The MOSFET is a powerful semiconductor device, Metal Oxide Semiconductor Field Effect Transistor is the full form of MOSFET, so technically it is a transistor but in different terms.

MOSFET is a four-terminal device and like NPN, PNP transistors, the MOSFETs are been divided into P- channel and N- channel MOSFET, and division will change in terms of technical specification.

In this post we are going to explain about MOSFET region of operation, as same as the operation at the transistor, the MOSFET has a cutoff, linear, and saturation region.

MOSFET characteristics curve

MOSFET characteristics curve
MOSFET characteristics curve

The figure shows the testing circuit and characteristics curve of MOSFET, here we can see three regions of operation by the MOSFET.

Cut-off region

  • This is the initial most region at the MOSFET operation, here the MOSFET is at OFF condition.
  • The gate to source voltage is zero and the VGS < VTH, at this MOSFET region of operation there is no current flow is present.
  • In this region, the MOSFET acts as an open switch.

Linear/ Ohmic region

  • This is the progression region of MOSFET, in this region where drain to source current increases with represent to drain to source voltage.
  • At this region, VGS becomes ON condition
  • In the MOSFET characteristics curve, this region is indicated in a curved shape.
  • In this region, MOSFET acts as a closed switch

 

Saturation region

  • This is the last and main region for the operation of MOSFET, at this region drain to source current becomes constant without support from drain to source voltage.
  • This particular will happens on the condition where drain to source terminal voltage increases towards a fixed limit.
  • At this region where VGS > VTH.
  • At this condition the device functioned as a closed switch and this region will be utilized at the amplifier circuit.

MOSFET region of operation equation

At the MOSFET region of operation, the N-channel and P-channel division is not important but at the operation equation, the division is needed.

At N-channel MOSFET

Cut-off – VGS ≤ VT

Linear   – VGS > VT, VDS ≤ VGS – VT  

Saturation – VGS > VT, VDS > VGS – VT

At P-channel MOSFET

Cut-off – VSG ≤ |VT|

Linear – VSG > |VT|, VSD ≤ VSG – |VT|

Saturation – VSG > |VT|, VSD > VSG – |VT|

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