Gunn diode

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.

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 practical oscillators, an electronic resonator is usually added to control frequency in the form of a waveguide, microwave cavity, or YIG sphere.

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.

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.

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

Gunn diode oscillators generate microwave power for:[6] airborne collision avoidance radar, anti-lock brakes, sensors for monitoring the flow of traffic, car radar detectors, pedestrian safety systems, "distance travelled" recorders, motion detectors, "slow-speed" sensors (to detect pedestrian and traffic movement up to 85 km/h (50 mph)), traffic signal controllers, automatic door openers, automatic traffic gates, process control equipment to monitor throughput, burglar alarms and equipment to detect trespassers, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors.

An additional "mixer diode" is inserted into the waveguide, and it is often connected to a modified FM broadcast receiver to enable listening of other amateur stations.

On most commercial units, this part is protected with a parallel resistor and other components, and a variant is used in some Rb atomic clocks.