Simplified structure, behaviour and characteristics.

The idea

A MOSFET is a voltage-controlled device built on a semiconductor substrate. In the simplified enhancement n-channel picture, two n-type regions are separated by a p-type channel gap, with a thin insulating oxide layer and a metal gate above. With zero gate-source voltage no conducting channel exists, so the device is off. Raising the gate-source voltage above the threshold voltage attracts electrons to the surface, forming an n-type inversion channel that connects the two regions and allows current to flow. The gate draws almost no steady current because the oxide insulates it, giving very high input resistance. Increasing the gate-source voltage increases channel conductivity, so a small input voltage controls a larger drain current. This makes the MOSFET useful as a voltage amplifier and as a fast electronic switch.

What earns the marks

• Identifies the three terminals and the role of each: gate controls conduction, source and drain are the main current-carrying terminals.
• Describes the insulating oxide layer between gate and channel, explaining the very high input resistance and negligible steady gate current.
• Explains enhancement behaviour: no channel at zero gate-source voltage, and channel formation once the gate-source voltage exceeds the threshold voltage.
• Relates gate-source voltage to channel width and conductivity, so drain current increases as gate-source voltage increases.
• Distinguishes voltage-controlled operation from current-controlled operation in a bipolar transistor.
• Applies the device as a switch (off below threshold, on above it) or as an amplifier (small gate signal controlling a larger drain current).

Common mistakes and how to put them right

• Treating the gate as a terminal that passes a large steady current.

  Put it right by stressing that the oxide insulator makes gate current negligible.

• Assuming a conducting channel exists at zero gate-source voltage.

  Put it right by stating that an enhancement MOSFET needs a gate-source voltage above threshold.

• Confusing the source and drain roles or reversing the polarity needed for an n-channel device.

  Put it right by linking terminal names to the direction of conventional current and carrier type.

Exam tips

• Sketch a labelled cross-section showing substrate, oxide, gate, source and drain before writing any explanation.
• Use the phrase voltage-controlled in answers and contrast it explicitly with current-controlled bipolar operation.
• When describing switching, state the threshold condition and what happens to drain current on each side of it.