# 9. Investigation of a tunnel diode ```9. INVESTIGATION OF A TUNNEL DIODE
9.1. Objective of the test
Knowledge of semiconductor diodes classification, their structure, principles of operation, characteristics and parameters; learning
to measure I-U characteristic, also called volt-ampere characteristic
(VAC) and some other major parameters of a tunnel diode (TD).
9.2. Peculiarities of tunnel diodes
The p and n regions of a tunnel diode are heavily doped. Then
materials are degenerate, the pn junction is thin, and an additional
current component, the quantum-mechanical tunnelling current, can
flow across the junction. It is important that VACs of tunnel diodes
have useful negative-resistance portions.
Minority carriers are not injected when tunnel current flows. For
this reason tunnel diodes are useful in high-frequency circuits. They
are used in low-noise microwave amplifiers, high-speed logic circuits
and other high-speed applications.
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9.3. Preparing for the test:
Using lecture-notes and referenced literature [2, p. 3–19], consider semiconductor diodes classification, structure, principle of
operation and major parameters.
Consider section “9.4. In laboratory” of this test.
Explain the structure and properties of a point-contact diode.
Name and discuss manufacturing methods of junction-type
diodes.
Describe structures, manufacturing and properties of planar
diodes.
Describe structure, manufacturing and properties of mesa diodes.
Name major parameters of semiconductor diodes. On what do
they depend, and how?
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6.
Draw the model (equivalent circuit) of a semiconductor diode at
high frequency. Comment on elements of the circuit. Explain
how it is possible to improve frequency properties of semiconductor diodes.
7. Describe semiconductor diode operation in switching regime.
8. What is a degenerate semiconductor?
9. Describe the structure and energy band diagram of a tunnel diode.
10. Draw the VAC of a TD and comment on it. Indicate characteristic points of the characteristic and draw the corresponding
energy band diagrams.
11. What is a backward diode? Draw its energy band diagram and
VAC.
9.4. In laboratory:
Familiarize with measurement devices and laboratory model.
Caution. Before any measurements the control knobs of all the
potentiometers of the laboratory model must be turned counterclockwise to the final position. During measurements voltages
and currents must not exceed the highest allowed values for used
devices. In the case of this laboratory test the maximum TD
current Imax is 10 mA.
3. Connect the measurement circuit shown in Fig 9.1. Turn on the
current source and set the bias voltage as shown in Fig 9.1
(+ 5V). 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.
4. Measure the forward VAC of the tunnel diode.
To this end turn the control knob of the potentiometer R2
counter-clockwise to the final position. Note values of TD current
and voltage while increasing TD current. Repeat the measurements with the current being decreased. Find extremes of the
characteristic and register the values of current and voltage in the
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Fig 9.1. Measurement circuit
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extreme points. Register also the diode voltage U  when the
diode current is I   0.6  I max .
Measure the reverse VAC of the tunnel diode.
To this end turn the potentiometer R2 control knob counterclockwise to the final position. Connect the TD in the reverse
direction and repeat the measurements.
Draw the graph of the TD VAC.
Repeat the measurement of the forward VAC of the tunnel diode
with a resistor R3 connected in parallel to the TD.
To this end turn the control knob of the potentiometer R2
counter-clockwise to the final position and connect the resistor
R3 in parallel with the TD. Note values of TD current and
voltage, while increasing TD current.
During this measurement the TD voltage must not exceed U  .
Find TD current I D using equation:
I D  I U / R ,
where U and I are the voltmeter and milliammeter readings; R is
resistance of the resistor (27 ), U / R – current through the
resistor R3.
Fill Table.
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Table. Measurement results
I / mA
0
U/V
U/I / mA
ID / mA
…
In accordance with measurements and calculation results draw
the forward VAC of the diode.
8. Calculate the average value of TD negative differential
resistance. Use formula:
RD  U 2  U1  I 2  I1  ,
where U1, I1, U2 and I2 are voltages and currents corresponding to
the extreme points of the tunnel diode VAC.
9. Examine the results. Repeat measurements if it is necessary.
10. Prepare the report.
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9.5. Contents of the report
Objectives.
Results of measurements (in Table).
Graphs of measured VACs.
Calculation of TD negative differential resistance.
Conclusions (comments on the measurement and calculation
results).
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