What is a Gunn Diode?

A Gunn diode is a passive semiconductor device with two terminals, which composes
of only an n-doped semiconductor material, unlike other diodes which consist of a p-n
junction. Gunn diodes can be made from the materials which consist of multiple,
initially-empty, closely-spaced energy valleys in their conduction band like Gallium
Arsenide (GaAs), Indium Phosphide (InP), Gallium Nitride (GaN), Cadmium Telluride
(CdTe), Cadmium Sulfide (CdS), Indium Arsenide (InAs), Indium Antimonide (InSb) and
Zinc Selenide (ZnSe).

General manufacturing procedure involves growing an epitaxial layer on a degenerate

n+ substrate to form three n-type semiconductor layers (Figure 1a), where-in the
extreme layers are heavily doped when compared to the middle, active layer.
Further the metal contacts are provided at either ends of the Gunn diode to facilitate
biasing. The circuit symbol for Gunn diode is as shown by Figure 1b and differs from
that normal diode so as to indicate the absence of p-n junction.

When a DC voltage is applied to a Gunn diode, an electric field develops across its
layers, especially in the central active region. Initially, conduction increases as
electrons move from the valence band to the lower valley of the conduction band.

The associated V-I plot is shown by the curve in the Region 1 (coloured in pink) of Figure
2. However, after reaching a certain threshold value (Vth), the conduction current
through the Gunn diode decreases as shown by the curve in the Region 2 (coloured in
blue) of the figure.

This is because, at higher voltages the electrons in the lower valley of the conduction
band move into its higher valley where their mobility decreases due to an increase in
their effective mass. The reduction in mobility decreases the conductivity which leads
to a decrease in the current flowing through the diode.

As a result, the diode exhibits a negative resistance region in the V-I characteristic
curve, spanning from the Peak point to the Valley Point. This effect is known as the
transferred electron effect, and Gunn diodes are also called Transferred Electron
Devices
Further it is to be noted that the transferred electron effect is also called Gunn effect
and is named after John Battiscombe Gunn (J. B. Gunn) after his discovery in 1963
which showed that one could generate microwaves by applying a steady voltage across
a chip of n-type GaAs semiconductor. However it is important to note that the material
used to manufacture Gunn diodes should necessarily be of n-type as the transferred
electron effect holds good only for electrons and not for holes.

Since GaAs is a poor conductor, Gunn diodes generate excessive heat and need a heat
sink. At microwave frequencies, a current pulse travels across the active region,
initiated at a specific voltage. This pulse movement reduces the potential gradient,
preventing further pulse formation.

A new current pulse can only be generated when the previous pulse reaches the far end
of the active region, increasing the potential gradient again. The time it takes for the
current pulse to travel across the active region determines the pulse generation rate and
the operational frequency of the Gunn diode. To vary the oscillation frequency, the
thickness of the central active region must be adjusted.

Further it is to be noted that the nature of negative resistance exhibited by the Gunn
diode enables it to work as both an amplifier and an oscillator, the latter of which is
known as a Gunn diode oscillator or Gunn oscillator.
Applications of Gunn Diode

Gunn diodes are valuable in applications requiring stable and high-frequency

microwave signals. Some of the key applications include:

1. Microwave Oscillators: Gunn diodes are commonly used in microwave

oscillators to generate stable and coherent microwave signals. These oscillators
are essential in radar systems, satellite communications, and microwave
transmitters. The simplicity and efficiency of Gunn diodes make them ideal for
these applications.

2. Local Oscillators in Receivers: In microwave and millimetre-wave receivers,

Gunn diodes serve as local oscillators. They provide the necessary local
oscillator signal for frequency conversion, allowing the receiver to process high-
frequency signals effectively.

3. Frequency Modulation: By varying the bias voltage applied to the Gunn diode,
the frequency of oscillation can be modulated. This makes Gunn diodes suitable
for frequency modulation (FM) systems used in various communication
applications.

