A junction diode is formed by two semiconductor pieces, one of which is p-type and the other n-type. A junction transistor is a ‘sandwich’ of three. By using three connections, one current going into the device can be used to control the size of another current. There are two possible ‘sandwiches’; npn and pnp. The npn type is the one used in this book.
The two transistor symbols shown in Fig. 45.12 are similar, but the arrow is drawn differently. The connection to the central slice is the base. The arrowed connection is the emitter. The arrow points in on a pnp transistor, and out on an npn transistor. The arrow on the emitter shows the main current direction through the transistor. The other connection is the collector.
Transistors vary in current-carrying capacity. The larger ones, which control more power, may have a heat sink to remove heat energy dissipated within the transistor and prevent its destruction by overheating. Some have air-convection cooling vanes while others have a thick metal base to remove heat by conduction.