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Should Vgs-Vth>3*VT? and also vdsat?

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hi guys,in most cases,wo should make vdsat>3*VT,is it necessary to make Vgs-Vth>3*VT,if not,what is the proper value for the Vgs-Vth?thanks!
 

lwjbh said:
hi guys,in most cases,wo should make vdsat>3*VT,is it necessary to make Vgs-Vth>3*VT,if not,what is the proper value for the Vgs-Vth?thanks!
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In the large signal model for weak inversion, vdsat=3*VT marks the transition voltage from the triode region to the saturation region, s. e.g. Rincon-Mora's workshop on "Weak Inversion Operation of Mosfets", p.5 , s. below. So in order to operate a mosfet in saturation, your above equation is correct.
 
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thanks erikl,but if in order to operate a mosfet in saturation,is it necessary to make Vgs-Vth>3*VT?I mean that in short channel ,the Vdsat is not equal to Vgs-Vth,if i want to ensure the mosfet operate in saturation,i should make Vdsat or Vgs-vth>3*VT?
 

lwjbh said:
... if i want to ensure the mosfet operate in saturation, i should make Vdsat or Vgs-vth>3*VT?
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Vdsat ≥ 3*VT for saturation region operation is only valid for very small Id current. The transition from the linear (or "triode") region to the saturation region is soft, by convention the transition point is Vdsat = Vds = Vgs-vth for arbitrary Id currents, s. this Id vs. Vds plot from Wiki. So with Vgs-vth = Vds + 3*VT you are on the safe side of the saturation region, valid for any Id. And this means Vdsat = Vgs-vth-3*VT . Most suitably retrace this relation by studying the a.m. plot.
 

thanks,erikl,gm=Id/(Vgs-Vth),if i want to get bigger value of gm/Id,i should make Vgs-Vth smaller,but i am afraid i can not get the correct simualtion result as the mosfet operates in the transition region,the bsim3 has not an accurate model for that mosfet.what should i do?
 

lwjbh said:
... the mosfet operates in the transition region, the bsim3 has not an accurate model for that mosfet. what should i do?
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Don't blame BSIM3 for that, it definitely has the ability to create a smooth (continuous) transition from weak to moderate to strong inversion, s. the PDF below. It's more likely that the special MOS model you're using doesn't exploit this feature. Do you use Lvl 49?

Also BSIM4 (Lvl 56) and the EKV models handle the smooth transition very well.
 
appreciate your help!
 

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