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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,
 

868.png 

where,
IDSS = Saturation current at VGS = 0V

Vp = pinch off voltage

IDS = Saturation current at given VGS.


The drain resistance rd, is given by

 

874.png 

where,
ro = resistance with VGS = 0V

rd = drain resistance at particular level of VGS.


Amplification factor
μ is given by
 

879.png 

where,
rd = drain resistance

gm = transconductance


Transconductance is given by

gm = 884.png


General equation for VGS is

VGS = Vp 889.png

MOSFET

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


where,

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


where,
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

where,
VTP = threshold voltage for p-type transistor


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

ID = 925.png





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