Working:

• Any device which exhibits negative resistance for dc will also exhibits it for ac i.e., If an ac voltage is applied current will rise when voltage falls at an arc rate. Hence negative resistance can also be defined as that property of a device which causes the current through it to be 1800 out of phase with voltage across it.
• Thus is the kind of negative resistance exhibited by IMPATT diode i.e., If we show voltage and current have a 1800 phase difference, then negative resistance in IMPATT diode is proved.
• A combination of delay involved in generating avalanche current multiplication together with delay due to transit time through a different space provides the necessary 1800 phase difference between applied voltage and the resulting current in an IMPATT diode.