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.
On applying a DC voltage across the terminals of the Gunn diode, an electric field is developed across its layers, most of which appears across the central active region. At initial stages, the conduction increases due to the movement of electrons from the valence band into the lower valley of the conduction band.
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 (Vth), the conduction current through the Gunn diode decreases as shown by the curve in the Region 2 (colored 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 is said to exhibit negative resistance region (region spanning from Peak point to Valley Point) in the V-I characteristic curve. This effect is called transferred electron effect and thus the 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.
Moreover as the GaAs is a poor conductor, Gunn diodes generate excessive heat and thus are usually provided with a heat sink. In addition, at microwave frequencies, a current pulse travels across the active region which is initiated at a particular voltage value. This movement of current pulse across the active region reduces the potential gradient across it, which in turn avoids the formation of further current pulses.
The next current pulse can be generated only when the pulse previously generated reaches the far-end of the active region, increasing the potential gradient once again. This indicates that the time taken by the current pulse to traverse across the active region decides the rate at which the current pulses are generated and thus fixes the operational frequency of the Gunn diode. Thus in order to vary the oscillation frequency, one has to vary the thickness of the central active region.
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 oscaillator.
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:
- In electronic oscillators to generate microwave frequencies.
- In parametric amplifiers as pump sources.
- In police radars.
- As sensors in door opening systems, trespass detecting systems, pedestrian safety systems, etc.
- As a source for microwave frequencies in automatic door openers, traffic signal controllers, etc.
- In microwave receiver circuits.
- In radio communications.
- In military systems.
- As remote vibration detectors.
- In tachometers.
- In Pulsed Gunn Diode Generator.
- In microelectronics as control equipments.
- In radar speed guns.
- As microwave relay data link transmitters.
- In Continuous Wave Doppler Radars.
