Enhancement-type MOSFET (E-MOSFET): Can it be turned on when the channel is already depleted, or does it require an induced channel via a threshold gate voltage?

Introduction / Context:Metal-oxide-semiconductor field-effect transistors (MOSFETs) come in depletion and enhancement varieties. Enhancement-type MOSFETs are normally off at VGS = 0 V and require a gate voltage exceeding a threshold to create (enhance) a conductive inversion channel. Misstating this behavior leads to biasing errors and device misuse.

Given Data / Assumptions:

• Enhancement MOSFET: no strong channel at VGS = 0 V.
• Turn-on occurs when VGS reaches or exceeds VTH (n-type positive, p-type negative).
• Depletion MOSFETs differ: they have a channel at VGS = 0 V and can be pinched off by opposite VGS polarity.
• Gate is insulated; gate current in normal operation is negligible.

Concept / Approach:For an E-MOSFET, conduction begins only after the gate-to-source voltage induces an inversion layer. Saying it turns on “when the channel is depleted” confuses the depletion device mechanism with enhancement behavior. An enhancement device cannot rely on a pre-existing channel; it creates one by field effect once |VGS| reaches VTH and grows with further |VGS| increases.

Step-by-Step Solution:

Verification / Alternative check:Look at ID–VGS transfer curves: for enhancement devices, ID ~ k*(VGS − VTH)^2 (simple square-law model) for VGS above VTH, and ~0 below threshold—reinforcing that an induced channel is required.

Why Other Options Are Wrong:

Common Pitfalls:Confusing depletion and enhancement types; assuming any MOSFET conducts at VGS = 0 V; ignoring threshold spread and temperature effects.

Final Answer:Incorrect