The MOSFET channel is the thin layer of mobile charge — electrons in an n-MOSFET, holes in a p-MOSFET — that forms right at the silicon surface under the Gate oxide when the gate voltage is large enough. The channel is the conducting path that connects source to drain; with no channel there is no path and the device is off.
How the channel forms (n-MOSFET)
Start with source, drain, and body all grounded, and slowly raise the gate voltage of an n-MOSFET. The substrate under the gate is p-type — its majority carriers are holes, with only a tiny number of electrons. The gate is one plate of a capacitor (see Gate oxide); the silicon surface is the other. As the gate goes positive, the field across the oxide acts on the surface in three stages:
- Accumulation pushed away → depletion. The positive gate first repels the positively charged holes away from the surface. What is left behind is the fixed, ionised negative acceptor atoms locked in the lattice — a depletion region, directly analogous to the one in a PN junction. There are still essentially no mobile carriers at the surface, so still no channel.
- Inversion. Keep raising the gate voltage. The field gets strong enough to attract electrons to the surface. The source and drain are heavily n+ doped, so they are reservoirs of electrons; electrons stream in from them and collect in a thin sheet right under the oxide. The surface, originally p-type, now behaves locally as n-type — it has been inverted. This electron sheet is the channel.
- Channel established. Once the channel exists it bridges the n+ source and n+ drain with a continuous conducting path, and current can flow.
The gate-to-source voltage at which the channel just appears is the Threshold voltage . How strongly the channel conducts is set by how far past threshold you are — the Overdrive voltage . More overdrive means more attracted charge means a thicker, more conductive channel.
The channel is not uniform once current flows
Apply a drain-to-source voltage and the channel stops being uniform along its length. The drain end sits at a higher potential than the source end, so the gate-to-channel voltage — the thing that actually holds charge in the channel — is smaller near the drain. The channel is therefore thinner at the drain end than at the source end. Push all the way up to and the drain-end channel thins to nothing: this is Channel pinch-off, and it is the boundary between the MOSFET triode region and the MOSFET saturation region. The whole behaviour of the MOSFET — voltage-controlled resistor below pinch-off, voltage-controlled current source above it — is just bookkeeping on the shape of this charge layer.