A transistor is a three-element, two-junction device used to control electron flow. By varying the amount of voltage applied to the three elements, the amount of current can be controlled for purposes of amplification, oscillation, and switching.
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.