A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor’s terminals changes the current through another pair of terminals. Because transistors can be used as amplifiers, they are widely employed in electronic equipment such as radios, television sets, and computers. In addition, digital logic circuits use transistors as switches because they can turn on and off rapidly in response to an electric signal.

Bipolar junction transistors (BJTs) are the most common type of transistor in electronic equipment today. They are made from two types of semiconductor materials sandwiched together into a single crystal structure. The first BJT was invented by Walter Houser Brattain and John Bardeen at Bell Laboratories in 1947 while working on point-contact germanium crystals. The first practical silicon transistor was developed by Morris Tanenbaum at Bell Labs in 1954. MOSFETs (metal oxide semiconductor field effect transistors) were later invented by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959 which led to modern integrated circuit chips that contain millions or billions of MOSFETs per chip.