Consider next the situation when a small positive voltage V_G. is applied to the gate. The direction of the electric field across the oxide will now be towards the substrate. The holes (majority carriers) are now driven back into the substrate, leaving the negatively charged immobile acceptor ions. Lack of majority carriers create a depletion region near the surface. Almost no mobile carriers are found near the semiconductor-oxide interface under this bias condition.

Next, let us investigate the effect of further increase in the positive gate bias. At a voltage V_GS = V_th , the region near the semiconductor surface acquires the properties of n-type material. This n-type surface layer however, is not due to any doping operation, but rather by inversion of the originally p-type semiconductor owing to the applied voltage. This inverted layer, which is separated from the p-type substrate by a depletion region, accounts for the MOS transistor operation. That is, the thin inversion layer with a large mobile electron concentration, which is brought about by a sufficiently large positive voltage between the gate and the source, can be effectively used for conducting current between the source and the drain terminals of the MOS transistor. Strong inversion is said to occur when the concentration of the mobile electrons on the surface equals that of the holes in the underlying p-type substrate.

As far as the electrical characteristics are concerned, an nMOS device acts like a voltage-controlled switch that starts to conduct when V_G (or, the gate voltage with respect to the source) is at least equal to V_th (the threshold voltage of the device). Under this condition, with a voltage V_DS applied between the source and the drain, the flow of current across the channel occurs as a result of interaction of the electric fields due to the voltages V_DS and V_GS. The field due to V_DS sweeps the electrons from the source toward the drain.As the voltage V_DS increases, a resistive drop occurs across the channel. Thus the voltage between the gate and the channel varies with the distance along the channel. This changes the shape of the channel, which becomes tapered towards the drain end.