CMOS vs. Bipolar

CMOS technology and the bipolar transistor represent two fundamentally different ways of controlling and amplifying electrical signals. While the bipolar transistor dominated digital electronics for decades (TTL logic), the field-effect transistor in the form of CMOS technology has almost completely displaced it since the 1980s – not because it is inherently "better", but because its properties suit mass integration far more effectively. In analogue, radio-frequency, and power applications, however, the bipolar transistor remains relevant to this day.

The real reason for this technology shift lies less in individual device properties than at the system level: digital circuits consist of millions to billions of individual transistors, and there it is mainly the power dissipation per switching event, together with the achievable packing density, that decides whether a technology is practical. As the following paragraphs show, CMOS offers decisive advantages here through its near-lossless standby state and easier scalability. Bipolar transistors, in turn, play to their strengths wherever high current drive capability, low noise, or high cutoff frequencies matter – properties that are secondary in digital electronics but continue to be decisive in analogue and RF circuits. The following paragraphs compare both device families in detail: first the basic operating principle, then power dissipation and switching behaviour, and finally integration density and scalability.

The MOSFET is a unipolar device: a voltage at the gate generates a conductive channel between source and drain via an electric field. Because the gate is insulated from the channel by an oxide layer, practically no control current flows in steady state – the MOSFET is voltage-controlled.

The bipolar transistor, by contrast, works with two types of charge carriers simultaneously (hence "bipolar"). A base current controls a much larger collector current through carrier injection and diffusion. It is therefore current-controlled and requires a continuous control current during operation.

This distinction between field control and charge-carrier control runs through practically all further electrical properties of both device families. In a MOSFET, it is essentially the gate capacitance that determines how quickly the channel can be built up and removed – a purely electrostatic process that requires no minority-carrier injection. In a bipolar transistor, by contrast, charge carriers must actually be injected into the base region for every switching event and then swept out again on turn-off, which costs additional time and is known as the storage-time effect. This differing physical basis is also what directly gives rise to the differences in power dissipation and switching behaviour covered in the next paragraph.