The bipolar junction transistor (BJT), the first practical solid-state amplifying device, represents one of the watershed achievements in human history. Transistors are essential to electronics and hence modern life. They have achieved this notoriety because they possess two fundamental capabilities: 1) to function as an amplifier of time-varying voltage or current signals, and 2) to function as a regenerative switch, enabling digital logic based on a binary (base 2) system. Both amplifiers and regenerative switches work well only because the transistor possesses the unique ability to produce "gain". In this chapter, we address the conception and operation of BJTs, discuss how gain is achieved in practice, and highlight how one optimizes BJTs for high speed, including its natural evolution to heterojunction bipolar transistor (HBT) implementations such as the SiGe HBT.
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