Chapter 17: pnpn and Other Devices

In Chapter 17, a number of important devices not discussed in detail in earlier chapters are introduced. The two-layer semiconductor diode has led to three-, four-, and even five-layer devices. A family of four-layer pnpn devices will first be considered: the SCR (silicon-controlled rectifier), the SCS (silicon-controlled switch), the GTO (gate turn-off switch), the LASCR (light-activated SCR), and then an increasingly important device—the UJT (unijunction transistor). Those four-layer devices with a control mechanism are commonly referred to as thyristors, although the term is most frequently applied to the SCR. The chapter closes with an introduction to the phototransistor, opto-isolators, and the PUT (programmable unijunction transistor).

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