How does a bipolar junction transistor work?

A bipolar junction transistor (BJT) works by controlling the flow of current through a semiconductor.

A BJT is made up of three regions: the emitter, base, and collector. These regions are doped with different types of impurities to create either an excess or a deficiency of electrons. The emitter is heavily doped with impurities to create a large number of free electrons, while the base is lightly doped to create a small number of free electrons. The collector is moderately doped to create a moderate number of free electrons.

When a voltage is applied to the base, it creates an electric field that attracts electrons from the emitter to the base. This causes a flow of current from the emitter to the collector, which is controlled by the voltage applied to the base. As the voltage applied to the base increases, the flow of current from the emitter to the collector also increases.

BJTs are commonly used as amplifiers and switches in electronic circuits. As an amplifier, the BJT amplifies the input signal by controlling the flow of current through the transistor. As a switch, the BJT either allows or blocks the flow of current through the transistor, depending on the voltage applied to the base.