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 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. How it works 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