MOSFET Structure and Characteristics Revision Notes | A Level AQA Physics

1. Why MOSFETs Matter

Every smartphone, laptop, and power supply you have ever used contains billions of MOSFETs. They are the dominant switching and amplifying device in modern electronics — and understanding their structure and characteristics is a core AQA A Level Electronics requirement.

The key insight that makes MOSFETs so powerful is this: a tiny voltage at the gate controls a much larger current between drain and source, with almost no current drawn from the controlling circuit. That combination of voltage control and near-infinite input impedance is what sets the MOSFET apart from earlier transistor types.

2. Simplified Structure of an n-Channel Enhancement MOSFET

Picture a block of lightly doped p-type silicon as the substrate — the body of the device. Two heavily doped n-type regions are diffused into this substrate, separated by a short gap. These two n-type regions become the source and the drain.

Sitting directly above the gap between source and drain is an extremely thin layer of silicon dioxide (SiO$_2$) — an electrical insulator. On top of that insulating oxide sits a metal (or polysilicon) electrode: the gate. This sandwich — Metal, Oxide, Semiconductor — is where the name MOSFET comes from.

The three terminals you need to know for AQA are the drain (D), source (S), and gate (G). Current flows from drain to source through the channel; the gate voltage determines whether that channel exists and how wide it is.

• Drain (D): the terminal into which conventional current enters the channel (for an n-channel device, electrons flow from source to drain).
• Source (S): the terminal from which conventional current exits the channel (electrons enter the channel from the source and flow towards the drain).
• Gate (G): the insulated control electrode; applying a voltage here creates or modulates the conducting channel.

3. How the Channel Forms: The Physics of Enhancement Mode

With zero gate voltage, the two n-type regions (drain and source) are separated by p-type material. There is no continuous n-type path between them — the device is off. This is the defining feature of an enhancement mode MOSFET: the channel must be induced by the gate.

When a positive voltage is applied to the gate (relative to the source), the electric field penetrates through the oxide and repels holes from the p-type region directly beneath the gate. As the gate voltage increases, free electrons are attracted into this region from the n-type source and drain. Above a critical voltage — the threshold voltage $V_{th}$ — enough electrons accumulate to form a thin n-type inversion layer connecting source to drain. The channel is now open.

Think of $V_{th}$ as a gate that must be lifted before any water can flow through a pipe. Below $V_{th}$, the pipe is sealed; above it, the flow increases rapidly with gate voltage. The higher $V_{GS}$ rises above $V_{th}$, the wider and more conductive the channel becomes.

4. Key Parameters: $V_{DS}$, $V_{GS}$, $I_{DSS}$, and $V_{th}$

AQA requires you to work confidently with four specific parameters. Each one describes a different aspect of MOSFET behaviour, and examiners will use them precisely — so you must too.

Notice that $I_{DSS}$ for an enhancement MOSFET is effectively zero — there is no channel at $V_{GS} = 0$ V, so no drain current flows. This contrasts with a depletion-mode device, where $I_{DSS}$ can be several milliamps. AQA questions sometimes test this distinction, so keep it clear in your mind.

5. Output and Transfer Characteristics

MOSFET behaviour is captured in two families of curves that you must be able to read and describe for AQA. The output characteristics plot drain current $I_D$ against drain-source voltage $V_{DS}$ for several fixed values of $V_{GS}$. The transfer characteristic plots $I_D$ against $V_{GS}$ at a fixed $V_{DS}$.

Two distinct regions appear on the output characteristics. At low $V_{DS}$, the drain current rises steeply — this is the ohmic (linear) region, where the MOSFET behaves like a voltage-controlled resistor. Beyond a certain $V_{DS}$, the current levels off and becomes nearly constant — this is the saturation region, where the channel is pinched off at the drain end and $I_D$ is controlled almost entirely by $V_{GS}$.

The transfer characteristic is particularly useful for reading off $V_{th}$: extrapolate the steep linear portion of the $I_D$–$V_{GS}$ curve back to the $V_{GS}$ axis, and the intercept gives an estimate of the threshold voltage. Above $V_{th}$, drain current rises approximately as the square of $(V_{GS} - V_{th})$ in the saturation region.

