METAL-OXIDE-SEMICONDUCTOR FET (MOSFET)or INSULATED GATE FET (IGFET)

1. DEPLETION TYPE MOSFETAn n-channel depletion type MOSFET is shown below.

The metallic gate is insulated from the semiconductor channel by the silicon dioxide (SiO2) layer and thus has a extremely high resistance. The gate impedance is mainly capacitive. The source and drain regions are very heavily doped, and the n-channel allows electrons (and hence current) to flow from the source to the drain (when the drain is made positive wrt the source), even when the gate-source voltage is zero.

1.1 Variation of Channel Width with Gate Voltage (VGS)The width of the channel, and hence the conductivity, can be controlled by applying a voltage to the gate.

(a) Depletion Mode of OperationIn this mode a negative voltage is applied to the gate relative to the substrate (ie. Source) and the channel width is reduced.

The negative gate voltage drives electrons away from the channel, that gets narrower and narrower as the negative gate voltage is increased. Hence the drain current reduces with increasing negative potential on the gate as indicated in the transfer characteristic.

(b) Enhancement Mode of OperationIn this mode a positive voltage is applied to the gate relative to the source. The +ve gate charge attracts electrons from the p-type substrate and the n+ regions, into the channel area and thus the width of the channel is increased (enhanced). Hence the drain current reduces with increasing negative potential on the gate as indicated in the transfer characteristic shown below.

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1.2 Characteristics of Depletion type MODFETSThe transfer and output characteristics of a depletion type MOSFET are shown below.

2. ENHANCEMENT TYPE MOSFETAn n-channel enhancement type MOSFET is shown below.

2.1 Mode of OperationIn this case there is no channel to conduct current when the gate voltage is zero or negative. The device can only operate in the enhancement mode. A positive voltage needs to be applied to the gate in order to attract electrons into the area between the source and drain, and hence induce an n channel as indicated below.

The gate to source voltage at which the channel starts to conduct is called the transition voltage VT.2.2 Enhancement Type MOSFET Amplifier

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Bias Conditions

Vgs = VDD R2/(R1+R2)

ID is obtained from graph of ID against Vgs

gm is obtained from the slope of the transfer characteristic at the bias point.

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