# Re: [SI-LIST] : Idsat variation

Mike Degerstrom ([email protected])
Mon, 23 Aug 1999 08:45:05 -0500

DC,

IDS=IDsat(1-VDS/VT)^2 for VDS => VT

This is more of a "JFET-style" equation but the above equation
seems to lend itself to analysis better than what Gray/Meyer give
in their textbook for mosfets as:

IDS=(k'/2)*(W/L)*(1+lamba*VDS)*(VGS-VT)^2

where I have omitted the (1+lamba*VDS) from my equation
above to simply further analysis.

If we are satified with the top equation, then my question
still stands. How much +/- 3 sigma variation can we expect
for IDsat for a mainstream CMOS process at a fixed temperature?

I do have information from a .25u process that, when I do
the DC curves on their process corners, tells me to expect only
+/- 11% IDsat variation at 25C. This is from a big IC supplier
and they are known for their thoroughness so I assume that
the +/- 11% is a +/- 3 sigma number. However it is much
less variation than I expected. So maybe I did the simulation
incorrectly?

Mike

On Aug 20, 3:41pm, D. C. Sessions wrote:
> Subject: Re: [SI-LIST] : Idsat variation
> 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.
>
> Of course it is. You even point it out, below.
>
> > 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
> -------------^^^
>
> This doesn't work. The dimensions are wrong. Perhaps you meant
>
> Id=Idsat*Vds/(Vgs-Vt)
>
> which is equivalent to
>
> Id=K*Vds*(Vgs-Vt)
>
> But in all cases Id depends on Vgs which is generally dependent on Vdd.
>
> > Also, I haven't been able to find the relationship between Idsat
> > and temperature in my text books.
>
> It's not simple because Vt and transconductance have opposite
> temperature responses, so the effect on Idsat depends on the
> bias point. Clever designers (this should come as no surprise)
> take advantage of this to produce low tempco current references.
>
> --
> D. C. Sessions
> [email protected]
>
> **** To unsubscribe from si-list: send e-mail to [email protected]
In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
>
>-- End of excerpt from D. C. Sessions

```--
_______________________________________________________________
Mike Degerstrom         Email:    [email protected]
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
_______________________________________________________________

**** To unsubscribe from si-list: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
```