# 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,

where,
IDSS = Saturation current at VGS = 0V

Vp = pinch off voltage

IDS = Saturation current at given VGS.

The drain resistance rd, is given by

where,
ro = resistance with VGS = 0V

rd = drain resistance at particular level of VGS.

Amplification factor
Î¼ is given by

where,
rd = drain resistance

gm = transconductance

Transconductance is given by

gm =

General equation for VGS is

VGS = Vp

# MOSFET

In case of N-channel MOSFET

if  then transistor is on

if  then transistor is off

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

If VDS â‰¥(VGS â€“ VT) then it is in saturation region and if VDS â‰¤ (VGS â€“ VT) then it is in triode (linear) region.

In saturation region the current is given by,

IDS =

where,

Î¼ = charge-carrier effective mobility

Cox = gate oxide capacitance

W = gate width

L = Gate Length

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

IDS = Cox

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 â‰¥ (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 =

where,
VTP = threshold voltage for p-type transistor

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

ID =