Device testing – n-channel D-MOSFET: An ohmmeter check shows these resistances (measured with device out of circuit): G–D = ∞, G–S = ∞, D–SS = ∞ or about 500 Ω depending on meter polarity, and D–S = about 500 Ω in both directions. Based on these readings, what fault (if any) is indicated?

Introduction / Context:
Technicians often perform quick ohmmeter checks on MOSFETs removed from a circuit. For an n-channel enhancement/depletion MOSFET with body/substrate tied to source (SS), the gate is insulated (very high resistance), and an intrinsic body diode exists between drain and source. Understanding expected readings helps identify shorts and opens.

Given Data / Assumptions:

• n-channel D-MOSFET, substrate shorted to source (SS).
• Measured out of circuit with a typical handheld ohmmeter.
• Readings: G–D = ∞, G–S = ∞, D–SS = ∞ or ~500 Ω depending on polarity, and D–S ≈ 500 Ω regardless of polarity.

Concept / Approach:
Ideal expectations: Gate to any terminal should read essentially infinite resistance (oxide insulation). Drain–Source should show a diode effect: low resistance one way (forward body diode) and very high the other. If D–S reads a similar low resistance in both directions, that suggests a bilateral conduction path, i.e., a short.

Step-by-Step Solution:
Check gate insulation: G–D = ∞ and G–S = ∞ → gate is not shorted.Check intrinsic diode: D–SS should read low in one polarity and ∞ in the other → this matches the body diode behavior.Evaluate D–S: reading ~500 Ω both directions indicates conduction both ways → inconsistent with a single diode and points to D–S short/fault.

Verification / Alternative check:
Reverse meter leads across D–S. A healthy device should show one low and one very high reading. Similar low readings both ways corroborate a shorted junction/structure.

Why Other Options Are Wrong:
Open G–D or open D–SS would not cause bilateral low D–S readings.

“Nothing” ignores the symmetrical low D–S behavior.

Gate oxide short would show low G–D or G–S, which we do not see.

Common Pitfalls:
Testing in-circuit can back-feed through other parts; always test with at least one lead lifted or the device removed.

Final Answer:
short D to S