5.1.1 Transistor Operation (NPN)

Chapter 5.1.1 Transistor Operation (NPN)

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 5. TRANSISTOR CIRCUITS82b)a)EECCBBFigure 5.1: a) NPN bipolar transistor and b) PNP bipolar transistor.5.1.1Transistor Operation (NPN)If the collector, emitter, and base of an NPN transistor are shorted together as shown infigure 5.2a, the diffusion process described earlier for diodes results in the formation of twodepletion regions that surround the base as shown. The diffusion of negative carriers intothe base and positive carriers out of the base results in a relative electric potential as shownin figure 5.2b.NPNCBEa) DepletionRegion+VCEBb)Figure 5.2: a) NPN transistor with collector, base and emitter shorted together, and b)voltage levels developed within the shorted semiconductor.When the transistor is biased for normal operation as in figure 5.3a, the base terminalis slightly positive with respect to the emitter (about 0.6 V for silicon), and the collector ispositive by several volts. When properly biased, the transistor acts to make ICIB.Thedepletion region at the reverse-biased base-collector junction grows and is able to supportthe increased electric potential change indicated in the figure 5.3b.For a typical transistor, 95% to 99% of the charge carriers from the emitter make it to