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