GUNN DIODE

GUNN DIODE

A Gunn diode is also known as a transferred electron device (TED). It  is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down.

In practice, this means a Gunn diode has a region of negative differential resistance.

The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. In effect, the negative differational resistance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely.  Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonator is usually added to control frequency. 

Operation of the Gunn diode

The operation of the Gunn diode can be explained in basic terms. When a voltage is placed across the device, most of the voltage appears across the inner active region. As this is particularly thin this means that the voltage gradient that exists in this region is exceedingly high.

It is found that when the voltage across the active region reaches a certain point a current is initiated and 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, and hence it determines the frequency of operation.

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.

Gunn diode tuning

The frequency of the signal generated by a Gunn diode is chiefly set by the thickness of the active region. However it is possible to alter it somewhat. Often Gunn diodes are mounted in a waveguide and the whole assembly forms a resonant circuit. As a result there are a number of ways in which the resonate frequency of the assembly can be altered. Mechanical adjustments can be made by placing an adjusting screw into the waveguide cavity and these are used to give a crude measure of tuning.

However some form of electrical tuning is normally required as well. It is possible to couple a varactor diode into the Gunn oscillator circuit, but changing the voltage on the varactor, and hence its capacitance, the frequency of the Gunn assembly can be trimmed.