The field-effect transistor is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type operation of bipolar (junction) transistors (BJT). The concept of the FET predates the BJT, though it was not physically implemented until after BJTs due to the limitations of semiconductor materials and relative ease of manufacturing BJTs compared to FETs at the time.
Source: Wikipedia
URL: http://en.wikipedia.org/wiki/Field_Effect_Transistor