When a third layer is added to a semiconductor diode, a device is produced that can amplify power, current, or voltage. The device is called a bipolar transistor, also referred to as a junction transistor or transistor. The term transistor will be used here.

A transistor, like a junction diode, can be constructed of germanium or silicon , but silicon is more popular. A transistor consists of three alternately
doped regions (as compared to two in a diode). The three regions are arranged in one of two ways.