Asimov wrote:

I never tested a zener when used as varactor but I think these have a
much greater reverse saturation current (even far below breakdown
threshold) and it is this that might cause comparatively more noise
than a conventional diode with a tiny leakage current. Well, at least
that is what the junction noise equations would seem to indicate.

They also have a whale of a lot more capacitance than a conventional
diode. So I don't think it's a fair comparison.

Roy Lewallen, W7EL