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Quesion on Vth of mosfet of TSMC( 0.18um 1.8Vsupply )

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benchen

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There are two type of nmos in TSMC 0.18um 1.8Vdd. one is norminal Vt and the other is medium Vt.
When I use norminal Vt nmos, I found when nmos is diode connected, its Vgs is even smaller than Vth and in region 3. Why? Usually in this case the nmos is in saturation region and Vgs is larger than Vth.

What is the difference between these two nmos besides the different Vth value?

For analog design, which nmos should we choose?

Thanks!
 

Perhaps you are not providing enough current for the mos and hence it operates in sub-treshold region.
 

dose the value of the supply current has relevence here?
The schematic is below?
 

My guess is still the same, I think the mos is probably in sub-treshold. What is the W/L ratio? If this ratio is really big, the 12u current is probably not enough to saturate the transistor.
 

the ration of W/L is about 20. I agree with analogguru. I did not know in the diode connected case the mosfet can still be in subthreshold region. The region is showed in 3 region, who knows what region 3 is?(normally subthreshold region is showed as region 0 in spectrel).

For analog design, what kind of mosfet do you choose? the norminal Vt or the medium Vt?
 

As far as I know, region 3 is the subthreshold region. So my guess is that the ratio of I/(w/L) of your device is small causing it to operate in subthreshold.

Subthreshold has the advantage og high gm/id ratio. however, the device would have a smaller output impedance and worse linearity ( as well as possibility of inaccurate modeling )
 

elbadry said:
As far as I know, region 3 is the subthreshold region. So my guess is that the ratio of I/(w/L) of your device is small causing it to operate in subthreshold.

Subthreshold has the advantage og high gm/id ratio. however, the device would have a smaller output impedance and worse linearity ( as well as possibility of inaccurate modeling )
Click to expand...

Agreed with elbadry.... If you are using bsim3v3, transition from subtreshold region to saturation region doesn't well covered by the bsim itself....
 

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