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An n-channel MOSFET has two heavily n-doped source and drain regions in a p-type body, with an insulated gate above the intervening surface. A positive gate voltage first depletes surface electron holes and then induces an electron semiconductor inversion layer. Above threshold $V_T$ this layer connects source to drain. Gate voltage controls the channel charge through the oxide capacitance, while the insulating gate permits a very small steady gate current.

For the long-channel MOSFET current law, let $C_{\mathrm{ox}}$ be oxide capacitance per area, $W,L$ channel width and length, and use constant mobility ${\mu }_n$. In the gradual-channel approximation,At larger drain voltage the ideal channel pinches off near the drain and the current saturates atCarriers still flow through the pinch-off region; pinch-off does not mean zero drain current. These formulas neglect channel-length modulation, short-channel effects and velocity saturation. A p-channel device reverses the carrier type and operating polarities. Gate control makes MOSFETs useful as electronic switches and amplifiers.

A field-effect transistor uses a gate voltage to control the conducting channel between source and drain. In a MOSFET, the gate is insulated from the semiconductor: an electric field changes the channel carrier population with little steady gate current. An n-channel device creates an semiconductor inversion layer when the gate sufficiently attracts electrons to a p-type surface. The drain supply provides the output power; the gate controls that flow.

The long-channel MOSFET current law follows from the gradual-channel approximation. With gate capacitance per area $C_{\mathrm{ox}}$, threshold voltage $V_T$, channel width $W$, length $L$, constant charge carrier mobility $\mu$ and $V_{mov}=V_{GS}-V_T>0$, the local sheet charge magnitude is $C_{\mathrm{ox}}(V_{mov}-V(x))$. Integrating the drift current along the channel givesAt $V_{DS}=V_{mov}$, the drain end reaches pinch-off. The ideal long-channel saturation current is $I_D=\mu C_{\mathrm{ox}}W{V}_{mov}^2/(2L)$; this assumes negligible channel-length modulation. A junction field-effect transistor instead changes the channel width through a reverse-biased depletion region.

Experimentally, measure $I_D$ against $V_{GS}$ at fixed drain bias to see gate control, and against $V_{DS}$ at several fixed gate biases to distinguish the approximately linear and saturation regimes. Simultaneously checking gate leakage tests the insulated-gate assumption.