I did the exponential ideality factor test on a IN4001 diode over the weekend - following Millman and Grabel *Microelectronics*

Came out pretty much dead on two

Bob Pease published results from a bunch of different diodes. The 1N4001 he tested had lower conductance than a good diode.

Yep, what Bob's classic paper shows is what old folks have known for multiple decades, the simple 2N3904 Vbe is about as good a PN junction regarding following the classic diode equation as it gets:

Ib = Is[e^Vbe/Vt) -1], where Vt = KT/q

deltaVbe = Vt[ln(ratio Ib)]

Ironically almost all the diodes are really poor in this respect

If you look at the 2N3904 Vbe at <10na and >1ma (L), it's perfectly straight with the correct slope of ~59.2mv/decade. We've used this characteristic in so many ways over the years for all sorts of useful functions from log amps, to temp sensors, even log domain filters and many applications we can't remember.

Edit: The important point here is the

*slope* of the 2N3904 Vbe vs. Ib will be

*identical* for all devices, regardless of the highly device and process variable Is or Vbe. So one should be able to take devices from various lots, manufacters, time frames and achieve identical results wrt the Vbe

*slope*. Theory predicts this and our experience confirms such (altho we haven't measured any devices in decades), quite an amazing parameter that few folks realize.

BTW this is one of the most important aspects of bipolar transistors that analog IC designers utilize. Bob Widlar utilized this "feature" in his brilliant Bandgap Reference, very valuable feature indeed

If one looks at the various plots, the K, L, M show the ideal junction behavior with proper

*slope*, and all are bipolar Vbe junctions!! Also, note the old HP SBD HP5082-2811 plot C, this shows good junction behavior up to ~100ua.

Best,