Let's take example of "
Enhancement type N-MOS", to turn it on a positive voltage is required at gate. As soon as positive voltage is applied at gate it places positive charge at gate terminal. This positive charge repels free holes in the p-substrate under the oxide layer, these displaced hole move towards the bottom of substrate leaving behind bound negative charges. Hence a layer of negative charge appears there. A vertical field exists between two oppositely charged layer (positive charge at gate electrode and negative charge in induced layer of electrons). This field controls the depth of conductive channel. This is why FETs are known as
"FIELD EFFECT TRANSISTOR".
