondingly.










11.3.

Preparing for the test:



Using your lecture
-
notes and referenced literature [2, p. 58

75],
consider FET structures, classification, principles of operation,
characteristics and parameters; clear up how parameters

g
m

and
R
o

can be found.



Consider section “11.4. In laboratory” of this test.



Prepare squared millimetre paper for graphs. Sketch the co
-
ordinate systems
I

versus
U

on the paper. The scale must be
chosen taking into account types of the characteristics. D
raw lines
corresponding to the FET ratings: drain current
I
D
max

=

20

mA,
drain
-
source voltage
U
DS
max

=

25 V, power
P
max

=

200

mW.



Prepare to answer the questions:

1.

Discuss and explain classification of FETs.

2.

Draw schematic symbols of FETs.

3.

Discuss the str
ucture of a JFET. Explain the principles of
operation of the transistor.

4.

Show polarities of the bias voltages for drain and gate circuits of
the JFET with
n
-
type channel.

5.

Draw the output VACs of a JFET and explain their shape.

6.

Draw the transfer VACs of a
JFET and explain their shape.

7.

Draw the structure (section view) of a normally
-
off (enhan
-
cement) MOSFET. Explain the principles of operation.

8.

Draw the structure (section view) of the normally
-
on (depletion
-

enhancement) MOSFET. Explain the principles of
operation.


47

9.

Name the major parameters of a FET and show how they can be
found using VACs of the FET. Demonstrate this (the teacher will
point corresponding parameter).

10.

Compare the properties of bipolar and field
-
effect transistors.


11.4. In laboratory:

1.

Ans
wer the test question.

2.

Familiarize with measurement devices and laboratory model.

Before any measurements the control knobs of all the
potentiometers of the laboratory model must be turned
counter
-
clockwise to the final position. During measurements,
volt
age and current must not exceed the highest values
allowed for the used devices. In the case of this laboratory
test: drain current

I
D
max

=

20 mA
, drain
-
source voltage
U
DS
max

=

25

V
, power
P
max

=

200

mW
.

3.

Connect the measurement circuit (Fig

11.1). Turn on
the current
source and set voltages as shown in Fig

11.1 (

9

V and +60

V).
After the teacher has checked the circuit connect the
measurement circuit to the current source.


48

After any change of the measurement circuit ask th
e teacher
to check it.

4.

Carry out measurements of the static VACs:

a)

Measure and draw the output VAC
I
D

versus
U
DS

of the FET
for the values of the gate
-
source voltage
U
GS

pointed out by
the teacher.

To this end turn the control knobs of the potentiomete
rs R2,
R3, R5 and R6 counter
-
clockwise to the final position.

Note the values of
I
D

and
U
DS

while increasing the voltage
U
DS

and sketch the characteristic. During this measurement the
voltage
U
GS

must be constant.

b)

Measure and draw the transfer characteris
tics
I
D

versus
U
GS

of the FET for the two values of the voltage
U
DS

pointed out
by the teacher. These two values of
U
DS

must exceed
saturation voltage.

To this end turn the control knobs of the potentiometers R2,
R3, R5 and R6 counter
-
clockwise to the fina
l position.




Fig 11.1.

Measurement circuit


49

Note the values of
I
D

and
U
GS

while increasing the voltage
U
GS

till
I
D

becomes 0, and sketch the characteristic. During this
measurement, the voltage
U
DS

must be constant.

5.

Examine the results. Consider the measured characteristics.
Repeat the

measurements if it is necessary.

6.

Using the measured output characteristics find FET parameters
g
m

and
R
o

at the operation point indicated by the teacher.
Find
transconductance
g
m

and pinch
-
off voltage
U
P

using the measured
transfer characteristics of the
FET.


11.5. Contents of the report

1.

Objectives.

2.

Output and transfer characteristics of the FET.

3.

Calculation of
g
m

and
R
o
. Value of
U
P
.

4.

Conclusions (explanation of the measured output and transfer
characteristics and physical meaning of major

parameters of
the
FET; comments on values of the parameters).