Re: [SI-LIST] : Idsat variation

Peng Fei Zhang (pzhang@fmi.fujitsu.com)
Mon, 23 Aug 1999 10:52:51 -0700 (PDT)

Hi,

There is a very important factor, the mosfet geometry (W, L), one needs to
look at to understand the Idsat variation in a standard CMOS process.

Ruling out temperature and power supply variation (which are of course
important factors in determining the current drive especially for deep
submicron generations), W and L are the major process variations that
would give rise to delta-Idsat. (threshold voltage Vt variation is primarily
due to the channel length L variation).

Process engineers can control L much better than W. So If the devices you are
interested in are >5um in width, I would be supprised to see a 10%(3sigma) Idsat
variation within a batch. I think the spice corner cases took into account
the batch to batch variation also.

For narrow devices, (W<1um), which are more often used in analog circuits,
the variation can be quite large. I've seen more than 50%.

Peng

>
> Jim,
>
> Thanks for the insight on temperature variation. I did omit the
> (1+lambda)*VDS part of the equation intentionally to simplify the
> equation. Otherwise your equation matches the one that I have so
> maybe we have the same textbook (Gray/Meyer). I simply chose to
> rewrite the mosfet equation in the saturation region to make for
> easier analysis.
>
> As D.C. Sessions pointed out, my simpler equation was wrong. So
> for an n-channel in the saturation region with an equation as:
>
> IDS=IDsat*(VDS/VT-1)^2,
>
> would you have a good idea how much IDsat will vary at a fixed
> temperature (say 25C)?
>
> Mike
>
>
> On Aug 20, 6:49pm, Jim Freeman wrote:
> > Subject: Re: [SI-LIST] : Idsat variation
> >
> > 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
> > > Mayo Clinic
> > > 200 1st Street SW
> > > Gugg. Bldg. RM 1042A Phone: (507) 284-3292
> > > Rochester, MN 55905 FAX: (507) 284-9171
> > > WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> > > _______________________________________________________________
> > >
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> >
> >-- End of excerpt from Jim Freeman
>
>
>
> --
> _______________________________________________________________
> Mike Degerstrom Email: degerstrom.michael@mayo.edu
> Mayo Clinic
> 200 1st Street SW
> Gugg. Bldg. RM 1042A Phone: (507) 284-3292
> Rochester, MN 55905 FAX: (507) 284-9171
> WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> _______________________________________________________________
>
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>
>

Peng-Fei

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