How Does a Power Transistor Work?

Transistor Composition

A transistor is made of three external terminals on a piece of semiconducting material. The three terminals are labeled as base (b), collector (c) and emitter (e). The most important part of a transistor is the semiconductor, which is generally made of silicon. This semiconductor is the rectifier (director of electric current) and amplifier (magnifier of electric current) for the system. During the construction of a transistor, certain impurities are placed in different parts of the semiconducting material. These impurities are either positively or negatively charged. The way they are placed in the semiconducting material determines the amplifying power of the transistor. The positive or negative impurities are generally placed to create junctions between the layers, so no two positive impurities are placed next to each other. The arrangement of positive (p) impurities and negative (n) impurities can follow different arrangements to create junctions. Some sample arrangements are n-p-n and p-n-p. The junction between the negative and positive sections is called an n-p junction.

The N-P Junction

The junctions between n and p sections of the semiconductor are the amplifiers for the transistor. As a note, the n-p junction is the rectifier of the transistor. In the explanation of a transistor, the p-n-p transistor type works well as an example. In a p-n-p transistor, there's a small layer of n material between two layers of p material. This means that the emitter terminal is connected to the p, the base is connected to the n and the collector is connected to the other p. It's very important that in all three terminal transistors, the base is connected to the opposite type of semiconducting material than the collector or emitter. The emitter, connected to the p, has a slightly negative voltage relative to the base terminal, which has a slightly positive voltage. The collector, connected to the other p, has a positive voltage. Electricity flows from the emitter to the base due to their difference in voltage. The flow of current between the emitter and the base has a low resistance, meaning that the current flows easily. The collector, which is connected to an output circuit, attracts the current flowing through the base because the collector is positively charged. The resistance between the base and collector is very high.

Significance of the Difference in Resistance

This difference in the resistance between the base and emitter and the base and collector is the central reason the transistor is such a good amplifier of electricity. Because the resistance is low in the current flowing from the emitter to the base and the resistance is high in the current flowing from the base to the collector, a very small change in voltage from the emitter to the base will be magnified between the base and the collector. Hence, this transistor can adequately amplify electricity running through it. A good analogy to this is how water pressure can build up when flowing from a large to a small pipe. The pressure of the water (the magnitude of the current) gets greater because the water has less space to travel through. This is similar to how the magnitude of the electricity increases because it goes from a low to high resistance.

Resources

• Encarta: Transistor