Question

With suitable sketch and explain the working of IMPATT diode.

Answer 1

Any device which exhibits negative resistance for dc will also exhibit for ac i.e. if an ac voltage is applied current will rise when voltage falls at an ac 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.

This kind of negative resistance is exhibited by IMPATT diode. A combination of delay involved in generating avalanche current multiplication together with delay due to transit time through drift space provides the necessary 180° phase difference between applied voltage and resulting current in an IMPATT diode. The cross section of the active region of this device is shown in figure above. It is a diode with the junction between the + and n layers. An extremely high-voltage gradient is applied to the IMPATT diode, of the order of 400kV/cm, eventually resulting in a very high current. A normal diode would very quickly breakdown under such conditions, but the IMPATT diode is constructed so as to be able to withstand such conditions repeatedly. Let us consider application of a RF ac voltage superimposed on top of the high dc voltage. Increased velocity of electrons and holes result in additional electrons and holes by knocking them out of the crystal structure by so called impact ionization. These additional carriers continue the process at the junction and it now snowballs into an avalanche. If the original dc field was just at the threshold of allowing this situation to develop, his voltage will be exceeded during the whole of the RF positive cycle and the avalanche current multiplication will be taking place during this entire time. Since it is a multiplication process avalanche is not instantaneous. This process in fact takes a time such that current pulse maximum at the junction occurs at the instant when RF voltage across the diode is zero and going negative. A 90° phase shift or phase difference between voltage and current has then been achieved. The current pulse as shown in figure below is situated at the junction. It does not stay there but moves towards the cathode due to applied reverse bias at a drift velocity dependent upon the presence of high dc field.