The Gunn diode is a special diode used in microwave oscillators and detectors. They are made on a gallium arsenide substrate. Then an epitaxial layer of n-doped gallium arsenide is applied. Epitaxial means that the crystaline structure of this layer has the same orientation as the substrates. The substrate is placed against a heatsink. This is also the anode of the diode. The cathode is connected to the epitaxial layer.

When th voltage over the diode becomes so high that the electric field in the epitaxial layer rises above about 350V/mm2, an area with high field strength is formed. This field moves towards the cathode. The time it takes to reach the cathode dictates the frequency of the microwave pulses. By using the gunn diode in a resonating circuit you get a sinusoidal signal.

One of the most common uses for the gunn diode is in automatic door openers at shoping centers. Here there is used two gunn diodes one to transmit and one to recive. By measuring the variations in the reflected microwave signal, the device can 'see' if someone is approaching. This is similar to a radar, but with much lower effects(about 10 mW).

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