MOSFET (metal-oxide semiconducting field-effect transistor) (A-level only) — AQA A-Level Physics

This topic covers the structure, operation and applications of the metal-oxide-semiconductor field-effect transistor (MOSFET), a voltage-controlled device central to modern electronics. You will study the n-channel enhancement-mode MOSFET, learning how the gate voltage creates an electric field that induces a conducting channel between source and drain, and how this controls the drain current. The transfer characteristic (I_D against V_GS) and the output characteristic (I_D against V_DS) are key graphs that reveal threshold voltage, saturation and transconductance.

Understanding the MOSFET is essential because it underpins digital logic, memory and amplification in almost all electronic devices. In the AQA A-Level Physics specification, it connects to earlier work on electrical circuits, resistance and potential dividers, and extends into operational amplifiers and logic gates. Exam questions often require you to interpret characteristic curves, calculate voltage gain, or explain switching behaviour, making it a high-value topic for both problem-solving and extended writing.

• →The MOSFET is a voltage-controlled device: the gate-source voltage V_GS controls the drain current I_D via an electric field across the insulating oxide layer.
• →An n-channel enhancement-mode MOSFET requires a positive gate-source voltage above the threshold voltage V_th to induce a conducting channel between source and drain.
• →The transfer characteristic (I_D vs V_GS) shows a threshold voltage and a linear region where transconductance g_m = ΔI_D / ΔV_GS is constant.
• →In the saturation region, the drain current becomes almost independent of V_DS, allowing the MOSFET to act as a current source or amplifier.
• →As a switch, the MOSFET is OFF (high resistance) when V_GS < V_th and ON (low resistance) when V_GS > V_th, making it ideal for digital logic.

• 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).
• Names the three terminals as gate, source and drain and links each to its function in the device.
• States that the gate is the control terminal and is insulated from the channel by an oxide layer.
• States that the source is the terminal through which carriers enter the channel and the drain is the terminal that collects them.
• Uses the gate-source voltage as the controlling input and the drain-source path as the output current path.
• Interprets the arrow on the source terminal as showing the direction of conventional current for the device type.
• Applies terminal identification when connecting a MOSFET in a circuit as a switch or amplifier.
• 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.

• 💡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.
• 💡Label terminal names directly on any circuit or cross-section diagram you draw.
• 💡State the carrier type and then give the conventional current direction to avoid ambiguity.
• 💡Link each terminal to its function in one short clause rather than listing names alone.
• 💡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.
• 💡Always state that the MOSFET is voltage-controlled and that the gate is insulated; this distinguishes it from a BJT and is often worth a mark.
• 💡When sketching or interpreting transfer characteristics, label the threshold voltage and the axes clearly, and use the gradient to find transconductance with correct units (A V^-1 or S).
• 💡For switching questions, explicitly link the logic states (0 and 1) to the gate voltage relative to the threshold voltage, and mention the high and low output resistances.

• Treating the gate as a terminal that passes a large steady current; correct this by stressing that the oxide insulator makes gate current negligible.
• Assuming a conducting channel exists at zero gate-source voltage; correct this 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; correct this by linking terminal names to the direction of conventional current and carrier type.
• Labelling the gate as the output terminal; correct this by identifying the drain-source path as the output current path and the gate as the control input.
• Reversing source and drain in an n-channel circuit; correct this by tracking electron flow from source to drain and conventional current from drain to source.
• Ignoring the oxide layer when describing the gate; correct this by stating that the insulation gives the gate its very high input resistance.
• Treating the gate as a forward-biased junction that draws steady current; the correction is that 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; the correction is that an enhancement MOSFET requires a gate-source voltage above threshold to form its channel.
• Students often think the gate current controls the drain current, but the gate is insulated by an oxide layer so virtually no current flows; control is by the electric field due to the gate voltage.
• Many confuse the MOSFET with a bipolar junction transistor (BJT), incorrectly stating that it is current-controlled. The MOSFET is voltage-controlled and has a much higher input impedance.
• Students sometimes believe that the drain current increases indefinitely with drain-source voltage, but in saturation the current levels off and is primarily set by V_GS.

1. 1Day 1-2: Learn the structure of an n-channel enhancement-mode MOSFET and the function of each terminal (gate, source, drain). Draw and label a cross-section diagram.
2. 2Day 3-4: Study the formation of the induced channel and how V_GS controls I_D. Watch animations or simulations to visualise the electric field effect.
3. 3Day 5-6: Master the transfer and output characteristics. Practice sketching them and extracting V_th and g_m from graphs.
4. 4Day 7-8: Work through calculations of transconductance and voltage gain in common-source amplifiers. Use past-paper questions to apply the formulas.
5. 5Day 9-10: Revise switching applications and compare MOSFETs with BJTs. Create a summary table of differences and test yourself with active recall questions.

• 📋Explanation questions: Describe how the gate voltage controls the drain current in an n-channel enhancement-mode MOSFET. Advice: use precise terms like 'electric field', 'induced channel', and 'threshold voltage'.
• 📋Graph interpretation: Given a transfer characteristic, determine the threshold voltage and calculate the transconductance. Advice: read the axes carefully, use a large triangle for gradient, and include units.
• 📋Calculation questions: Calculate the voltage gain of a common-source amplifier using g_m and R_L. Advice: remember the negative sign for phase inversion and that gain is a ratio (no units).
• 📋Application questions: Explain how a MOSFET can be used as a switch in a digital circuit. Advice: clearly state the OFF and ON conditions with reference to V_th and the resulting resistance states.