What is IMPATT diode? How does the negative resistance arise in this diode?
It is possible to make a microwave diode exhibit negative resistance by having a delay between voltage and current in an avalanche together with transit time through the material. The IMPATT diode is stand for Impact Ionization Avalanche Transit Time Device.
A combinational of delay involved in generating avalanche current multiplication together with delay due to transit time through a different space provides the necessary 180° phase difference between applied voltage and resulting current in an IMPATT diode.
Fig. 9(a) IMPATT diode internal structure and Fig. 9(b) resonant circuit with IMPATT diode.
Any device which exhibits negative resistance for d.c. will also exhibits it for a.c. i.e., if an a.c. voltage is applied current will rise when voltage falls at an a.c. rate. Hence negative resistance can also be defined as that property of a device which causes the current through it to be 180° out of phase with the voltage across it. Thus is the kind negative resistance exhibited by IMPATT diode i.e., if we show voltage and current have a 180° phase difference, then negative resistance in IMPATT diode is proved.
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Fig.23