2-27
Figure 2-22.—MOSFET (enhancement type) equivalent circuit.
Q-17.
Why is it not advisable to use a solder sucker when working on MOSFETs?
MOSFET (Depletion/Enhancement Type) Test
Using an ohmmeter set to the R 100 scale, measure the resistance between the MOSFET drain and
the source; then reverse the ohmmeter leads and take another reading. The readings should be equal,
regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the
negative lead, measure the resistance between the gate and the drain and between the gate and the source.
Both readings should show infinity. Disconnect the positive lead from the gate and connect the negative
lead to the gate. Using the positive lead, measure the resistance between the gate and the drain; then
measure it between the gate and the source. Both readings should show infinity. Disconnect the negative
lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance between
the substrate and the drain and between the substrate and the source. Both of these readings should
indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the
substrate. Using the negative lead, measure the resistance between the substrate and the drain and
between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms).
MOSFET (Enhancement Type) Test
Using an ohmmeter set to the R 100 scale, measure the resistance between the drain and the
source; then reverse the leads and take another reading between the drain and the source. Both readings
should show infinity, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the
gate. Using the negative lead, measure the resistance between the gate and the drain and then between the
gate and the source. Both readings should indicate infinity. Disconnect the positive lead from the gate and
connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and
the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the
negative lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance
between the substrate and the drain and between the substrate and the source. Both readings should
indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the
substrate. Using the negative lead, measure the resistance between the substrate and the drain and
between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms).
