MOSFET Applications: Switch and High Input Resistance Device Revision Notes | A Level AQA Physics

1. Why MOSFETs Matter: The Voltage-Controlled Switch

Picture a light switch on your wall. You apply a small mechanical force and a large current flows to power a lamp. A MOSFET does the same job electronically: a small voltage at one terminal controls whether a much larger current flows through the device. That single idea underpins almost every digital circuit, power controller, and signal amplifier in modern electronics.

What makes the MOSFET particularly powerful is that its control terminal — the gate — draws virtually no current from the circuit driving it. This is the high input resistance property, and it has profound consequences for circuit design. Before unpacking why, it helps to understand exactly what kind of MOSFET the AQA specification requires you to know.

2. The N-Channel Enhancement Mode MOSFET: Structure and Behaviour

The AQA specification requires knowledge of the N-channel, enhancement mode MOSFET only. Enhancement mode means the device is normally off: with zero gate-source voltage ($V_{GS} = 0$), no conducting channel exists between drain and source, and no drain current flows. You have to actively enhance the channel into existence by applying a positive gate voltage.

The three terminals are the gate (G), drain (D), and source (S). The gate is separated from the semiconductor body by an extremely thin layer of silicon dioxide — an insulator. This is the physical reason for the enormous input resistance: charge cannot flow through an insulator, so the gate draws negligible current from whatever is driving it.

Here is the key sequence to hold in mind. When $V_{GS}$ is below the threshold voltage $V_{GS(th)}$, the channel does not form and the MOSFET is off — drain current $I_D \approx 0$. When $V_{GS}$ exceeds $V_{GS(th)}$, a conducting N-type channel forms between drain and source, and $I_D$ can flow. The larger $V_{GS}$ is above the threshold, the lower the channel resistance and the more current can flow.

This on/off behaviour is exactly what makes the MOSFET so useful as a switch. The next section examines that switching application in detail, including what the circuit looks like and how to reason about each state.

3. MOSFET as a Switch: Circuit Operation

In a switching circuit, the MOSFET is driven between two extremes: fully off and fully on. There is no intention to operate it in the linear region between these states. Think of it like a relay — either the contacts are open (off) or closed (on) — but with no moving parts and switching times measured in nanoseconds.

A typical circuit places a load (for example a resistor, motor, or LED) in series with the drain, with the supply voltage $V_{DD}$ connected to the top of the load. The source is connected to the 0 V rail. The gate voltage $V_{GS}$ is the control input. When $V_{GS}$ is low (below $V_{GS(th)}$), the MOSFET is off: no current flows through the load, and the full supply voltage appears across the drain-source terminals. When $V_{GS}$ is driven high (above $V_{GS(th)}$), the MOSFET turns on: current flows through the load, and the drain-source voltage drops to a very small value (ideally 0 V for a perfect switch).

• OFF state: $V_{GS} < V_{GS(th)}$, $I_D \approx 0$, $V_{DS} \approx V_{DD}$ (all supply voltage across the MOSFET)
• ON state: $V_{GS} > V_{GS(th)}$, $I_D$ determined by load, $V_{DS} \approx 0$ V (MOSFET acts like a closed switch)
• Gate current $\approx 0$ in both states due to the insulating oxide layer

Worked example: Analysing a MOSFET Switch Circuit

An N-channel enhancement mode MOSFET is connected in series with a 100 $\Omega$ load resistor and a 12 V supply. The threshold voltage is 3 V. The gate is driven to 5 V. Determine: (a) whether the MOSFET is on or off, (b) the approximate drain current, and (c) the voltage across the load resistor.

1. $V_{GS} = 5\text{ V}$, $V_{GS(th)} = 3\text{ V}$ $V_{GS} > V_{GS(th)}$ — Because the applied gate voltage exceeds the threshold, the conducting channel forms and the MOSFET is ON. [B1]
2. $V_{DS} \approx 0\text{ V}$ (MOSFET fully on, treated as closed switch) — In the fully on state, the MOSFET's drain-source resistance is very small, so we approximate $V_{DS} = 0$ V for this analysis. [M1]
3. $V_{load} = V_{DD} - V_{DS} = 12\text{ V} - 0\text{ V} = 12\text{ V}$ — All of the supply voltage appears across the load when the MOSFET is fully on. [M1]
4. $I_D = \frac{V_{load}}{R} = \frac{12\text{ V}}{100\text{ }\Omega} = 0.12\text{ A}$ — Current through the series circuit equals voltage across the load divided by load resistance. [A1]
5. If MOSFET were off: $I_D = 0$, $V_{load} = 0$ V, $V_{DS} = 12$ V. Here $I_D = 0.12$ A, consistent with ON state. — Checking the off-state prediction against the on-state result confirms the analysis is internally consistent.

