What is Gunn diode ?

A Gunn diode is a type of semiconductor device which exhibits a unique property called "negative differential resistance". This means, as voltage across diode increases, current through it decreases which is opposite to normal behaviour observed in most of the electronic components. It is also known as transferred electron device (TED). It is used to generate microwave signals in the frequency range of a few GHz to terahertz.

This type of diode is widely used at high frequency electronic circuits. It is also known as transferred electron device or TED. Unlike other diodes which usually will have both N and P doped regions, Gunn diode has only N-type of semiconductor doping material.

As shown in the figure below, Gunn diode is made of 3 regions out of which two are heavily doped(N+) and one is lightly doped(N-). Upon application of voltage across the Gunn diode, large electrical gradient will develop acoss middle layer (N-) of the device. But at high potential, middle layer conductive properties can be altered and hence will reduce the gradient across the device. This will prevent the conduction slowly after current will get reduced. The figure depicts Gunn diode symbols.

Some key points about Gunn diodes are as follows.
• Gunn diode operates in negative differential resistance region which is observed in semiconductor materials such as GaAs (Gallium Arsenide).
• Due to above characteristic, when it is biased with DC voltage, it can spontaneously start oscillating at microwave frequencies. This mechanism is called "Gunn Effect".
• Gunn diodes can generate CW or pulsed microwave signals in few GHz to terahertz range. Hence it is used as microwave signal sources in radar, communication systems and electronic warfare systems.
• It is a two terminal device with one terminal acts as anode and the other as cathode.

Gunn diodes generate relatively low output power and narrow tuning range compared to other microwave signal sources (e.g. klystrons, magnetrons).

Gunn diode Oscillator circuit diagram

A Gunn diodes utilize negative differential resistance characteristic to generate microwave oscillations. The circuit provides the necessary bias and feedback to sustain the oscillations at the desired frequency.

Figure depicts Gunn diode layers, transfer characteristics and circuit diagram of Gunn diode oscillator. Gunn diode diagram depicts negative resistance region.

This negative resistance along with lead inductance and some stray capacitance result into oscillations. This relaxation type of oscillation in most of the cases will have large amplitude which will burn out the diode. Large capacitor is connected across the Gunn diode which will help avoid and minimize this failure.

This feature is used to build oscillators at higher frequencies in GHz and THz bands. Here resonator is added in order to control the frequency.

Conclusion : It has to be noted that tuning a Gunn diode oscillator circuit is complex due to sensitivity of circuit's parameters such as diode's bias voltage, feedback path, resonator dimensions etc. Advanced designs incorporate extra components for modulation, frequency stabilization and power control. Technology has evolved and we have FETs or VCOs find applications in modern microwave systems due to their improved performance and reliability compared to Gunn diode oscillators.

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