Here we explain the operation of the MOSFET, referring to Fig. 3-6(a).
(1) Apply voltage between drain and source with positive drain polarity. (Drain-source voltage: VDS)
(2) Apply voltage between gate and source with positive gate polarity. (Gate-source voltage: VGS)
(3) As a result, electrons are attracted to the p-type layer just under the gate insulator film, and part of the p-type layer is turned into n-type region.(This n-type region in this p-type layer is called the "inversion layer (channel)".)
(4) As this inversion layer is completed, an n-layer path is formed from the drain to the source of the MOSFET.
(n + ⇔ n - ⇔ inversion layer (n) ⇔ n +)
(5) As a result, the MOSFET works at low resistance, and drain current is determined by applied VDS and load flows.