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Field Effect Transistors

The primary difference between BJT and FET are as under.


Junction Field Effect Transistor (JFET)


In saturation region current through JFET is given by,


IDSS = Saturation current at VGS = 0V

Vp = pinch off voltage

IDS = Saturation current at given VGS.

The drain resistance rd, is given by



ro = resistance with VGS = 0V

rd = drain resistance at particular level of VGS.

Amplification factor
μ is given by


rd = drain resistance

gm = transconductance

Transconductance is given by

gm = 884.png

General equation for VGS is

VGS = Vp 889.png


In case of N-channel MOSFET

if 894.png then transistor is on

if 899.png then transistor is off

If transistor is ON then it is either in saturation or in triode region.

If VDS (VGS – VT) then it is in saturation region and if VDS  (VGS – VT) then it is in triode (linear) region.

In saturation region the current is given by,

IDS = 904.png


μ = charge-carrier effective mobility

Cox = gate oxide capacitance

W = gate width

L = Gate Length

In triode (linear) region the current is given by,

IDS = 909.pngCox914.png

VDS = Drain to source voltage.

In p- channel MOSFET

if VSG > |VTP| then the transistor is on

and if VSG < |VTP| then the transistor is off

If transistor is on then it can be in linear mode or saturation mode

if VSD  (VSG + VTP) the transistor is in saturation region

if VSD < (VSG + VTP) the transistor is in linear region.

In saturation region the current ID through transistor is given by ;

IDS = 919.png

VTP = threshold voltage for p-type transistor

In linear (triode) region, the current is given by

ID = 925.png

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