Difference between bipolar junction transistor amplifier and field effect transistor amplifier?

Bipolar transistors are so named because in which holes & electrones takes part in conduction.as opposed to unipolar transistors, such as field-effect transistors, in which only one type of Majority carrier takes part in conduction. FET Advantage over BJT FET requires vertually no input corrent and gives extreamly high input resistance.

FET's have a 4th source called a body and are a little bit more complicated the simple emitter, base, and collector of BJT's. they are more semiconductor and can't handle as much power as BJT.

http://en.wikipedia.org/wiki/Field_effect_transistor

http://en.wikipedia.org/wiki/Bipolar_junction_transistor