4. Test and Measurement Equipment: Gunn diodes are used in signal generators
and other test and measurement instruments that require stable microwave
sources. Their ability to generate consistent microwave frequencies makes them
valuable in laboratory and industrial settings.

5. Microwave Sensing and Imaging: In microwave sensing and imaging systems,

Gunn diodes are used as microwave sources. They play a critical role in systems
such as microwave radiometers, which are used for remote sensing and imaging
applications. Their stable output ensures accurate measurements and high-
resolution images.
Other Applications of Gunn Diode

The applications of a Gunn Diode include:

1. In parametric amplifiers as pump sources.

2. In police radars.

3. As sensors in door opening systems, trespass detecting systems, pedestrian

safety systems, etc.

4. As a source for microwave frequencies in automatic door openers, traffic signal

controllers, etc.

5. In radio communications.

6. In military systems.

7. As remote vibration detectors.

8. In tachometers.

9. In Pulsed Gunn Diode Generator.

10. In microelectronics as control equipment’s.

11. In radar speed guns.

12. As microwave relay data link transmitters.

13. In Continuous Wave Doppler Radars.

Advantages
1. High-Frequency Operation:

• Microwave Generation: Gunn diodes can generate microwave frequencies

from a few GHz up to over 100 GHz. This makes them ideal for
applications in radar, satellite communications, and microwave
transmitters.

• Stability: They provide stable frequency oscillations, which is crucial for

applications requiring precise signal generation.

2. Simple Construction:

• No P-N Junction: Gunn diodes are constructed from a single type of

semiconductor material without the need for P-N junctions, simplifying
their design and fabrication.

• Ease of Integration: Their relatively simple structure allows for easy

integration into various microwave circuits.

3. Wide Range of Applications:

• Versatility: They are used in a variety of applications, including oscillators,

local oscillators in receivers, frequency modulators, and microwave
sensing devices.

4. Tuneable Frequency:

• Voltage Control: The frequency of oscillation can be adjusted by varying

the applied voltage, offering flexibility in applications where different
frequencies are needed.

5. Reliable Performance:

• Durability: Gunn diodes are known for their reliable performance and
longevity, making them suitable for continuous operation in demanding
environments.

The disadvantages of Gunn diodes include:

• They have a high turn-on voltage

• They are less efficient below 10 GHz

• They exhibit poor temperature stability.

How a Gunn diode acts as a Microwave Oscillators

Whilst the Gunn diode has a negative resistance region, it is interesting to see a little
more about how this happens and how it acts as an oscillator.

At microwave frequencies, it is found that the dynamic action of the diode incorporates
elements resulting from the thickness of the active region.

When the voltage across the active region reaches a certain point, a current is initiated
that travels across the active region. During the time when the current pulse is moving
across the active region the potential gradient falls preventing any further pulses from
forming. Only when the pulse has reached the far side of the active region will the
potential gradient rise, allowing the next pulse to be created.

It can be seen that the time taken for the current pulse to traverse the active region
largely determines the rate at which current pulses are generated. It is this that
determines the frequency of operation.

To see how this occurs, it is necessary to look at the electron concentration across the
active region. Under normal conditions the concentration of free electrons would be the
same regardless of the distance across the active diode region. However, a small
perturbation may occur resulting from noise from the current flow, or even external
noise - this form of noise will always be present and acts as the seed for the oscillation.
This grows as it passes across the active region of the Gunn diode.

The increase in free electrons in one area cause the free electrons in another area to
decrease forming a form of wave. The peak will traverse across the diode under the
action of the potential across the diode, and growing as it traverses the diode as a
result of the negative resistance.

A clue to the reason for this unusual action can be seen if the voltage and current curves
are plotted for a normal diode and a Gunn diode. For a normal diode the current
increases with voltage, although the relationship is not linear. On the other hand, the
current for a Gunn diode starts to increase, and once a certain voltage has been
reached, it starts to fall before rising again. The region where it falls is known as a
negative resistance region, and this is the reason why it oscillates.