# 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,

I_{DSS} = Saturation current at V_{GS} = 0V

V_{p} = pinch off voltage

I_{DS} = Saturation current at given V_{GS}.

The drain resistance r_{d}, is given by

where,

r_{o }= resistance with V_{GS} = 0V

r_{d} = drain resistance at particular level of V_{GS}.

Amplification factor Î¼ is given by

where,

r_{d} = drain resistance

g_{m} = transconductance

Transconductance is given by

g_{m} =

General equation for V_{GS} is

V_{GS} = V_{p}

# 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 V_{DS} â‰¥(V_{GS} â€“ V_{T}) then it is in saturation region and if V_{DS} **â‰¤** (V_{GS} â€“ V_{T}) then it is in triode (linear) region.

In saturation region the current is given by,

I_{DS} =

where,

Î¼ = charge-carrier effective mobility

C_{ox} = gate oxide capacitance

W = gate width

L = Gate Length

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

I_{DS} = *C _{ox}*

where,

V_{DS} = Drain to source voltage.

In p- channel MOSFET

if V_{SG }> |V_{TP}| then the transistor is on

and if V_{SG} < |V_{TP}| then the transistor is off

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

if V_{SD} â‰¥ (V_{SG} + V_{TP}) the transistor is in saturation region

if V_{SD} < (V_{SG} + V_{TP}) the transistor is in linear region.

In saturation region the current I_{D} through transistor is given by ;

I_{DS} =

where,

V_{TP} = threshold voltage for p-type transistor

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

I_{D} =