INTRODUCTION

In this appendix we will review some important aspects of the diode, which forms the basis of many of the optoelectronic devices discussed in Chapter 5. Most optical detectors and light emitters are based on p–n diodes.

P–N JUNCTION

As noted above the p–n diode is one of the most important optoelectronic devices. It forms the basis of most detectors and light emitting devices. Light detection occurs when photons create electrons and holes, while light emission occurs by e–h recombination.

Unbiased P–N junction

The p–n junction is one of the most important junctions in solid-state electronics. The fabrication techniques used to form p- and n-type regions involve (i) epitaxial procedures, where the dopant species are simply switched at a particular instant in time: (ii) ion-implantation in which the dopant ions are implanted at high energies into the semiconductor (the junction is obviously not as abrupt as in the case of epitaxial techniques); and (iii) diffusion of dopants into an oppositely doped semiconductor.

We will assume in our analysis that the p–n junction is abrupt, even though this is really only true for epitaxially grown junctions. Let us first discuss the properties of the junction in the absence of any external bias where there is no current flowing in the diode.

What happens when the p- and n-type materials are made to form a junction and there is no externally applied field? We know that, in absence of any applied bias, there is no current in the system and the Fermi level is uniform throughout the structure.