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The word IMPATT stands for impact avalanche transit time. IMPATT diodes are made on silicon, silicon carbide, GaAs and InP.
IMPATT diodes not that long ago were an important technology. IMPATT holds many of the world's records for solid state power, either as oscillators or as amplifiers. However, the complexity of creating an IMPATT transmitter is such that few could afford one. Waveguide is most often used to combine many IMPATTs.
The diode is constructed on P+, N and N+ layers. It is operated at reverse bias. Impact ionization happens when the applied electric field causes a free electron to hit an atom in the lattice and release an electron-hole pair.
An ISIS diode is a close cousin of IMPATT. These are used in frequency multipliers as opposed to amplifiers.
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