3 - Junctions

Summary

In this chapter we discuss different types of junctions that are of importance in semiconductor devices. Specifically, we examine p–n junctions, Schottky barriers, and ohmic contacts. We will delay discussing metal–insulator–semiconductor, MIS, junctions until Chapter 6. We begin our discussion with p–n homojunctions. Heterojunctions, junctions formed between two dissimilar materials, are discussed briefly in Chapter 11. Devices made using heterojunctions are also presented in Chapter 11.

p–n homojunction in equilibrium

Before we begin our study of junctions let us make a few definitions. The bulk region is the area far from the junction where the carrier concentrations are equal to their equilibrium values. The metallurgical junction is the physical location of the junction between the n- and p-type regions. The depletion region is the area surrounding the metallurgical junction. It is called the depletion region since the action of the built-in field within the junction sweeps out the free carriers leaving behind immobile space charge.

For simplicity we make the following assumptions:

1. (i) The junction is one-dimensional, and a one-dimensional analysis can be employed.

2. (ii) The metallurgical junction is located at x = 0.

3. (iii) The p–n homojunction is a step or abrupt junction with uniformly doped p and n regions.

4. (iv) There exist perfect ohmic contacts far away from the metallurgical junction.

Let us first consider the p–n junction in equilibrium. As is always the case, in equilibrium no net current flows. For simplicity let us assume that our p–n junction is formed by putting an n-type layer into contact with a p-type layer.