Overview

Historically, bipolar integrated circuits used to be much more popular than MOS integrated circuits, particularly for small-scale logic circuits. There were two major reasons for this: first, bipolar-junction transistors (BJT) originally could be manufactured more reliably than MOS transistors, and second, they were faster. As the reliability of MOS transistors improved, and as integrated circuits became more complex, which made the lower power and smaller size of MOS logic more important, the popularity of BJT logic decreased; however, BJT technology is still popular for the highest frequency logic circuits. Also, due to the recent emergence and growing popularity of BiCMOS ICs, where both MOS and bipolar gates can be realized in the same IC, bipolar logic design is growing in importance.

The main reason for the high speed of BJTs, and therefore BJT logic circuits, is the high transconductance of BJTs. Remember from Chapter 3 that for a BJT we have the transconductance, g _m, given by

At room temperature a bipolar transistor has a transconductance of approximately 4 mA/V for a 0.1-mA bias current. This is more than an order of magnitude larger than the typical transconductances of MOS transistors having comparable sizes. This large transconductance results in a large unity current gain frequency (i.e., f _t) for the transistor. In addition, it allows parasitic capacitances to be charged and discharged quickly. Finally, it also allows BJT logic gates to be operated with small voltage changes, which is another major reason why modern bipolar logic circuits...