P-I-N Diode

Abstract

A P-i-N diode is a two terminal device as shown in Fig. 4.1. It blocks a negative voltage on the anode terminal with respect to the cathode terminal but conducts current when a positive voltage is applied to the anode. It consists of an intrinsic layer sandwiched between a pn junction. The intrinsic layer has a very low concentration of (generally) n-type in the order of 1013cm-3. It’s thickness ranges from 10μm to 500μm depending on the rated breakdown voltage of the device. The outside p- and n- layers are usually very highly doped. The wide intrinsic layer provides unique features of high breakdown voltage in reverse bias and charge storage in the forward bias. In the reverse bias, the lightly doped intrinsic layer is fully depleted and the maximum breakdown voltage achievable is the product of the critical electric field of silicon and the thickness of the intrinsic layer. In the forward bias, both types of carriers are injected into the intrinsic layer. The current conduction is through carrier recombination in the middle layer. In the steady state, the electron and hole concentrations are equal in the middle region in accordance with the charge neutrality condition. The hole and electron concentrations are the highest at the P+ - i and the N+ - i junctions respectively with a minimum closer to the cathode side as a result of the difference in the mobility of electrons and holes.