Vbe stuff

Ebers-Moll, Gummel-Poon - have you looked at what the classical transistor models predict?

As far as I know, the Gummel-Poon 64-parameter model of the bipolar transistor is the one normally realised in Spice simulators, while the simpler (and less accurate) Ebers-Moll model is the one taught by lazy academics to undemanding students.

The evil behaviour of the 1N4006 is well known - I haven't a clue why rectifier diodes behave oddly, but I am well aware that they do.

Something to do with being PIN diodes, perhaps?

Probably

...Jim Thompson

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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 ;-)

...Jim Thompson

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

The diode equation (Shockley, "Theory of p-n Junctions", Bell System Technical Journal #28, p 435-489 , 1949) applies to planar semiconductor junctions.

I = Is * (exp( (Qe*V) / (k * T) ) - 1)

Is is dependent on the current-conducting area, T is the junction temperature

The base-emitter junction of a modern transistor (MPS-A42) is a good fit.

A 1N4006 is intended for high current, and there's a fault condition called thermal runaway that could funnel ALL the current in the diode through one small hot spot. To keep that fault from occurring, this kind of diode has some distributed electrical resistance added to the diode; the diode equation might still fit well, but you have to consider that you're measuring both the diode and the series resistor that's built-in.

At the high end of diode-equation compliance, low noise transistors can follow the equation for about eight decades in current (the deviations from the equation behavior are one part in a hundred million). At the low end, items like point-contact diodes have (literally) different diode area at different biases. Your crystal radio diode might not follow the formula well at all.

In addition to 1N4006 diodes not being good logarithmic-response elements, high current transistors are similarly treated. The 1N4006 has a surge rating of 10A, and the highest current rating I could find for a near-ideal transistor was about the 2A current rating (TIP 31).

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

...Jim Thompson

Well, in the theory there is a factor I=Is e^(Vf/mVt) with m being 1...2 for diodes, which predicts this result. On the upper current range there comes a point where the density of the injected minority carriers equals the impurity concentration, so only majority carriers can flow more. Higher blocking voltage has lower doping and this effect starts early. For very low currents the recombination current(long charge carrier lifetime) becomes higher than the diffusion current and forces m again to 2.

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

Jon

Certainly. I believe someone (was it you, Jon?) mentioned PIN diodes. I believe that the 1N4xxx series is very PIN-like in doping. I'll ask someone at ON Semi.

...Jim Thompson

No, that was me. I learned it here, during a discussion I started about Tom McEwan avalanching them (at LLNL) to get 100ps pulses peaking around 100KW. He used a 1000V step of a couple of ns into a pulse-forming network, and the MOSFET step generator was also extensively discussed.

OK; scratch the 1N4006; the MPSA42 did not conformeither; it just was "closer".

The MPSA42 has a multi-milliamp rating,and should follow the log relationship to at least 10mA or more; i measured at 100uA max - so internal resistance is not significant.

That would explain the large discrepancy noted for the 1N4006; thanks. But the MPSA42????

Did you tie base to collector before measuring as a diode?

...Jim Thompson

Cockpit error ?:-)

...Jim Thompson

At 100uA, the IR drop is not significant.