Emitter Region of a Bipolar Junction Transistor (BJT)

Small Signal Transistor

A bipolar junction transistor has 3 regions: the base, the collector, and the emitter.

This is illustrated below:

Doping of BJT Regions

The emitter region is the most heavily doped area of the transistor.

The emitter contains the largest amount of charge carriers out of all regions in the transistor.

Role of Emitter Region

The role of the emitter region is to emit or inject current carriers into the base region, which then moves to the collector region of the transistor, where the transistor outputs the current signal.

In a bipolar junction transistor, current flows from the emitter to the collector and then out from the collector.

The reason the emitter is the most heavily doped region is because it serves to inject a large amount of charge carriers into the base, which then travels into the collector, so that switching or amplification can occur.

In npn transistors, the n-type emitter injects free electrons into the base. In pnp transistors, the p-type emitter injects holes into the base.

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