Transistors are made of three layers, each capable of conducting electricity. These layers consist of a semiconductor material like silicon or germanium. N-type silicon, containing phosphorus impurities, gives a negative charge. P-type silicon with boron impurities added, results in a positive charge. Electrons flow from layer to layer in the transistor, increasing in intensity from input to output, producing a gain in current.
Transistors come in unipolar and bipolar configurations.
The middle layer is known as the base, while one outer layer is called the emitter, and the other the collector. In FETs, the middle layer is known as the channel or gate, while the outer regions are called the source and drain. Get a detailed history on the development of the transistor from Lucent Technologies.