Transistors are electronic devices used in all types of appliances and machines, which operate in all forms of business. They combine at least two semiconductors to benefit from the surface effects between their interface. They usually have three terminals. The current of one pair controls the current of the other pair of terminals. (The three-terminal setup works like this because one terminal is in common to the input and the output.)
There are dozens of different types of transistors. To explain any differences between them, semiconductors must first be explained. Semiconductors are insulating lattices usually made of silicon, with conducting atoms sprinkled in, "doping" or charging the insulator. The n-type semiconductors have an excess of electrons (the "n" stands for negative), while the p-type (p = positive) semiconductors have a deficiency of electrons. The amount of "excess" and "deficiency" describes how far the atom is from having a full outer electron shell, which is a highly stable configuration.
The Junction Transistor
A bipolar transistor, or bipolar junction transistor, combines three semiconductors: a thin "base" in between a "collector" and an "emitter," each with its own terminal. The order of semiconductor type can be n-p-n or p-n-p. In either case, there are two p-n junctions. The bipolar transistor is a current magnifier, often found in telecommunications equipment. A Darlington transistor combines two bipolar transistors into one device, providing high current gain with less space than if two actual bipolar transistors were combined.
The Field-Effect Transistor
The field-effect transistor (FET), by contrast, uses an electric field to determine the conductivity of a channel. There are over half a dozen types of FET. Three or four terminals are used, with one or two "gate" terminals connected to the conduction channel, and the other two connected to what are called the "drain" and the "source," analogs of the collector and emitter found in the junction transistor. A voltage is applied to the gates to impede the input current from traversing the conducting channel. A demonstration can be found at Concord.org.
The Metal-Oxide Semiconductor Field-Effect Transistor
MOSFET transistors are FETs that, being made of metal-oxide, are suitable for heavy-duty use, such as in radio transmitters, television monitors and stereo amplifiers.
The Silicon-Controlled Rectifier
Silicon-controlled rectifiers (SCR) contain four layers of alternating semiconductor: n-p-n-p. They are used to transform alternating current (AC) into direct current (DC), allowing current through when the AC is operating in one direction. This is the opposite function of a transformer, which blocks DC noise.