I dont get it exactly....Consider the case of a n-channel MOSFET....
If its a depletion MOSFET,it is ON for V_GS=0...As V_GS is decreased the n channel gets depleted and beyond the threshold voltage the MOSFET turns OFF....On the other hand if V_GS>0, electrons from the bulk are enter the conducting channel and increase the conductivity,thus ensuring enhancement mode operation....
Now if its an enhancement MOSFET,if V_GS is greater than the thermal voltage,a conducting channel(inversion layer) is formed and the device is ON....But how can this MOSFET operate in the depletion mode????There is simply no conducting channel which can be depleted for V_GS<0....