CHAPTER 5 - THE p–n JUNCTION

Abstract

This chapter describes what occurs when a p-type semiconductor material forms a junction with n-type material. It describes the diffusion of charge carriers across the junction of p-type and n-type semiconductor material. The holes in the p-type experience a relatively high concentration of their own kind and, therefore, diffuse away to the right and into the n-type region, leaving behind the negative acceptor ions. The opposite applies to the electrons in the n-type material. There is a region in the crystal where it is depleted of its normal complement of charge carriers. This region, which can be as small as 10-4 m wide, is called the depletion layer, and it is a space charge region wherein the p-type side exhibits a net negative charge, and vice versa. The charges occur in the region of the junction; however, if the diffusion process continued, the number of diffused electrons would eventually equal the number of diffused holes. However, this cannot occur, due to the finite lifetime of free electrons and holes. As soon as an electron enters the p-type material, the probability that it will recombine with a hole increases enormously. The converse is true of holes entering the n-type material.