Subject: circuit origins of the non-monotonic pull-up curve

From: [email-masked]
Date: Thu, 10 Dec 1998 07:27:45 -0600

We got into a discussion the other day about the origin of the
non-monotonic IV curve depicted on p. 20 of the cookbook.  In my mind, the
"output enabled" IV curve should be a sum of the clamp (ESD diode and/or
parasitic well diode) and the FET characteristics.  Therefore, when you
subtract the clamp curve from it, you should get the FET curve, with the
possible exception of some round-off errors.  However, the "hump" you
really see in the pull-up or pull-down curve is much bigger than round-off
error.  Is this easy to explain, or is it fairly subtle?  Where did my
reasoning go wrong?

Greg Edlund
Advisory Engineer, Critical Net Analysis
IBM
3650 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
[email-masked]

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From: Jon Powell <[email-masked]>
Date: Thu, 10 Dec 1998 08:25:23 -0800

If this is the "hump" I think you are talking about it accurs from the
subtraction of two really huge currents as they tend toward MAXFLOAT
(sometimes called infinity). We had a big argument and scientific
discussion about this when we were first deciding how to approach this
and there were many little papers and emails sent around. I think that
my final resolution on the matter was, "The hump occurs so far outside
of the active region that the device would have melted anyway so it's
not worth arguing about".

regards,
jon

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From: Stephen Peters <[email-masked]>
Date: Thu, 10 Dec 1998 09:32:24 -0800

Hello Greg, IBIS list:

     Ummm...  this is a place where a picture is worth a thousand 
words, so I will try a little ANSI art.

Ground Clamp Data = *
Pulldown Data     = +
                       (I)
                      |
                      |
                      |   ++++++++++++++++
                      |  +
                      | +
                      |+
            -------**********************  (V)
                  *  +|
                 *  + |
                *  +  |
                * +   |
   point A -->  *     |
                *     |
                      |

 The picture above is an attempt to show a pulldown curve and ground 
clamp curve on the same set of axis.  What I've seen many time is that
at point A, the pulldown curve and the ground clamp become very close to
one another, even converging so that they lie on top of each other.
Physically, I'm not exactly sure what is happening, but apparently
when the pulldown FET starts acting as the clamp diode is stops acting
like a FET.  Note that I've seen this convergence (or near to it) even in
devices that have ESD protection diodes.  Apparently the current in the ESD
structure is small compared with the main clamping action of the
pulldown FET.  At any rate, when you subtract the two curves you end up
with the curve shown in the cookbook.  Hope this helps.

         Regards,
         Stephen Peters
         Intel Corp.