Re: [SI-LIST] : Idsat variation

Jim Freeman (freeman@mavericknetworks.com)
Fri, 20 Aug 1999 18:49:04 -0700

Hi Mike,
IDS does increase with Vds by the value (1+lambda)*VDS where lambda is the slope of the ID curve in the saturated region. The increase current is due to a phenomenon called channel length modulation. In addition to this
change in current, there are two other temperature effects that have to do with mobility of carriers in the channel and the doping level in the channel. Since the mobility is a function lattice scattering, it is inversely
proportional to temperature and the k factor is directly proportional to the mobility.

The current equation is IDS = k' w/l((Vgs-Vt)**2)*(1+lambda)*Vds in saturation.

The linear region of operation is a little simpler and is the final region of operation when an n-channel device has pulled a node fully to a low.

Mike Degerstrom wrote:

> Jim,
>
> Thanks for your reply. I'm no device physicist but I don't
> think Idsat should be a function of voltage. Certainly voltage
> variation affects a CMOS buffer drive strength. However
> I was asking for Idsat for the following equation relating drain current
> to the gate-to-source voltage for a CMOS device in the ohmic region:
>
> ID=IDsat(VGS-VT)^2
>
> Also, I haven't been able to find the relationship between Idsat
> and temperature in my text books.
>
> BTW, for anyone else that wants to contribute - please don't
> supply PROPRIETARY information that you have from foundrys that
> you work with.
>
> Mike
>
> > HI Mike,
> >
> > You can expect a 4:1 variation over Process,voltage, and temperature for
> 3 sigma process models.
> >
> > Jim Freeman
> >
> >
> > Mike Degerstrom wrote:
> >
> > > Can anyone tell me what reasonable Idsat variations one could
> > > expect from a typical CMOS line? Could you please qualify your
> > > answers such as "these are +/- 3 sigma numbers" if at all possible?
> > >
> > > Mike
>
> --
> _______________________________________________________________
> Mike Degerstrom Email: degerstrom.michael@mayo.edu
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