Basic Buck Converter Switching Mosfet
With an N-channel MOSFET, a positive change in gate voltage means an increase in conduction in the FET D-S channel, whether you are using a D-MOSFET (Depletion mode) or an E-MOSFET (Enhancement mode). 

The difference between D- and E- is the level at which the change takes place. For instance an E-MOSFET is off with zero gate volts, and (depending on the specific model) on with 2v to 10v on the gate. A D-MOSFET will be on with zero gate volts. To turn it off you take the gate to a negative voltage, just like with a vacuum tube grid. In both cases, a positive change increases conduction.

As you see on the waveforms picture, on is a more positive voltage then off. The difference between using a D- and E- device would be where you draw the zero on the control voltage graph. With E- it would be the lower voltage, with D- it would be the higher voltage.

With a P-channel MOSFET, the polarities are opposite, and the states for zero gate voltage are the same as for N-channel.