Characteristics of Enhancement Type MOSFET

The drain characteristics of enhancement type MOSFET is given below.This depicts the variation of drain current(ID)with drain to source voltage(VDS)for different values of gate to source voltage(VGS).

The lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almost zero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device can operate in the ohmic,active or cutoff(break down)region.The rising part of curve (fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a resistor,when operated in this region.The drain current is almost constant when the device operates in the active region.when VDS exceeds the rated value,avalanche breakdown occurs and the device is in the breakdown region.

Transconductance curve of enhancement MOSFET is shown below

This curve start from VGS(Th) because the device is off and drain current is  zero when VGS<VGS(Th).In addition two other quantities specified  are VGS(ON) and ID(ON).Drain current is given by

ID =[(VGS -VGS(TH)) / (VGS(ON)- VGS(TH))] × ID(ON)

MOSFEts have a very thin silicon dioxide layer.This layer is kept very thin to ensure that the gate has good control over the gate current.This layer could be destroyed if a voltage higher  than rated value is applied to the gate.If the MOSFET has a  rated VGS of -30V,we should never apply a voltage higher than +30V or lower than -30V.Moreover they should not be connected or disconnected in the circuit when the circuit is ON.

Written by arjun on October 16th, 2008 with 1 comment.
Read more articles on Enhancement type MOSFET and MOSFET and Power Electronics.

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