Field-effect transistor operation: The point at which a JFET channel just stops allowing further increase of controlled drain current with more negative VGS (i.e., the device can no longer control current) is called the:

Introduction / Context:
In a JFET, the gate–channel junction is reverse-biased, and controlling the channel width with VGS regulates drain current ID. A key operating point is “pinch-off,” where the depletion region expands enough to restrict the channel such that increasing VDS no longer significantly increases ID at a fixed VGS.

Given Data / Assumptions:

• Junction FET (JFET) device.
• Gate reverse-biased for control.
• We are naming the condition where the JFET ceases to control current growth in the usual linear sense.

Concept / Approach:
At pinch-off, the channel is constricted; the device enters the constant-current (saturation) region where ID ≈ IDSS * (1 − VGS/VP)^2 for an n-channel device (qualitative). The term “pinch-off” specifically refers to the VGS and VDS condition where channel cross-section effectively pinches.

Step-by-Step Solution:
Start from ohmic region at small VDS.Increase VDS at fixed VGS until the channel narrows enough → pinch-off.Beyond that, ID is roughly constant; further VDS increases drop across the pinch-off region.

Verification / Alternative check:
Look at the JFET output characteristics ID vs VDS at several VGS; you will see flat “saturation” plateaus beginning at pinch-off.

Why Other Options Are Wrong:
Breakdown is an excessive reverse-bias phenomenon.

“Depletion region” is a structure, not the operating point name.

“Saturation point” is a broader term; the JFET’s entry into saturation is defined by pinch-off.

Common Pitfalls:
Confusing MOSFET and JFET terminology; for JFETs, pinch-off defines entry to the constant-current region.

Final Answer:
pinch-off region