Transistors are components in electronic devices that control and amplify the flow of electricity in the device and are considered one of the most important inventions in the development of modern electronics. Important transistor characteristics that affect how the transistor operates include the transistor's gain, structure, and polarity, as well as construction materials. Transistor characteristics can vary widely according to the transistor's purpose.
Transistors are useful because they can use a small amount of electricity as a signal to control the flow of much larger amounts. The transistor's ability to do this is called the transistor's gain, which is measured as the ratio of the output the transistor produces to the input required to produce that output. The higher the output relative to input, the higher the gain. This ratio can be measured in terms of the electricity's power, voltage, or current. Gain decreases as operating frequency rises.
Transistor characteristics vary according to the transistor's composition. Common materials include the semiconductors silicon, germanium, and gallium arsenide (GaAs). Gallium arsenide is often used for transistors that operate at high frequencies because its electron mobility, the speed at which electrons move through the semiconductor material, is higher. It can also safely operate at higher temperatures in silicon or germanium transistors. Silicon has lower electron mobility than the other transistor materials, but is commonly used because silicon is inexpensive and can operate at higher temperatures than germanium.
One of the most important transistor characteristics is the transistor's design. A bipolar junction transistor (BJT) has three terminals called the base, collector, and emitter, with the base lying between the collector and emitter. Small amounts of electricity move from the base to the emitter, and the small change in voltage causes much larger changes in the flow of electricity between the emitter and collector layers. BJTs are called bipolar because they use both negatively charged electrons and positively charged electron holes as charge carriers.
In a field-effect transistor (FET), only one type of charge carrier is used. Every FET has three semiconductor layers called the gate, drain, and source, which are analogous to BJTs base, collector, and emitter, respectively. Most FETs also have a fourth terminal referred to as the body, bulk, base, or substrate. Whether an FET uses electrons or electron holes to carry charges depends on the composition of the different semiconductor layers.
Each semiconductor terminal in a transistor can have positive or negative polarity, depending on what substances the transistor's main semiconducting material has been doped with. In N-type doping, small impurities of arsenic or phosphorous are added. Each atom of the dopant has five electrons in its outer shell. The outer shell of each silicon atom has only four electrons, and so each arsenic or phosphorous atom provides an excess electron that can move through the semiconductor, giving it a negative charge. In P-type doping, gallium or boron, both of which have three electrons in their outer shell, are used instead. This gives the fourth electron in the outer shell of the silicon atoms nothing to bond with, producing corresponding positive charge carriers called electron holes into which electrons can move.
Transistors are also classified according to the polarity of their components. In NPN transistors, the middle terminal—the base in BJTs, the gate in FETs—has positive polarity, while the two layers to either side of it are negative. In a PNP transistor, the opposite is the case.