Perhaps the most important application of compound semiconductor materials to date is in optoelectronic devices. The types of optoelectronic devices that are discussed here can be classified into two main categories, photonic detectors and emitters. In this section, we discuss photonic semiconductor detectors. In Chapter 13 we will discuss emitters.

Many compound semiconductors, such as GaAs, InP, GaInAs, GaN, ZnS, etc., are direct-bandgap materials. As discussed in Chapter 10, optical absorption and emission processes occur to first order in direct-gap systems. Therefore these materials are extremely useful as both detectors and emitters of electromagnetic radiation. Depending on the bandgap energy, different semiconductors can be used to detect radiation from the far infrared to the ultraviolet portion of the spectrum.

In this chapter, we discuss photonic detectors, concentrating on MIS structures (particularly charge-coupled devices, or CCDs), photoconductors, photodiodes, and avalanche photodiodes (APDs). To begin with, some issues in detection are discussed; later it is shown how each of the above-mentioned device types operate.

Basic issues in Photonic Detection

The fundamental purpose of any photonic detector is to convert an input photonic signal into an electrical signal. The application in which the detector is used greatly affects the performance criterion of the detector. For example, a detector can be used for imaging. In these applications, a good detector is defined as one that provides a high degree of spatial resolution, gray-scale resolution (the ability to distinguish different shades on a totally white to totally black contrast scale), etc.