Editing Gunn diode (section)

== Principle ==

The [[electronic band structure]] of some [[semiconductor]] materials, including [[gallium arsenide]] (GaAs), have another energy band or sub-band in addition to the [[Valence band|valence]] and [[conduction band]]s which define a semiconductor material and which is exploited to design [[semiconductor devices]]. This third band (there could be more of them) is at higher energy than the normal conduction band and is typically empty at room temperature until energy is supplied to promote electrons to it. The energy comes from the kinetic energy of [[Ballistic conduction|ballistic electrons]], that is, electrons in the conduction band but moving with sufficient kinetic energy such that they are able to reach the higher band. The additional kinetic energy is typically provided by an electric field, applied externally to the device.

These electrons either start 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 (solid-state physics)|effective mass]] of the electrons in the third band is higher than those in the usual conduction band, so the [[electron mobility|mobility]] or drift velocity of the electrons in that band is lower. As the forward voltage increases, more and more electrons can reach the higher energy band, causing them to move slower (though they have higher energies), and the current through the device decreases. This creates a region of negative differential resistance in the voltage/current relationship.

When a high enough potential is applied to the diode, the charge carrier density along the cathode becomes unstable and will develop small segments of low conductivity, with the rest of the cathode having high conductivity. Most of the cathode voltage drop will occur across the segment so that it will have a high electric field. Under the influence of this electric field, it will move along the cathode to the anode. It is impossible to balance the population in both bands, so thin slices of high-field strength will always be in a background of low-field strength. So in practice, with a slight increase in forward voltage, a low conductivity segment is created at the cathode, resistance increases, the segment moves along the bar to the anode, and when it reaches the anode, it is absorbed, and a new segment is created at the cathode to keep the total voltage constant. Any existing slice is quenched if the voltage is lowered and resistance decreases again.

In this context, ballistic electrons—those that travel with minimal scattering—play a crucial role. They can move through the semiconductor with a long mean free path, effectively gaining the necessary energy to transition to the higher energy states.

The laboratory methods used to select materials for manufacturing Gunn diodes include [[ARPES|angle-resolved photoemission spectroscopy]].