aryajur
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The subthreshold equation that is given in many references is like this:
Id = µ K W/L Vt² (exp(Vgs-Vth)/(ζVt) ) (1 - exp(-Vds/Vt) )
ANd when Vds becomes a little large then the last exponential term reduces to 0. But for this equation the term Vgs -Vth will be negative so the curve should be exponentially decreasing ! Razavi says that the equation can be approximated to:
Id = W/L Is exp(Vgs/(ζVt))
I don't understand how can Vt (Upper limit of the threshold) can be ignored here?
Id = µ K W/L Vt² (exp(Vgs-Vth)/(ζVt) ) (1 - exp(-Vds/Vt) )
ANd when Vds becomes a little large then the last exponential term reduces to 0. But for this equation the term Vgs -Vth will be negative so the curve should be exponentially decreasing ! Razavi says that the equation can be approximated to:
Id = W/L Is exp(Vgs/(ζVt))
I don't understand how can Vt (Upper limit of the threshold) can be ignored here?
