In the analog world of continuously varying signals, a transistor is a device used to amplify its electrical input. In the digital world of computing, a transistor is mostly a binary switch and the fundamental building block of digital circuitry. Like a light switch on the wall, the transistor acts as a simple on/off switch, either preventing or allowing current to flow through.

Made of Semiconductor Material
The active part of the transistor is made of silicon or some other semiconductor material that can change its electrical state when pulsed. In its normal state, the material may be nonconductive or conductive, either impeding or letting current flow. When voltage is applied to the transistor's gate, it changes its state.

Transistors to Gates to Circuits to Systems
Transistors, as well as resistors, capacitors and diodes, are wired in patterns that make up logic gates. Logic gates wired in patterns make up circuits, and circuits wired in patterns make up electronic systems. To learn more about the transistor, see chip. See phototransistor and High-K/Metal Gate.