Gunn diode

A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal passive semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the 'Gunn effect' discovered in 1962 by physicist J. B. Gunn. Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers. A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal passive semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the 'Gunn effect' discovered in 1962 by physicist J. B. Gunn. Its largest use is 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 does not conduct in only one direction 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 of those are heavily N-doped on each terminal, with a thin layer of lightly n-doped material between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. If the voltage is increased, the current through the layer will first increase, but 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. 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 negative differential resistance of the diode cancels the positive resistance of the load circuit, 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 loss resistance of the resonator, so it produces oscillations at its resonant frequency. The frequency can be tuned mechanically, by adjusting the size of the cavity, or in case of YIG spheres by changing the magnetic field. Gunn diodes are used to build oscillators in the 10 GHz to high (THz) frequency range. Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz. The Gunn diode is based on the Gunn effect, and both are named for the physicist J. B. Gunn who, at IBM in 1962, discovered the effect because he refused to accept inconsistent experimental results in gallium arsenide as 'noise', and tracked down 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. The electronic band structure of some semiconductor materials, including gallium arsenide (GaAs), have another energy band or sub-band in addition to the valence and conduction bands which are usually used in semiconductor devices. This third band is at a higher energy than the normal conduction band and is empty until energy is supplied to promote electrons to it. The energy comes from the kinetic energy of ballistic electrons, that is, electrons in the conduction band but moving with sufficient kinetic energy such that they are able to reach the third band. These electrons either start out below the Fermi level and are given a sufficiently long mean free path to acquire the needed energy by applying a strong electric field, or they are injected by a cathode with the right energy. With forward voltage applied, the Fermi level in the cathode moves into the third band, and reflections of ballistic electrons starting around the Fermi level are minimized by matching the density of states and using the additional interface layers to let the reflected waves interfere destructively. In GaAs the effective mass of the electrons in the third band is higher than those in the usual conduction band, so the mobility or drift velocity of the electrons in that band is lower. As the forward voltage increases, more and more electrons can reach the third band, causing them to move slower, and current through the device decreases. This creates a region of negative differential resistance in the voltage/current relationship.