Peak Atlas DCA Pro
User Guide
June 2019
–
Rev 1.7
Page 26
Junction FETs (JFETs)
Junction FETs are conventional Field Effect Transistors. Here
we refer to them as JFETs.
The voltage applied across the gate-source terminals controls current between
the drain and source terminals. N-Channel JFETs require a negative voltage on
their gate with respect to their source, the more negative the voltage, the less
current can flow between the drain and source.
Unlike Depletion Mode MOSFETs, JFETs have no insulation layer on the gate.
This means that although the input resistance between the gate and source is
normally very high (greater than 100M
Ω
), the gate current can rise if the
semiconductor junction between the gate and source or between the gate and
drain become forward biased. This can happen if the gate voltage becomes
about 0.6V higher than either the drain or source terminals for N-Channel
devices or 0.6V lower than the drain or source for P-Channel devices.
The internal structure of JFETs is
essentially symmetrical about the gate
terminal, this means that the drain and
source terminals of the JFET are often
indistinguishable by the
DCA Pro
. The
JFET type, gate terminal and measured parameters are displayed however.
Pinch-Off
A common parameter to be specified for JFETs is
“
Pinch-Off
”
. This is the
voltage needed between the gate-source
to turn off the JFET. The
DCA Pro
will
determine that the JFET is off when the
drain current is less than 5
μ
A.
