JFET drain-current parameter IDSS: Which definition correctly describes IDSS for a JFET (shorted-gate drain current)?
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Athe minimum possible drain current
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Bthe maximum possible current with VGS held at –4 V
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Cthe maximum possible current with VGS held at 0 V
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Dthe maximum drain current with the source shorted
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Ethe drain current at the threshold voltage
Answer
Correct Answer: the maximum possible current with VGS held at 0 V
Explanation
Introduction / Context:IDSS is a key datasheet parameter for depletion-mode JFETs. It sets the upper bound of drain current when the gate-source PN junction is unbiased (VGS = 0 V). Understanding IDSS is essential for biasing, small-signal design, and interpreting spread in device characteristics.
Given Data / Assumptions:
- Depletion-mode n-channel JFET.
- VGS = 0 V (gate shorted to source) while sweeping VDS into the saturation region.
- Temperature/time effects ignored.
Concept / Approach:
By definition, IDSS is the drain current at VGS = 0 V with the device biased so that it operates in the constant-current (saturation) region. Any negative VGS reduces channel width and decreases drain current below IDSS; more negative VGS drives the device toward cutoff at VGS(off).
Step-by-Step Solution:
Set VGS = 0 V (short gate to source).Increase VDS until the device is in saturation (beyond pinch-off).Measure ID; this value is IDSS.Note that for VGS < 0, ID < IDSS; for VGS → VGS(off), ID → 0.Verification / Alternative check:
Device transfer characteristics (ID vs. VGS) show maximum current at VGS = 0 V; datasheets label this as IDSS. Curve tracers confirm this condition practically.
Why Other Options Are Wrong:
- Minimum current (option a) is cutoff, not IDSS.
- VGS = –4 V (option b) is arbitrary; any negative VGS reduces ID below IDSS.
- “Source shorted” (option d) is ambiguous; the standard definition is the gate shorted to source (VGS = 0).
- “At threshold” (option e) is MOSFET terminology; JFET uses VGS(off) rather than a positive threshold.
Common Pitfalls:
- Mixing MOSFET threshold concepts with JFET depletion behavior.
- Forgetting that IDSS varies widely between parts and is temperature dependent.
Final Answer:
the maximum possible current with VGS held at 0 V