Answer: (a) MOSFET is ON. (b) $I_D = 0.12$ A. (c) $V_{load} = 12$ V.

Notice that throughout the switching analysis, the gate drew no current from the driving circuit. That is not an approximation made for convenience — it is a direct consequence of the insulating oxide layer. This property deserves its own section, because it changes what is possible in circuit design.

4. Very High Input Resistance: What It Means and Why It Matters

The input resistance of a device is the resistance seen looking into its input terminals. For a MOSFET, the input is the gate-source terminal pair. Because the gate is separated from the channel by silicon dioxide — a near-perfect insulator — the gate input resistance is typically greater than $10^9$ $\Omega$, and often quoted as $10^{12}$ $\Omega$ or higher in data sheets.

To appreciate why this matters, compare it with a bipolar junction transistor (BJT). A BJT is current-controlled: to keep it on, the base must be supplied with a continuous base current. That base current is drawn from the driving circuit, which must be capable of supplying it. A MOSFET is voltage-controlled: once the gate is charged to the required voltage, virtually no current is needed to maintain that state. The driving circuit is not loaded by the MOSFET input at all.

The practical consequence is significant. A sensor producing a very small signal — for example a microphone, a photodiode, or a pH electrode — can drive a MOSFET gate directly without the signal being distorted or attenuated by the input resistance of the amplifying device. The MOSFET does not steal current from the sensor. This is why MOSFETs are the preferred choice wherever the source signal has high output impedance or very limited current capability.

There is one subtlety worth noting. Although steady-state gate current is negligible, the gate does have a small capacitance. When the gate voltage changes rapidly — as it does in high-speed switching — a brief charging current flows to change the voltage on that capacitance. This is a dynamic effect, not a steady-state one, and does not undermine the high input resistance property in normal operation.

5. Connecting the Two Applications: Switch and High Input Resistance

The switching application and the high input resistance property are not separate topics — they are two consequences of the same physical structure. The insulating gate oxide is what gives the MOSFET its high input resistance, and that same structure allows the gate voltage to control the channel without drawing current. Both applications exploit this.

When used as a switch, the high input resistance means the logic gate or microcontroller driving the MOSFET gate does not need to source significant current. A standard logic output can switch a MOSFET controlling a motor drawing several amperes — the logic circuit only needs to charge the gate capacitance, which requires minimal energy. This is why MOSFETs dominate in digital power control, motor drivers, and switched-mode power supplies.

When used as a high input resistance device — for example at the input stage of a voltmeter or an instrumentation amplifier — the MOSFET ensures that the measuring instrument does not disturb the circuit being measured. A voltmeter with very high input resistance draws negligible current from the circuit under test, so the voltage it reads is the true circuit voltage rather than a loaded-down version of it.

6. Exam Technique: Scoring Full Marks on MOSFET Questions

AQA questions on MOSFETs tend to fall into a small number of patterns. Recognising them early lets you structure your answer efficiently. The most common patterns are: describe the switching action for a given $V_{GS}$; explain why the input resistance is high; compare MOSFET and BJT behaviour; and analyse a circuit to find voltages or currents in each switching state.

For describe questions about switching, work through the states in order. State whether $V_{GS}$ is above or below the threshold, state the consequence for the channel, then state the consequence for drain current and drain-source voltage. Examiners award marks for each logical step, so do not skip from input to output without naming the channel formation.

For explain questions about high input resistance, the mark-winning chain is: gate insulated by oxide layer → negligible gate current → very high input resistance. Each arrow represents a mark. Students who write only "the gate is insulated" without connecting it to the consequence of negligible current often lose the second mark.

Bringing everything together: the N-channel enhancement mode MOSFET is a voltage-controlled switch whose gate draws negligible current due to its insulating oxide layer. These two properties — controllable switching and very high input resistance — make it the dominant active device in both digital electronics and precision analogue circuits, and both properties trace back to the same physical feature of the device structure.