JFET drain-current parameter IDSS: Which definition correctly describes IDSS for a JFET (shorted-gate drain current)?

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:

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