Opening Thoughts on Bipolar Transistors

You can build most of the components we have discussed so far using a basic CMOS process.

As we saw in the CMOS layout section, the inherent Gate capacitance in CMOS transistors slows our device. However, in what we call a Bipolar transistor, the switching region can be made much smaller. Making regions smaller reduces capacitance.

Bipolar transistors, therefore, help solve the capacitance problem by their size. Also, with their smaller RC time constant, they operate much faster than a CMOS transistor. Fast is good. Bipolar is good. This is a powerful chapter.

We use the name Bipolar because these transistors use both electrons and holes at the same time during its operation. It's as if one pole is attracting electrons, while another pole attracts holes. Two ( bi) poles ( polar).

You could call a CMOS transistor a unipolar device because it only uses one type of carrier during operation. A P Type CMOS transistor uses holes as its main conductor, for instance.

Doping levels of the N and P Type diffusions in a CMOS process are optimized for the CMOS transistor operation, not for Bipolar transistor operation. Extra processing steps, implants and diffusions optimize N and P levels for Bipolar devices. Therefore, you do not produce very good Bipolar devices using plain CMOS.

You will find manufacturers typically offer either pure CMOS processes, or pure Bipolar processes. If you want a mixture of the two types of transistors...