How Do MOSFETs Work as Voltage-Controlled Switches?

Metal-oxide-semiconductor field-effect transistors, commonly known as MOSFETs, serve as the fundamental building blocks of modern classical computing by acting as microscopic, electrically operated switches. By applying a voltage to a control terminal called the gate, a MOSFET can either allow electrical current to flow between two other terminals or block it completely. This binary switching behavior enables processors to represent and manipulate the ones and zeros of digital logic at billions of times per second.

Structure of a MOSFET

A standard enhancement-mode n-channel MOSFET (nMOS) is built on a lightly doped p-type silicon substrate, which has an excess of positive charge carriers called holes. Two heavily doped n-type regions, rich in electrons, are embedded into the substrate to form the source and the drain.

Between the source and the drain lies a thin insulating layer of silicon dioxide (\(\text{SiO}_2\)), topped by a conductive gate electrode. Because the silicon dioxide layer acts as a dielectric, no direct current flows from the gate into the semiconductor substrate. Instead, the gate and the substrate form a parallel-plate capacitor.

The Switching Mechanism

The operation of a MOSFET as a switch depends on the voltage applied between the gate and the source, denoted as \(V_{GS}\), relative to a device-specific threshold voltage, \(V_{th}\).

Cutoff Region (Switch Off)

When \(V_{GS} < V_{th}\) (including when \(V_{GS} = 0\)), there is no conductive path between the source and the drain. The p-type substrate between the two n-type regions forms back-to-back p-n junctions, which block current flow even if a voltage \(V_{DS}\) is applied between the drain and the source. In this state, the drain current \(I_D\) is effectively zero:

\[I_D \approx 0\]

The transistor acts as an open switch, corresponding to a logical off state.

Triode and Saturation Regions (Switch On)

When a positive voltage \(V_{GS} > V_{th}\) is applied to the gate, the resulting electric field penetrates the oxide layer and repels the holes in the p-type substrate beneath the gate. Simultaneously, it attracts free electrons toward the surface. Once \(V_{GS}\) exceeds \(V_{th}\), a thin layer of electrons forms directly under the oxide, creating an inversion layer or conductive n-type channel that bridges the source and the drain.

With this channel established, applying a drain-to-source voltage \(V_{DS} > 0\) causes electrons to flow from the source to the drain. For small \(V_{DS}\) (where \(V_{DS} < V_{GS} - V_{th}\)), the MOSFET operates in the triode (linear) region, behaving much like a low-value resistor:

\[I_D = \mu_n C_{ox} \frac{W}{L} \left[ (V_{GS} - V_{th})V_{DS} - \frac{1}{2}V_{DS}^2 \right]\]

Here, \(\mu_n\) is electron mobility, \(C_{ox}\) is the gate oxide capacitance per unit area, and \(W/L\) is the width-to-length ratio of the channel. In digital circuits, this low-resistance path represents a closed switch, or a logical on state.

Complementary Logic in Classical Computing

Modern digital processors rarely use nMOS transistors alone; instead, they pair nMOS with p-channel MOSFETs (pMOS) in a configuration called CMOS (Complementary Metal-Oxide-Semiconductor).

In a pMOS transistor, the substrate is n-type, the source and drain are p-type, and the channel conducts when the gate voltage is pulled low relative to the source. By combining nMOS and pMOS transistors into logic gates—such as inverters, NAND gates, and NOR gates—CMOS circuits ensure that one type of transistor is always off when the other is on in steady state. This complementary pairing minimizes static power consumption while allowing rapid, reliable switching between high and low voltage levels.