3.5 Bipolar-Junction Transistors

Historically, integrated circuits that originally dominated the market were realized using bipolar-junction transistors (BJT transistors). Then around the late 1970s, ICs realized using MOS transistors began to dominate, although BJT ICs remained somewhat popular for high-speed applications. More recently, BJT transistor understanding has become more important because of the growing popularity of BiCMOS technologies, where both BJT transistors and MOS transistors can be realized in the same IC.

Modern silicon BJT transistors can have unity-gain frequencies as high as 15 to 60 GHz, or more, as compared to a unity-gain frequency of 1 to 8 GHz for MOS transistors realized using a technology having a similar lithography resolution. However, BJT transistors do have the unfortunate characteristic that the control terminal, the base, has a finite input current when the transistor is conducting current (from the collector to the emitter for an npn transistor; for a pnp transistor the current goes from the emitter to the collector). Fortunately, at low frequencies, the base current is much less than the collector-to-emitter current it may be only 1/100th as large as the collector current for an npn transistor. For lateral pnp transistors, the base current may be as large as 1/20th the emitter-to-collector current. Another disadvantage of bipolar transistors is they require greater area than MOS transistors realized using similar resolutions.

A typical cross section of an npn bipolar-junction transistor is shown in Fig. 3.29. Although this structure looks quite complicated, it corresponds to the approximately...