The MOSFET

Nearly every modern circuit that switches or controls power — from a phone charger to a car's engine management system — leans on a tiny device that can turn a current on and off using nothing but a voltage. That device is the MOSFET, and understanding its structure is the starting point for this entire module.

A voltage-controlled semiconductor device

The MOSFET (metal-oxide-semiconductor field-effect transistor) is a semiconductor device whose conduction between two of its terminals is controlled by a voltage applied to a third. That single idea — control by voltage rather than by a continuous flow of current into a control terminal — is what sets it apart from many older device families, and it's the property that makes the MOSFET so useful throughout electronics.

MOSFETs come in several types, but this module deals with only one: the N-channel enhancement-mode MOSFET. Restricting the scope this way keeps the physical picture simple, and it also matches how the device is used in the vast majority of practical switching and interfacing circuits you'll meet later in this module.

Simplified structure: drain, source, gate

A MOSFET has three terminals:

• Drain (D) — where current enters the device (in the usual conventional-current sense) when it is conducting.
• Source (S) — where current leaves the device.
• Gate (G) — the control terminal. A voltage applied here determines whether a conducting path exists between drain and source.

The crucial structural feature is that the gate is electrically insulated from the rest of the device by a thin layer of oxide (an insulating material, historically silicon dioxide — the "oxide" in the device's name). The gate does not form a direct electrical connection to the channel beneath it; instead, it acts a little like one plate of a capacitor, influencing the semiconductor beneath it through an electric field rather than through direct contact. This insulating layer is the structural fact that gives the MOSFET its extremely high input resistance at the gate — a property explored in full in the next lesson, where it turns out to matter enormously for interfacing with sensors.

Enhancement-mode operation: switching on a channel

"Enhancement mode" describes how the conducting path between drain and source is created in the first place.

• With no gate voltage applied (or a gate-source voltage below a certain threshold), there is no conducting channel between drain and source. The device is off: negligible current flows between drain and source no matter what voltage is placed across them.
• As the gate-source voltage $V_{GS}$ is increased, at some point it crosses a threshold voltage $V_{th}$. Above this threshold, the electric field from the gate induces — "enhances" — a thin conducting channel in the semiconductor directly beneath the gate, linking drain to source.
• Once this channel exists, a drain current $I_D$ can flow between drain and source. Increasing $V_{GS}$ further beyond $V_{th}$ widens/strengthens the channel and allows a larger drain current to flow.

So the device behaves as a voltage-controlled switch at the most basic level: below $V_{th}$, off; above $V_{th}$, on and conducting. This on/off behaviour, controlled entirely through the gate voltage and drawing effectively no steady current into the gate itself, is what the next lesson builds into a full switching circuit.

The key quantities

Four quantities recur constantly when describing MOSFET behaviour and reading characteristic curves:

Symbol	Name	Meaning
$V_{DS}$	Drain-source voltage	Potential difference between drain and source terminals
$V_{GS}$	Gate-source voltage	The controlling input — potential difference between gate and source
$V_{th}$	Threshold voltage	Minimum $V_{GS}$ needed to induce a conducting channel and switch the device on
$I_{DSS}$	Saturation drain current	The (roughly constant) maximum drain current once the device is fully "on" for a given $V_{GS}$

Characteristic curves

MOSFET behaviour is usually summarised using two families of graphs.

The output (drain) characteristic plots drain current $I_D$ against drain-source voltage $V_{DS}$, with a separate curve drawn for each of several fixed values of $V_{GS}$. Each individual curve has the same overall shape: $I_D$ rises steeply from the origin as $V_{DS}$ first increases, then the curve bends over and flattens into a roughly constant, "saturated" current level for larger $V_{DS}$. Comparing curves for different $V_{GS}$ values, the saturation current is higher for larger $V_{GS}$ — a family of curves stacked on top of one another, each flatter section sitting higher than the last as $V_{GS}$ increases.

The transfer characteristic plots drain current $I_D$ against gate-source voltage $V_{GS}$ (at some fixed $V_{DS}$ large enough to keep the device in its saturated region). This curve shows essentially zero current for $V_{GS}$ below $V_{th}$, and then a rising current for $V_{GS}$ above $V_{th}$ — a direct graphical picture of the switch-on behaviour described above, and the curve most directly used to read off $V_{th}$ for a particular device.

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