A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative differential resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1962 by physicist J. B. Gunn. Its main uses are in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers. Its internal construction is unlike other diodes in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. It, therefore, conducts in both directions and cannot rectify alternating current like other diodes, which is why some sources do not use the term diode but prefer TED. In the Gunn diode, three regions exist: two are heavily N-doped on each terminal, with a thin layer of lightly n-doped material between them. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. If the voltage increases, the layer's current will first increase. Still, eventually, at higher field values, the conductive properties of the middle layer are altered, increasing its resistivity and causing the current to fall. This means a Gunn diode has a region of negative differential resistance in its current–voltage characteristic curve, in which an increase of applied voltage causes a decrease in current. This property allows it to amplify, functioning as a radio frequency amplifier, or to become unstable and oscillate when it is biased with a DC voltage.
Gunn diode oscillators
The negative differential resistance, combined with the timing properties of the intermediate layer, is responsible for the diode's largest use: in electronic oscillators at microwave frequencies and above. A microwave oscillator can be created simply by applying a DC voltage to bias the device into its negative resistance region. In effect, the diode's negative differential resistance cancels the load circuit's positive resistance, thus creating a circuit with zero differential resistance, which will produce spontaneous oscillations. The oscillation frequency is determined partly by the properties of the middle diode layer but can be tuned by external factors. In practical oscillators, an electronic resonator is usually added to control frequency in the form of a waveguide, microwave cavity, or YIG sphere. The diode is usually mounted inside the cavity. The diode cancels the resonator's loss resistance, producing oscillations at its resonant frequency. The frequency can be tuned mechanically, by adjusting the size of the cavity, or in the case of YIG spheres, by changing the magnetic field. Gunn diodes are used to build oscillators in the 10 GHz to THz frequency range. Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.
History
The Gunn diode is based on the Gunn effect, and both are named for physicist J. B. Gunn. At IBM in 1962, he discovered the effect because he refused to accept inconsistent experimental results in gallium arsenide as "noise", and determined the cause. Alan Chynoweth of Bell Telephone Laboratories showed in June 1965 that only a transferred-electron mechanism could explain the experimental results. It was realized that the oscillations he detected were explained by the Ridley–Watkins–Hilsum theory, named for British physicists Brian Ridley, Tom Watkins and Cyril Hilsum who in scientific papers in 1961 showed that bulk semiconductors could display negative resistance, meaning that increasing the applied voltage causes the current to decrease. The Gunn effect and its relation to the Watkins–Ridley–Hilsum effect entered electronics literature in the early 1970s, e.g., in books on transferred electron devices and, more recently, on nonlinear wave methods for charge transport.
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