Explain MOSFET operation and the difference between enhancement-mode and depletion-mode devices.

• A. MOSFET uses gate to modulate channel; Enhancement-mode requires Vgs above threshold to conduct; Depletion-mode conducts with zero gate bias and decreases with negative Vgs (for n-channel).
• B. MOSFET uses drain to modulate channel; Enhancement-mode requires Vds above threshold.
• C. Enhancement-mode conducts without any gate bias.
• D. Depletion-mode never conducts at zero gate bias.

Answer: (A)

The main idea being tested is how the gate voltage controls the channel in a MOSFET and how enhancement and depletion types differ in their default conduction state.

A MOSFET controls current by forming or modulating a channel between source and drain with the electric field from the gate. In an n-channel device, applying a positive gate-to-source voltage creates an inversion channel that allows electrons to flow from drain to source when a drain-source voltage is present. In enhancement-mode devices, there is no channel at zero gate bias, so the device is off until Vgs rises above a threshold value, at which point the channel forms and conduction begins. In depletion-mode devices, a channel already exists at zero gate bias, so the device conducts without any gate voltage; applying a negative gate voltage reduces the channel charge and can pinch off the current, while applying a positive gate voltage can enhance conduction. Keep in mind the signs swap for p-channel devices, but the same ideas apply with opposite polarity.

So the correct description emphasizes that the gate modulates (not the drain), enhancement-mode requires gate bias above threshold to conduct, and depletion-mode conducts with zero bias and is reduced by negative bias (for an n-channel).