Enhancement Type Power MOSFET
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(VGS>0)the gate attracts free electrons into the p-substrate region these free electrons recombine with holes near silicon dioxide layer.When VGS 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 VGS which can create inversion layer is known as gate threshold voltage(VGS(th)).
Thus the transistor is off when VGS<VGS(th) and on when VGS>VGS(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
Written by John on September 30th, 2008 with
1 comment.
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