Presence of depletion layer in an N-MOSFET
Because the source has N-type doping and the substrate has P-type doping, and together they form a P-N junction, or diode. Even with no external potential applied, such a junction forms a depletion layer.

An enhancement-mode MOSFET turns on by using the electrical field from the gate to shift charge carriers so that the substrate immediately below (the channel) temporarily becomes N-type (has an excess of electrons), creating a conductive path between the source and drain.