An Impact ionization avalanche transit-time (IMPATT) diode is a form of high-power semiconductor diode used in high frequency microwave electronics devices.
They have negative resistance and are used as oscillators to generate microwaves as well as amplifiers. They operate at frequencies between about 3 and 100 GHz or more.
IMPATT diode is made of silicon as it is cheaper and easier to fabricate using epitaxial growth. The gold alloy contact is used because of low ohmic and thermal resistance.
In IMPATT diode extremely high voltage gradient is applied which a normal p-n junction can’t withstand. Such a high potential gradient, back-biasing the diode cause a flow of minority carrier across the junction.
The ac current is approximately 180 degree out of phase with the applied voltage this gives rise to negative conduction and oscillation is resonant circuit.