User manual
m) Junction FETs are conventional eld-effect
transistors
The voltage at the gate-source terminal controls the current be-
tween the drain and source terminals. N-channel JFETs need a
negative voltage at their gate with regard to their source. As the
voltage becomes morenegative,less current canowbetween
the drain and the source.
In contrast to the depletion mode MOS
-
FETs, JFETs have no insulating layer
on the gate. This means that the input
resistance between gate and source is
normally very high, but the gate current
can increase if the semiconductor junc-
tion between gate and source or between
gate and drain is directed forward. This
can happen if the gate voltage is about
0.6 V higher than the drain or source pins
for N-channel devices or 0.6 V lower than
the drain or source pins for P-channel
devices.
The internal structure of JFETs is essen-
tially symmetrical around the gate termi-
nal, i.e., the tester cannot differentiate be-
tween the drain and source pin. However,
the JFET type and the gate terminal are
identied.
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