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-ampere 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 caused 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-oxidesemiconductor transistors (MOSTs), metal-oxide-semiconductor
FETs (MOSFETs) or metal-insulator-semiconductor FETs
(MISFETs). Extremely high frequency (microwave) amplifiers are
being designed using gallium arsenide-based FETs (GASFETs) that
are metal-semiconductor FETs (MESFETs).
BJTs are current-controlled devices while FETs are voltagecontrolled devices. FETs have extremely high input impedance, are
more temperature stable than BJTs, can tolerate 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:
I D  f (U GS , U DS ) .
According to this, there are two major parameters of a FET –
transconductance gm and output resistance Ro that are given by
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g m  I D / U GS when UDS is constant;
Ro  U DS / I D when UGS is constant.
Here ID is drain current, UGS and UDS are gate-source and drainsource voltages correspondingly.
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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 gm and Ro
can be found.
Consider section “11.4. In laboratory” of this test.
Prepare squared millimetre paper for graphs. Sketch the coordinate systems I versus U on the paper. The scale must be
chosen taking into account types of the characteristics. Draw
lines corresponding to the FET ratings: drain current
IDmax = 20 mA, drain-source voltage UDSmax = 25 V, power
Pmax = 200 mW.
Prepare to answer the questions:
Discuss and explain classification of FETs.
Draw schematic symbols of FETs.
Discuss the structure of a JFET. Explain the principles of
operation of the transistor.
Show polarities of the bias voltages for drain and gate circuits of
the JFET with n-type channel.
Draw the output VACs of a JFET and explain their shape.
Draw the transfer VACs of a JFET and explain their shape.
Draw the structure (section view) of a normally-off (enhancement) MOSFET. Explain the principles of operation.
Draw the structure (section view) of the normally-on (depletionenhancement) MOSFET. Explain the principles of operation.
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).
Compare the properties of bipolar and field-effect transistors.
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Fig 11.1. Measurement circuit
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11.4. In laboratory:
Answer the test question.
Familiarize with measurement devices and laboratory model.
Before any measurements the control knobs of all the
potentiometers of the laboratory model must be turned counterclockwise to the final position. During measurements, voltage
and current must not exceed the highest values allowed for the
used devices. In the case of this laboratory test: drain current
IDmax = 20 mA, drain-source voltage UDSmax = 25 V, power
Pmax = 200 mW.
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.
After any change of the measurement circuit ask the teacher to
check it.
Carry out measurements of the static VACs:
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a) Measure and draw the output VAC ID versus UDS of the FET
for the values of the gate-source voltage UGS pointed out by
the teacher.
To this end turn the control knobs of the potentiometers R2,
R3, R5 and R6 counter-clockwise to the final position.
Note the values of ID and UDS while increasing the voltage UDS
and sketch the characteristic. During this measurement the
voltage UGS must be constant.
b) Measure and draw the transfer characteristics ID versus UGS of
the FET for the two values of the voltage UDS pointed out by
the teacher. These two values of UDS must exceed saturation
voltage.
To this end turn the control knobs of the potentiometers R2,
R3, R5 and R6 counter-clockwise to the final position.
Note the values of ID and UGS while increasing the voltage UGS
till ID becomes 0, and sketch the characteristic. During this
measurement, the voltage UDS must be constant.
Examine the results. Consider the measured characteristics.
Repeat the measurements if it is necessary.
Using the measured output characteristics find FET parameters gm
and Ro at the operation point indicated by the teacher. Find
transconductance gm and pinch-off voltage UP using the measured
transfer characteristics of the FET.
11.5. Contents of the report
Objectives.
Output and transfer characteristics of the FET.
Calculation of gm and Ro. Value of UP.
Conclusions (explanation of the measured output and transfer
characteristics and physical meaning of major parameters of the
FET; comments on values of the parameters).
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