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Materials
An NPN transistor consists of a sandwich of three layers of specially treated silicon pressed together, with tiny wires connected to each layer. A plastic inner container surrounds and protects the delicate silicon from damage and contamination. An outer container, made of plastic or metal, holds the inner container and the transistor's three leads. Each lead connects to one of the small wires inside the transistor, and they serve as sturdy supports for the device.
Polarity
The outer two silicon layers contain traces of impurities called dopants that cause the silicon to prefer negative electric charges. The inner layer prefers positive charges. The order of the layers, Negative-Positive-Negative, gives the transistor its name. The other order, Positive-Negative-Positive, yields a PNP transistor. A transistor's polarity dictates how and where it works in a circuit.
Uses
Transistors perform two basic functions in electronic circuits: switching and amplifying. As a switch, a transistor can turn a large current on and off a small current controls it. A mechanical switch can also do this, but the transistor can switch at speeds up to several billions of times per second, and do it silently and without friction or wear. A transistor's amplifying action also controls large currents with small ones; in this case, it works over a range of currents. Its ability to reliably control currents is of fundamental importance to electronic design.
Size
The largest NPN transistors handle several hundred watts of power and have cases the size of a small cookie. Most discrete NPN transistors, those that have their own separate package, are the size of a pencil eraser. Integrated circuits, or ICs, have NPN transistors so small you need a high-powered microscope to see them.