Introduction

In Chapter 6, we saw that a semiconductor driven away from thermodynamic equilibrium can emit light (in addition to blackbody radiation) when excited carriers recombine from one band to another. We also derived the Bernard–Durrafourg condition, which the energy distributions of the carrier populations must satisfy before optical amplification can occur. We will now show how this light emission can be put to use in electroluminescent diodes (alternatively known as light emitting diodes or LEDs) and laser diodes. In doing so, we will draw upon the contents of no less than five of the previous chapters:

• Chapter 4, which describes the physics of laser oscillations;

• Chapter 7, which describes the various optical emission mechanisms in semiconductors;

• Chapter 8, which describes the physics of semiconductor heterostructures and quantum well structures;

• Chapter 9, which describes waveguiding in optical heterostructures;

• Chapter 10, which describes carrier injection mechanisms in p–n diodes.

This chapter is fairly complex (and exciting!) in that it brings into play many of the different physical concepts elaborated over the course of this book. While making frequent use of material developed in other chapters, we will take the time to recap many of the key concepts to allow the reader to progress through this chapter without breaking stride on too many occasions.

Electrical injection and non-equilibrium carrier densities

Light emission in a semiconductor usually proceeds from electron–hole recombination in regions where they are in excess in comparison with levels allowed by thermodynamic equilibrium.