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Modeling >> Semiconductor Devices >> Vth Shift( Positive or Negative?)
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Message started by jason_class on Feb 22^nd, 2006, 11:29pm

Title: Vth Shift( Positive or Negative?)
Post by jason_class on Feb 22^nd, 2006, 11:29pm

Hello All

I would like to seek advice on threshold voltage shift of cmos.
Let's say there is sodium contamination on the cmos device, will both nmos and pmos experience positve threshold voltage shift (both with increasing Vth?)?
I try to relate the sodium contamination with Qi/Ci where Qi is the charge in oxide.
As flat band voltage is Vfb= Vms - Qi/Ci , so I guess both will show the decreasing or increasing threshold voltage.

Kindly enlighten.
Thank you all

best regards
Jason

Title: Re: Vth Shift( Positive or Negative?)
Post by Marc Murphy on Feb 28^th, 2006, 1:49pm

Qi/Ci is positive for both PMOS and NMOS (the interface charge, Qi, is positive). So, the Vfb term you mentioned is more negative for a larger contamination (Qi/Ci is bigger)

Vfb = Vms - Qi/Ci
Vth = Vfb + ...

For NMOS, Vth is more negative (threshold is lower).
For PMOS, Vth is more negative (threshold is higher, since it has a more negative Vth)

I.e.

VthNMOS = 1 - 0.1 = 0.9 (better)
VthPMOS = -1 - 0.1 = -1.1 (worse)

I assume you want minimum threshold voltages. This might not be desirable for low leakage currents.

I hope that cleared things up.

Cheers,
Marc

Title: Re: Vth Shift( Positive or Negative?)
Post by jason_class on Mar 2^nd, 2006, 5:05am

Hi Murphy

That explains everything clearly !
Thank you so much

best regards
jason