Dave, your considerable effort to explain the nuances of diodes to
John
is commendable, but your explanation is rather misleading. It's not
true
that for a diode to conduct, the "barrier potential must be
exceeded,"
and "the junction becomes forward biased and conducts heavily."
Instead
the diode current has an exponential relationship to the voltage
across
it, and gradually turns on over many hundreds of millivolts, not
abruptly
at say 600mV. Here, examine some diode measurements I made a long
time
ago, http://www.picovolt.com/win/elec/com...de-curves.html

The theory confirms your results.

I diode ~K*exp(Vsignal/Vt) =K+ a*Vsignal + b*Vsignal^2+ .... by the
binomial theorem

So at low signal amplitudes the diode current follows a linear
relationship as predicted by the diode equation and confirmed by the
graphs there. At higher values the higher terms start dominating and
the exponential term takes over.