Field Effect Transistors
Part 1: Junction FET
junction FET takes advantage of the peculiarities of the PN junction
to make a more efficient transistor. The source and drain leads
are connected at either end of a layer of p-type or n-type semiconductor
(n-channel or p-channel). The gate which controls the flow of current
through the channel always reverse biased so that very little current
flows through the gate.
illustrations here show an N-channel JFET. To turn off the transistor,
a negative voltage is applied to the gate. The negative voltage
repels the free electrons in the N-channel, effectively shutting
off the flow of electrons from source to drain.
When the negative
voltage on the gate is removed, the electrons can flow unimpeded
towards the positive voltage on the drain. Although JFETs are far
more efficient bipolar transistors, the MOSFET
improves efficiency still further.
space space space space
transistors operate on the principle that like charges repel. A
negative gate voltage will repel electrons in n-type semiconductor,
producing a depletion zone and cutting off current flow.