VLSI (SUBTHRESHOLD CONDITION)

 SUBTHRESHOLD CONDITION:


  • when Vgs>Vth, we know that current will flow through the MOSFET.

  • when Vgs < Vth, the current that flows through the MOSFET should be '0'. But there is a small negligible current that is flowing through the MOSFET.

  • This current which is flowing through MOSFET when Vgs < Vth is called SUBTHRESHOLD CONDITION.

  • when Vgs goes less than Vth the current isn't abruptly dropping down to 0 rather the current is exponentially decreasing which is observing by the below equation.

  •  the subthreshold current leads to static power dissipation.

  • the subthreshold current equation is:

                Ids = Idso * e^[(Vgs-Vth)/nVt] * [1-e^(-Vds/Vt)]
                            Idso=beta * Vt^2 * e^1.8
Idso is current at threshold voltage depends on process and device geometry.
n is non ideality factor(n>1)(for cmos 1.4 - 1.5)

FROM THE ABOVE EQUATION,
as Vth decreases( Vgs is less than Vth) the subthreshold current increases exponentially.
when Vgs is greater than Vth it behaves as a Square law device.





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