Power transistor

Transistors are three-terminal semiconductor electronic devices that can be used as switches or amplifiers. They can be of NPN or PNP polarity, and many different types are available with different power and switching speed ratings.

The three transistor terminals are known as base, emitter and collector respectively. If a battery and light are connected in series across an NPN transistor’s collector and emitter, the transistor will not pass current and the lamp won’t light. However, if a voltage, typically of about 0.7 V, is applied across the base and emitter, a small current will flow between these terminals. This turns the transistor on, so the lamp will light.

If the base-emitter voltage is varied, for example if it is driven by an audio signal, the large current flowing from collector to emitter will vary accordingly; this is how the transistor can be used as an amplifier as well as a switch.

If a transistor designed to these concepts can handle more than 1 A of collector current, it is generally considered to be a power transistor. Such transistors must have low output resistance to deliver large load currents, and good junction insulation to resist high voltages. They must also dissipate heat very quickly to avoid overheating. The collector/base junction must be as large as possible, as most heat is generated in this area.