Field Effect Transistor

A Field Effect is a type of Transistor.  It is different from other type of the transistor, the Bipolar junction Transistor (PNP and NPN) in that there is only only junction.  Field Effect Transistors are unipolar transistors.  

A Field Effect Transistor consists of a Channel which conducts the Current.  One one end of the Channel is the Source and on the other is the Drain.  The Channel is made of N-type material.   Between the Source and the drain is a substrate of P-type material.    A terminal called the Gate is connected toe P-type material.   

The current from the source and the drain can be controlled by the voltage applied at the Gate.  This is the basic principle behind the Field Effect Transistor.When a negative voltage is applied at the P-type material, the electrons which are the majority charge carriers in the substrate are repelled, the depletion channel at the junction between the gate and the substrate increases, this decreasesthe available area for the electrons to pass through.  The current flow is thus reduced.  Thus, the current can be controlled by the gate voltage.

Reducing the negative gate voltage will cause the current flow to increase.  

Care should be taken that the gate voltage does not become positive with respect to the source.  This would reduce the depletion layer and cause current to flow between the substrate and the channel resulting in heating and failure of the Field Effect Transistor.