Semiconductor Basics

4Infrared Detectors

OBJECTIVES OF THIS CHAPTER

Let's take a pause from the “semiconductor theory” that I covered in the previous three chapters and discuss one of the applications, infrared detectors, that can be easily understood after we have learned about the concept of energy bands and energy gaps for both intrinsic and extrinsic semiconductors. We do not need to know how semiconductor devices such as transistors or diodes work.

In this chapter, after I explain what infrared radiation is and some infrared applications, I discuss the radiation spectrum and how we use semiconductors, both intrinsic (pure) and extrinsic (doped), to “see” the infrared images. I also cover a couple of compound semiconductors that we use as infrared detectors.

4.1 What is Infrared Radiation?

In 1800 Frederick William Herschel (1738–1822) performed the experiment I show in Figure 4.1. Hershel passed light through a prism that separates the white light into different colors (in a prism, each color bends at slightly different angle, resulting in the color separation, see Appendix 4.1). Hershel placed a thermometer outside the visible radiation on the left of the color red and detected an increase in temperature. Although he did not see anything, it was obvious that there was an “invisible” radiation below (infra) the red color. Very soon scientists realized that the radiation spectrum was considerably wider than just the narrow band of what we call light and that we are able to see.

Figure ...