Flat-band voltage (Vfb): definition

Beginner

The gate voltage that leaves the silicon under the gate just as it would be with no gate at all.

Novice

The gate voltage at which the energy bands in the silicon are flat right up to the oxide: no carriers are attracted or repelled. It is set by the difference between the gate material and the silicon, plus any charge trapped in the oxide.

Expert

$V_{\mathrm{fb}} = \psi_{\mathrm{g}} - \psi_{\mathrm{s}} - Q_{\mathrm{ox}}/C_{\mathrm{ox}}$ . For an $\mathrm{n^+}$ poly gate ( $\psi_{\mathrm{g}} = \chi_{\mathrm{Si}} = 4.05\,\mathrm{V}$ ) on p-type silicon, $V_{\mathrm{fb}} = -(E_{\mathrm{c}} - E_{\mathrm{F}})/q$ , roughly −1 V. It is the first term in the threshold equation.