I thought I was going to do saturation next, but somehow I've diverted off into looking at the transistor as a diode. This came about from my
referring to Bob Pease's book, Troubleshooting Analog Circuits, to look up something else and then thinking that I'd like to measure for
myself the graph he shows of Vf against If for various diodes, including transistors connected as diodes.
A bipolar junction transistor contains two PN junctions, one between the base and the emitter (B-E) and one between the base and the collector (B-C),
and either can be used as a diode. When used in a circuit, the base-emitter diode normally has the collector shorted to the base (CB-E). It is
also possible to join the collector and emitter (B-CE), giving a final possible configuration (then the two diodes are in parallel).
The following graph shows the B-C B-E and CB-E forms for a CB-E for a BD135(a medium power device intended for audio amplifiers
along with a few common diodes. I've also included a small-signal Schottky diode (BAT42) for comparison. This plot is in the usual form.
This is the same data with the current plotted on a log scale. This has the advantages that we can see the low current detail better and it allows us to see
and compare the forms of the curves.
All the curves are different. What I am measuring evidently isn't just a simple piece of semiconductor physics, though all the curves have a similar
form with a straight section at the lower currents and moving away from that higher up. I'm not going to sample lots of devices, but be aware that
there will be differences between different batches from the same manufacturer and differences between parts marked with the same part number
from different manufacturers.
The two CB-E transistor curves stand out as being almost straight lines over the five decades of current I've done the measurements for. In practice
the relationship holds much further and it points to one of the main uses of transistors in this configuration - linear to log and log to linear conversion,
often in conjunction with an op-amp.