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I have often seen it stated and discussed that a diode-connected BJT has an ideality factor (n) closer to 1 in Shockley's diode equation when compared with a simple pn diode's ~2/3.

Why is this?

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    \$\begingroup\$ High level injection and sometimes also bulk Ohmic resistance makes the diode less ideal. Diode connected BJTs usually aren't used at those kinds of levels and they are made differently. (Their reverse breakdown is pretty low.) The factor is a "tweak" of sorts that gets modeling them okay over their specified range of use. \$\endgroup\$ – jonk Dec 29 '19 at 1:44
  • \$\begingroup\$ If you want a much more serious answer, I can provide the old papers you can read on a variety of more physically real effects and associated construction details. But it complicates the analysis a lot and the factor suffices for most uses. \$\endgroup\$ – jonk Dec 29 '19 at 1:47
  • \$\begingroup\$ @jonk I'm eventually interested by your "old papers". Do you have references or ideally links ? \$\endgroup\$ – andre314 Dec 29 '19 at 10:02
  • \$\begingroup\$ @jonk Thanks for your response. I too would be interested in seeing these papers. \$\endgroup\$ – Benjamin Crawford Ctrl-Alt-Tut Dec 29 '19 at 13:58
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Diodes come in low-current types, that have a point-contact-like geometry, for high speed operation. The depletion region in such diodes is a hemisphere, not a uniform slab, and that deviates from the diode equation geometry. Diodes also come in high-current types, which often have intentional resistive layers, in order to prevent hot spot formation. The resistive features are not part of the diode equation either.

Small-signal transistors, however, have a large flat thin base region (which is the right geometry) and minimal resistance in the emitter, and low-resistance collector metallization, so the junctions DO fit the equation, even if not specifically designed to do so. It's accidentally the better 'ideal diode' because of features frequently designed into diodes.

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