0UziMonkeyBest Answer 12 years ago There are several types of transistors, but the most commonly referred to is the Bipolar Junction Transistor (BJT). A BJT is a semiconductor device with 3 pins: base, emitter and collector. In a very simplified view, the resistance across the emitter and collector pins is proportional to the voltage applied across the base and emitter. The more voltage applied to the base pin, the lower the resistance across the other two pins. The relationship is often exponential as well, a small change in voltage on the base pin creates a large change in resistance across the other two pins. A transistor can be used as a switch. When a certain condition occurs (such as enough light shining on a photo-resistor) and another devices causes the voltage on the base pin to go high, the transistor is said to be "on," meaning it has a low resistance and will conduct readily. When the voltage on the base pin goes low enough, the transistor is said to be "off," meaning the resistance is high enough you can think of it as an open switch. Though, since transistors create large changes in resistance for small changes in voltage, they're more often used as amplifiers. A small signal is applied to the base pin of a transistor. This small signal creates large changes in resistance between the other two pins, which creates an amplified version of the same signal. You've probably heard of "transistor radios." The transistors here were used to amplify the signals from the antenna, which are extremely weak. They're also used in the de-modulation of the radio signal, and again to amplify the resulting radio signal. Modern devices use another type of transistor called a Field Effect Transistor (FET). FETs are similar, but are constructed in a completely different way.