Sensors

Field effect

Principle

If you generate an electric field perpendicular to the surface of a conductor $\mathit{E.}$, this creates a surface charge:

$$Q={\epsilon }_r{\epsilon }_0\mathit{E.}$$

If this conductor consists of a thin, electron-conducting layer, the conductance of the layer increases by $\Delta \mathrm{L.}$. This results in a relative change in the conductance $\raisebox{1ex}{$\Delta \mathrm{L.}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{L.}$}\right.$ with the layer thickness $d$ and the carrier concentration $n_0$:

$$\frac{\Delta \mathrm{L.}}{\mathrm{L.}}=\frac{-{\epsilon }_r{\epsilon }_0\mathit{E.}}{q{n}_{0}d}$$

The relative effect is greater, the thinner the layer and the smaller the concentration of conductivity electrons in the layer. This explains why the field effect can best be examined on thin layers of semiconductors. When using p-semiconductors, the sign is reversed.