diode temperature coefficient

[kell]

Some references describe diode tempco as -2.1 mV per degree C and also
as 3300 parts per million.
If parts per million, then the tempco would vary with the forward
voltage of the diode, for example, with a very low forward current, Vf
could be half a volt; at 3300 parts per million that comes to about
-1.7 mV. So is it -1.7 or is it -2.1; in other words is tempco a
constant or is it a proportion of Vf?

[John Popelish]

The voltage across an ideal diode is the natural log of a ratio of
bunch of terms that involve doping densities and thicknesses and
widths multiplied by the diode current. And that log is multiplied by
k*T/Q.

The derivative of all that with respect to T is the same thing with
the T replaced by 1. So as long as the current is constant (and all
those other constants do not vary with temperature), the slope is
constant, or a fixed increment per degree, not a fixed fraction of Vf
per degree.

[Pooh Bear]

John Popelish wrote:

And don't forget to factor in the IR drop due to bulk resistance.

When using diodes or Vbe like that to monitor temp I typically run them @
around 1mA or less.

Graham

[pebe]

> kellwrote:

Temperature coefficient of the p/n junction is approx -2mV/deg. C
It does not depend on Vf.

[John Fields]

On Sat, 26 Nov 2005 00:01:08 -0600,
pe...@cullerne81.freeserve.co-dot-uk.no-spam.invalid (pebe) wrote:

>Temperature coefficient of the p/n junction is approx -2mV/deg. C
>It does not depend on Vf.

---
That's only true at a single temperature and at a single Vf, so yes,
it does depend on Vf.

--
John Fields
Professional Circuit Designer

[kell]

I didn't notice this before, but in John Popelish's response he says
that
"as long as the current is constant," tempco is constant.
Since Vf is a function of current, and tempco
varies with current (if I interpret correctly), then tempco would vary
with Vf.
But I guess we're not talking about anything simple or remotely linear.