Coverning the MOSFET transistor,
First, when apply Vgs , it causes a field that starts to repulse the majority carriers at the substrate .This causes "the depletion region" i.e depleted from majority carriers .On increasing Vgs more, the field starts to attract minority carriers from the substrate and from the 2 n+ drain and source regions .This is called "inversion" .These carriers collected during inversion form the channel which causes the current at the transistor .Strong inversion occurs above the threshold voltage of the transistor .