Gunn Diodes:

The Gunn diode is made up from a compound termed as gallium arsenide (GaAs). Whenever a voltage is applied to this device, it oscillates since of the Gunn Effect, named after J. Gunn of International Business Machines (IBM), who first noticed the phenomenon in the 1960s. Oscillation takes place as an outcome of a property termed as negative resistance. This is a misnomer since, as we have learned, there is no such thing as a device which conducts superior than perfectly. In this logic, negative resistance refers to the fact which over a certain limited portion of the characteristic curve, the current through a Gunn diode reduces as the voltage increases, on contrary to what generally occurs in electrical systems.