Re: Vbe stuff

On Mon, 16 Oct 2006 03:41:52 GMT, Robert Baer
<robertbaer@xxxxxxxxxxxxx> wrote:

Jonathan Kirwan wrote:

On Sun, 15 Oct 2006 12:09:17 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:


On Sun, 15 Oct 2006 17:50:26 GMT, Jonathan Kirwan
<jkirwan@xxxxxxxxxxxxxx> wrote:


On Sun, 15 Oct 2006 09:51:18 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:


On Sun, 15 Oct 2006 16:24:30 GMT, Fred Bloggs <nospam@xxxxxxxxxx>
wrote:


Jim Thompson wrote:


Density has nothing to do with vbe change per decade _provided_ you're
not into where resistance is affecting the measurement.

...Jim Thompson

I suppose it has more to do with a design goal minimizing conduction
losses in the high current diode thereby prohibiting the use of very
large ratios of majority doping material density as with the transistor.
This will cause a departure from the simplified minority carrier density
at the transition region boundaries as a function of forward bias
because the minority carrier density on both sides of the transition
region change significantly with a coupled dependence.

As someone else pointed out, the 1N4xxx devices are also very lightly
doped (almost PIN-diode-like) to get the "high-voltage" performance.

But, Fred, your dissertation sounds just like that... dissertation out
of the mouth of some _twisted_ PhD ;-)

All the same, it makes some sense to me. Shockley's equation develops

from an assumption that the forward current is entirely due to

minority carrier diffusion in neutral regions, I think. I need to
think more about it, but on first cut it sounds like the right
direction.

Jon

Large ratios of doping level are precluded by (primarily) breakdown
voltage and forward resistance considerations.

But isn't forward resistance a valid part of this measurement being
discussed?

Jon

At 100uA, the IR drop is not significant.

I was asking. In the case of high voltage _diodes_ (and this is one,
yes?), I was absorbing what I'm reading here and imagining the case
where the dopant levels are low and the effective resistance higher as
a result. How much higher, I've no idea. Keep in mind I'm merely a
hobbyist here learning from you folks.

You were talking about roughly 25mV (slightly more) than the usual
expectation of between 59mV and 60mV per decade of current at that
region. So about 25mV/90uA or about 270 ohms. That seems troubling
at first, but there is another place where this can appear in the
equation, which is:

I = Is*(e^(q(V-I*R)/kT)-1)

I hadn't worked that out for this case or any other, but I've seen it
present in some papers and I wondered.

Also, the emission coefficient of the 4006 is very close to 2. (1.984
in one model I see) and that definitely has an effect, I'd imagine.
Whether your actual diode has that value is another thing. But there
it is and that would _not_ be the resistance.

Anyway, I'd reading this with interest.

Jon
.

Relevant Pages

  • Re: Vbe stuff
    ... not into where resistance is affecting the measurement. ... This will cause a departure from the simplified minority carrier density ... because the minority carrier density on both sides of the transition ... But isn't forward resistance a valid part of this measurement being ...
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  • Re: Vbe stuff
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