Use in N-channel, enhancement mode only is required.

The idea

The specification restricts MOSFET work to the N-channel enhancement type, so you must recognise its structure and explain why it needs a positive gate-source voltage to conduct. In an N-channel device the source and drain are n-type regions in a p-type substrate; a thin insulating oxide layer separates the gate. With V_GS below the threshold voltage no conducting channel exists, so drain current is negligible. Raising V_GS above threshold attracts electrons to the surface, forming an induced n-channel that connects source and drain, and I_D then rises with V_DS. You should be able to describe this in terms of field effect and charge carriers, sketch the transfer characteristic, and use the device as a voltage-controlled switch or amplifier.

What earns the marks

• Identifies the source and drain as n-type regions formed within a p-type substrate, with a metal or polysilicon gate insulated by a thin oxide layer.
• Explains that a positive gate-source voltage above the threshold value induces an n-type conducting channel between source and drain.
• States that below threshold no channel exists, so the drain current is effectively zero and the device is off.
• Describes the MOSFET as a voltage-controlled device because the gate voltage, not a gate current, controls the drain current.
• Uses the transfer characteristic to show drain current rising once V_GS exceeds the threshold voltage.
• Applies the device as a switch, identifying the off state below threshold and the on state above it.

Common mistakes and how to put them right

• Treating the gate as a forward-biased junction that draws steady current.

  Put it right: The oxide insulates the gate, so gate current is negligible and control is by electric field.

• Assuming any positive gate voltage switches the device on; the correction is that conduction begins only above the threshold voltage.

• Confusing enhancement behaviour with depletion behaviour by expecting a channel at zero gate-source voltage.

  Put it right: An enhancement MOSFET requires a gate-source voltage above threshold to form its channel.

Exam tips

• Sketch the transfer characteristic with V_GS on the horizontal axis and I_D on the vertical axis, marking the threshold voltage clearly.
• Use the phrase voltage-controlled when contrasting the MOSFET with a bipolar transistor, and link it to the insulated gate.
• When describing switching, state the gate-source voltage condition for each state rather than saying simply on or off.