Transistors are the commonest components in a microchip. They are used mostly as switches, letting current through to represent the binary digit 1, or cutting it off to represent 0.
A widely used type of transistor has two islands of n-type semiconductor in a larger base of p-type. While the transistor is switched off. The free electrons from the layers drain into the p layer and are absorbed by the free holes. The transistor is switched on by applying a voltage from a separate low-power circuit to an aluminium gate above the p base. This voltage attracts the free electrons from the p base towards the gate. They then form a bridge between the two n islands and provide a path for the current through the circuit in which the switch is operating.
The transistor is switched off by cutting off the power. The free electrons then drain back to the p base and are absorbed by the free holes. Without the bridge they formed between the islands, current cannot flow through the circuit.
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