Inversion layer: definition · Chip Field Guide

Beginner

A thin sheet of electrons that forms just under the gate when the gate voltage is high enough. It is the transistor’s bridge.

Novice

A layer at the silicon surface where the minority carrier has become the majority, for example electrons at the surface of p-type silicon. It is only 1–2 nm thick and connects the source to the drain.

Expert

Defined as starting when surface electron density equals $N_{\mathrm{a}}$ ( $\phi_{\mathrm{s}} = 2\phi_{\mathrm{B}}$ ). Above threshold its sheet charge is $Q_{\mathrm{inv}} \approx -C_{\mathrm{ox}}(V_{\mathrm{g}} - V_{\mathrm{T}})$ because the depletion charge barely changes. Its finite thickness ( $T_{\mathrm{inv}}$ ) adds about $T_{\mathrm{inv}}/3$ to the electrical oxide thickness.