A transistor is a semiconductor device that can amplify or switch electronic signals. The transistor is a ubiquitous component, used in almost all modern electronic devices.
Transistors are useful for their gain property. A small signal applied to one pair of terminals can control a much larger signal at another pair of terminals. With this property, the transistor can be used in just about every amplifying and switching application, particularly in digital circuits.
The vast majority of transistors are built into integrated circuits. They are valued for their low power consumption, speed, and extremely small footprint. They are the primary requirement for advanced electronics like microchips and high-speed memory. Billions of discrete transistors are also produced every year, for industrial board design and equipment repair.