Semiconductor Basics

13Optoelectronics

OBJECTIVES OF THIS CHAPTER

Although I have already discussed some of the optical electronic devices when I talked about infrared detectors in Chapter 4, I’d like in this chapter to bring together semiconductor devices that interact with light, topics that include photoconductors, lasers, and light‐emitting diodes. I cover these devices in several sections, which all involve the effect of light on semiconductor and pn‐devices.

13.1 Photoconductors

The simplest optoelectronic device is the photoconductor. It consists of a piece of semiconductor material and just two contacts (Figure 13.1). Intrinsic semiconductors have very few electrons in the conduction band. In Section 2.4 I mentioned that intrinsic silicon has 1.45 × 10¹⁰ electrons per cm³ in the conduction band, which is a tiny number compared to the total number of silicon atoms, 5 × 10²² atoms per cm³. That means that the resistivity of pure silicon is 60 000 Ω‐cm. If we apply a voltage and light shines on the semiconductor, many electrons absorb the light and generate free electrons (and holes), the resistivity of the semiconductor decreases, and the ammeter measures the change in current. Since I have spent considerable time explaining infrared detectors in Chapter 4, I do not need say more here. The semiconductor lets me know that light is shining by the change in current: the more current, the stronger the light.

Figure 13.1 A simple photoconductor consists of a semiconductor with two ...