Roy Lewallen wrote:
Zener diodes work fine as varicaps, at least at HF. The lower the
zener voltage and higher the power dissipation rating, the higher the
C. As someone else mentioned, transistor emitter-base junctions can
be used as either zeners (typical zener voltage around 5 volts) or
varicaps.
A zener can be used as a broadband noise source. I've had the best
luck with zeners of 10 - 15 volt breakdown, with around 100 uA
current. Some are noisier than others, and they often have a critical
current where the noise is the greatest.

Tektronix used selected transistors to generate high voltage (~100
volts) fast steps (~100 ps rise time if I recall correctly) by
avalanching the collector. Some fraction of some common transistor
types worked satisfactorily in this application.

1N914 type diodes can be used as step recovery diodes to generate a
step with about a ns risetime -- maybe faster with a chip component
and some care. This could be the basis of a broadband harmonic
generator.
Roy Lewallen, W7EL

At a leading Ultrasonic flaw detector company we used simple low frequency
Motorola sot23 transistors in avalance mode for making a nice pulse
generator for 100MHz probes. These were better than the Zetex avalance
specified transistors.
--
ciao Ban
Apricale, Italy