A transistor is a simple electronic device that switches and amplifies electrical currents. Though scientists have invented many kinds of transistors, the junction transistor was developed first, and the PNP is one such type. A PNP transistor is one in which the main flow of current is controlled by altering the number of holes instead of the number of electrons in the base. The low cost, reliability and small size of transistors has made them one of the greatest inventions of the 20th century.
A typical PNP transistor has a plastic or metal case roughly the size of a pea. High-powered transistors are larger, about as big as a bottle cap. The device has three wires called leads that connect to other parts in a circuit. The leads are called the base, collector and emitter, and each has a specific function. The body of the transistor may have a part number and manufacturer's logo printed or stamped on it, along with the letters "E," "B" and "C" that identify the emitter, base and collector leads.
The transistor consists of three slivers of specially treated silicon, an element that conducts electricity when mixed with traces of other elements. The outside two layers have a treatment that makes them prefer positive electric charges. The inner layer prefers negative charges. The three layers together form a positive-negative-positive transistor, or PNP for short.
A small electrical current flowing to the transistor's base and emitter leads controls a larger current from the emitter to the collector. A PNP transistor turns on its emitter-collector connection if the voltage at the base is lower than that at the emitter. This valve-like action allows the transistor to control large currents with small ones, in effect amplifying the smaller currents.
A PNP transistor in a radio boosts the relatively small signal from an antenna, letting you tune in stations many miles away. Transistors in power amplifiers drive loudspeakers, which require large amounts of current. In computer circuits, they rapidly switch currents completely on and off. Transistors also generate stable, high-frequency signals used in radio and TV broadcasting.