A transistor is an electronic component used for amplification and switching electronic signals. Transistors commonly have three terminals: The base, the collector and the emitter. Current can only flow from the collector to the emitter when a small current is present at the base. Transistors are the main building blocks of all computers and most electronic devices.
Bipolar junction transistor (BJT)
The BJT transistor consists of three regions of semiconductor. One at the emitter, one at the base and one at the collector. These regions are p,n and p or n, p and n type depending on the type of transistor. Each region is connected to one lead of the transistor, labeled: emitter (E), base (B) and collector (C).
NPN and PNP transistors
BJTs are made in two types: NPN and PNP. The NPN variant is made from one p layer between two n layers. Usually, no current flows from the emitter to the collector. However, when a current is applied to the base, the current will start to flow and the transistor is "on". The transistor will turn "off" again when the current at the base stops flowing.
PNP transistors are made of one n layer between two p layers. The PNP transistor is "on" when the base is pulled to low. The current will stop flowing from the emitter to the base when a current is applied to the base.