This type of power MOSFET is widely used in digital computers.

The construction of enhancement type power MOSFET is shown in figure above .This is an n-channel device.However as seen in the figure,the p-substrate extends right up to the silicon dioxide layer.Thus there is no n-channel between drain and source.

The above figure shows the connections.When the gate voltage is zero,drain battery tries to push free electrons from the source to the drain.However the p-substrate has only a few thermally produced free electrons and some electrons due to surface leakage.Therefore the drain current is almost zero.When gate is positive(V_GS>0)the gate attracts free electrons into the p-substrate region these free electrons recombine with holes near silicon dioxide layer.When V_GS is large enough all these holes near the silicon dioxide layer have recombined and electron starts flowing from source to drain.The effect is as though a n-type layer between source and drain has been created.This n-type layer is n-type inversion layer.When this layer is formed,the transistor is turned on and free electron can easily flow from source to drain.This minimum value of V_GS which can create inversion layer is known as gate threshold voltage(V_GS(th)).

Thus the transistor is off when V_GS<V_GS(th) and on when V_GS>V_GS(th).Thus the conducting capability of enhancement type mosfet depends on the action n-type inversion layer.Vgsth varies from about 1to 5 V for these devices.

An enhancement mode mosfet is used in digital computer since it acts as a switch.When gate voltage is less than threshold it is off but when gate voltage is more than threshold it turns on like a switch.

Symbol

The channel is shown as a broken line to distinguish it from depletion mode also to indicate that normally it is in offstate

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