12.1 : バイポーラ接合トランジスタ

Transistors play a vital role in electronic circuits, enabling the functionality of amplifiers, memories, and microprocessors.

Transistors can be designed using either bipolar technology or unipolar devices that utilize the field effect.

A typical bipolar transistor comprises two p-n junctions and three distinct doped regions: the emitter, base, and collector.

The type of transistor, whether NPN or PNP, is determined by the doping of these three regions using N-type or P-type impurities.

An arrow in the circuit symbol of the transistor represents the conventional direction of current flow under forward bias conditions.

The term 'bipolar' signifies that both holes and electrons contribute to the flow of current in these transistors.

The emitter is a heavily doped region. The base, a moderately doped central region, is relatively narrow, especially when compared with the minority carrier diffusion length. The collector, a lightly doped region, is wider than both the emitter and base.

Bipolar junction transistors have diverse applications. In digital electronics, they can function as switches, while in analog electronics, they serve as amplifiers.