Introduction

As early as 1850, Antoine Cesar Becquerel discovered that certain materials generate an electrical current when exposed to a flux of light. It took, however, until about 1935 before a quantum theory of condensed matter could be developed to give a satisfactory account of this phenomenon. In spite of a lack of any firm theoretical understanding for these empirical observations, photodetectors were fashioned from these materials and put to work in photography and in military infrared detection applications.

The basic general principles behind the operation of semiconductor detectors are illustrated in Fig. 11.1. In the absence of photoexcitation, the carriers in these materials do not conduct electricity either because: (a) they are in a band where they cannot participate in conduction (e.g. a full valence band), (b) they are blocked by a potential barrier (as in a Schottky detector), or (c) they are trapped in quantum bound states (e.g. extrinsic photoconductors or quantum well detectors).

Optically driven transitions between two ensembles of quantum levels (one conducting and the other insulating), are at the origin of photodetection. For this reason, semiconductor detectors are sometimes referred to as quantum detectors.

Sections 11.3 and 11.4 describe (with reference to Fig. 11.1) type (a) photodetectors (photoconducting and photovoltaic), Section 11.5 describes type (b) internal emission Schottky photodetectors, and Section 11.6 describes type (c) quantum well photodetectors. We will see that all these detectors share a common feature.