JFET

Unlike the Bipolar junction transistor(BJT) the junction
field effect transistor is an unipolar transistor. This
means that the conducting area only have one polarity, either
positive or negative. Like the BJT it has three electrodes but
they have different names. The controlling electrode, that on
BJTs is called base, is in the JFET called gate. The equivalent
of the collector is called drain and the emitter is called
source.

schematic buildup of an N-canal JFET 

            
                 
     _____.---._____ 
 D  |     \_P_/     |   S
 ___|   N canal N   |___
    |      ___      | 
    |_____/ P \_____|
          `-+-'  
            |    
            G            

D  Drain
S  Source
G  Gate
P  Material with a positive doping(both the P
   areas is connected to the GATE electrode. 
N  Negatively doped material

The P-canal version has the same buildup, but
with a positive canal and negative control electrodes

The junction between the P and the N areas act as a diode. When a negative voltage is connected to the Gate, an electric field will be created in the junction. The higher the negative potential of the gate is, the further this field will extend out in the N-canal. This field prevents the flow of electrons from the Source to the Drain, and the stronger it is, the fewer electrons will be able to flow through(The P-canal JFET uses a positive control voltage to prevent the flow off positive charge carriers ). This also means that the JFET is voltage controlled, rather than current controlled like the BJT. The JFET often has a lower resistance when conducting than the BJT and a higher amplification. But JFETs are more expensive than the BJT, especialy high effect versions. There also exists another set of FET types that uses MOS technology, these are called MOSFETs.

Symbol

       N-canal      P-Canal

             D           D
            |           |
            |           |
        |___|       |___|
        |           |
        |           |
        |_\_        |_/_
   G  __| / |   G __| \ |
            |           |
            |           |
             S           S