JUNCTION FIELD-EFFECT TRANSISTOR (JFET) TESTS
The junction field-effect transistor (JFET) has circuit applications similar to those of a vacuum tube. The
JFET has a voltage-responsive characteristic with a high input impedance. Two types of JFETs that you
should become familiar with are the junction p-channel and the junction n-channel types, as shown in
figure 2-16. Their equivalent circuits are shown in figures 2-17 and 2-18, respectively. The only difference
in your testing of these two types of JFETs involves the polarity of the meter leads.
(2SK) (2SJ)
Fig. 2-16
N-Channel Test
Using an ohmmeter set to the R 100 scale, measure the resistance between the drain and the source; then
reverse the ohmmeter leads and take another reading. Both readings should be equal (in the 100- to 10,000-
ohm range for small signal devices, and 1 ohm or less for large power devices such as VFET’s), regardless
of the meter lead polarity. Switch the meter to read for a diode test, and connect the positive meter lead to
the gate. Using the negative lead, measure the voltage between the gate and the drain; then measure the
voltage between the gate and the source. Both readings should indicate a diode drop plus a bit more to
account for the substrate resistance of the drain or source, and be approximately the same. Disconnect the
positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the
voltage between the gate to the drain; then measure the voltage between the gate and the source. Both
readings should show infinity, or an open circuit depending on how your meter functions.
Fig. 2-17 N-Channel FET
P-Channel Test
Using an ohmmeter set to the R 100 scale, measure the resistance between the drain and the source; then
reverse the ohmmeter leads and take another reading. Both readings should be the same (in the 100- to
10,000-ohm range for small signal devices, and 1 ohm or less for large power devices such as VFET’s),
regardless of meter lead polarity. Switch the meter to read for a diode test, and connect the negative meter
lead to the gate. Using the positive lead, measure the voltage between the gate and the drain; then measure
the voltage between the gate and the source. Both readings should indicate a diode drop plus a bit more to
account for the substrate resistance of the drain or source, and be approximately the same. Disconnect the
negative lead from the gate and connect the positive lead to the gate. Using the negative lead, measure the
voltage between the gate to the drain; then measure the voltage between the gate and the source. Both
readings should show infinity, or an open circuit depending on how your meter functions.
Fig. 2-18 P-Channel FET
