
Semiconductor Components
303
The MOS type transistor is based on the modulation of the charge concentra-
tions in the semiconductor region below the layer of oxide of the gate, thanks to a
voltage applied to the gate terminal above the oxide.
In the case of a p−type semiconductor (with hole density equal to N
) a posi-
tive voltage V
applied to the gate reduces the concentration of holes in the semi-
conductor under the oxide, and increases the concentration of electrons. For a
sufciently high value of V
, the concentration of negative charge carriers below
the gate exceeds that of positive charges, and the so-called inversion layer is
formed. ...