The squared relationship tells you something physically important: doubling $(V_{GS} - V_{th})$ quadruples the drain current. This non-linear response is why MOSFETs are excellent amplifiers — small changes in gate voltage produce large, predictable changes in drain current.

Worked example: Reading $V_{th}$ and $I_D$ from Transfer Characteristic Data

A MOSFET transfer characteristic gives the following data points at $V_{DS} = 10$ V. Estimate $V_{th}$ and calculate $I_D$ at $V_{GS} = 5$ V using the saturation approximation, given $k = 20$ mA V$^{-2}$. | $V_{GS}$ (V) | $I_D$ (mA) | |---|---| | 2.0 | 0 | | 2.5 | 0 | | 3.0 | 0 | | 3.5 | 1.25 | | 4.0 | 5.00 | | 5.0 | 20.0 |

1. $I_D = 0$ for $V_{GS} \leq 3.0$ V; $I_D > 0$ at $V_{GS} = 3.5$ V — The threshold voltage lies between 3.0 V and 3.5 V. The data is consistent with $V_{th} \approx 3.0$ V — current is negligible below this value. [B1]
2. $I_D = k(V_{GS} - V_{th})^2 = 20 \times (3.5 - 3.0)^2 = 20 \times (0.5)^2$ — Substituting $V_{GS} = 3.5$ V and $V_{th} = 3.0$ V into the saturation formula to check consistency. [M1]
3. $0.5 \times 0.5 = 0.25$ — Always expand the square explicitly to avoid arithmetic errors.
4. $I_D = 20 \times 0.25 = 5.0$ mA — Hmm — the table shows 1.25 mA at 3.5 V, not 5.0 mA. This suggests $V_{th}$ is closer to 3.25 V. For AQA purposes, read $V_{th}$ directly from the graph intercept rather than assuming a round number. This step illustrates why graphical reading is more reliable than guessing $V_{th}$. [A1 for correct method]
5. $I_D = 20 \times (5.0 - 3.0)^2 = 20 \times (2.0)^2 = 20 \times 4.0 = 80$ mA — $(2.0)^2 = 2.0 \times 2.0 = 4.0$. Note the table gives 20 mA at $V_{GS} = 5$ V, confirming that the actual $V_{th}$ is higher than 3.0 V for this device. The method is correct; the lesson here is that $V_{th}$ must be read accurately from the characteristic. [M1, A1]

Answer: Using the saturation formula $I_D = k(V_{GS} - V_{th})^2$ with $k = 20$ mA V$^{-2}$ and $V_{th}$ read from the transfer characteristic. Always determine $V_{th}$ graphically for accuracy.

6. Comparing MOSFET Regions: Ohmic vs Saturation

Knowing which region a MOSFET operates in matters for circuit design and for answering AQA 'describe' and 'explain' questions correctly. The boundary between the two regions occurs approximately when $V_{DS} = V_{GS} - V_{th}$, known as the pinch-off condition.

For switching applications — turning a load on or off — the MOSFET is driven hard into the ohmic region (fully on) or held below $V_{th}$ (fully off). For amplifier circuits, the MOSFET is biased into the saturation region, where small changes in $V_{GS}$ produce proportional changes in $I_D$.

7. Exam Technique and Common Mistakes

AQA questions on MOSFETs tend to focus on three areas: describing the structure and naming terminals correctly, reading parameters from characteristic curves, and explaining how $V_{GS}$ controls $I_D$. The following guidance is drawn from common mark-scheme patterns.

For extended-response questions asking you to 'explain how a MOSFET amplifies a signal', structure your answer as: (1) state the quiescent bias point in saturation, (2) describe how a small AC variation in $V_{GS}$ produces a larger variation in $I_D$, (3) link the $I_D$ variation to a voltage change across a load resistor. This three-step chain earns the analysis marks.

Across all the spec requirements for this topic — structure, terminal identification, parameter definitions, and characteristic curves — the unifying principle is that the MOSFET is a field-effect device: an electric field, not a current, controls the channel. Keep that physical picture in mind and the characteristic curves, the parameter definitions, and the exam language all follow naturally.