An NMOS device operating with a small drain-source voltageserves as a resistor.

Question

An NMOS device operating with a small drain-source voltageserves as a resistor. If the supply voltage is 1.8V, what is theminimum on-resistance that can be achieved with W/L=20? thanks for all the help not sure where to start with thisquestion thanks
using the equation Ron=1/(UnCoxW/L(Vgs-Vth)) where Un, Cox are constants Vgs is the voltage accross thegate and source and Vth is the threshold voltage to turn on thetransistor and w/l is given, thanks An NMOS device operating with a small drain-source voltageserves as a resistor. If the supply voltage is 1.8V, what is theminimum on-resistance that can be achieved with W/L=20? thanks for all the help not sure where to start with thisquestion thanks
using the equation Ron=1/(UnCoxW/L(Vgs-Vth)) where Un, Cox are constants Vgs is the voltage accross thegate and source and Vth is the threshold voltage to turn on thetransistor and w/l is given, thanks

Explanation / Answer

Un = mobility of electrons Cox = ox / tox where ox = permittivityof the oxide and tox = thickness of the oxide layer.
So Ron = 1/(Kn*20*(Vgs-Vth)) For a NMOS device, Vth >0 in enhancement mode. Since we want to find minimum Ron, let us equate Vth =0. Now, Vgs = Vg -Vs = 1.8 -Vs 1.8V So maximum possible value of Vgs =1.8V To minimise Ron, we take the maximum possible value of Vgs. Thus minimum possible Ron = 1/(Kn*20*(1.8-0))          = 1/36Kn
Given the value of Kn, one can now find the minimum possible Ronunder the given circumstances.
I hope this helps.

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