Gunn Diode: Working Principle & Applications

What is a Gunn Diode?

A Gunn diode is a two-terminal semiconductor device made only of n-type material. It has no p-n junction. It is an active microwave source: a steady DC bias produces RF through negative resistance. Practical parts use GaAs, InP or GaN, whose conduction bands have two nearby valleys. Other III-V crystals such as CdTe, CdS, InAs, InSb and ZnSe have been studied.

Manufacture grows an epitaxial layer on a degenerate n+ substrate so three n-type semiconductor layers result (Figure 1a). The two outer layers are doped much more heavily than the middle active layer.

Metal contacts at both ends apply the bias. The Gunn diode symbol (Figure 1b) differs from a normal diode to show that there is no p-n junction.

A DC bias puts an electric field across the layers, mainly in the central active region. At low field, electrons stay in the high-mobility lower valley of the conduction band, so current rises with voltage.

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

At higher voltages those electrons transfer into the upper valley. Effective mass rises, mobility falls, conductivity falls and the current through the diode drops.

The V-I curve then shows a negative resistance from the peak to the valley. That intervalley transfer is the transferred-electron effect. Gunn diodes are also called transferred-electron devices.

The same effect is called the Gunn effect after J. B. Gunn, who in 1963 produced microwaves from a steady voltage across n-type GaAs. The crystal must be n-type. The transfer is an electron effect, not a hole effect.

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.

That negative resistance lets the same chip work as an amplifier or as an oscillator. The oscillator is a Gunn diode oscillator.

Advantages and Disadvantages of Gunn Diode

The advantage of Gunn diodes include:

• lies in the fact that they are the cheapest source of microwaves (compared to other options such as klystron tubes)
• They are compact in size
• They operate over a large bandwidth and possess high frequency stability.

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.

Applications of Gunn Diode

The applications of a Gunn Diode include:

1. In electronic oscillators to generate microwave frequencies.
2. In parametric amplifiers as pump sources.
3. In police radars.
4. As sensors in door opening systems, trespass detecting systems, pedestrian safety systems, etc.
5. As a source for microwave frequencies in automatic door openers, traffic signal controllers, etc.
6. In microwave receiver circuits.
7. In radio communications.
8. In military systems.
9. As remote vibration detectors.
10. In tachometers.
11. In Pulsed Gunn Diode Generator.
12. In microelectronics as control equipments.
13. In radar speed guns.
14. As microwave relay data link transmitters.
15. In Continuous Wave Doppler Radars.