Bipolar Transistors

Abstract

The previous two chapters have discussed the pn junction, the MOS capacitor, and the FET device. For the pn junction, we considered minority carrier injection into material of the opposite carrier type. In the MOSFET, we create a conducting channel between source and drain regions (both of opposite carrier type to the substrate) by inverting the carrier type of the substrate (or the well). In this chapter we consider the bipolar transistor and its operation in detail. Bipolar transistor operation hinges on the proximity of two pn junctions, separated by less than a diffusion length for minority carriers. If one junction is forward biased and the other reverse biased, it is possible to inject (or “emit,”) carriers from the forward biased junction and, “collect,” these carriers at the adjacent, reverse biased junction.