Those of us who are old enough...
Transistors operating in various breakdown conditions were a fad. Some significant early research (in the late 1950's) looked at it because it offered some interesting benefits.
This is one interesting example.
One feature which makes operating in this mode so interesting, is the exhibition of negative resistance. It should be noted that zener and avalanche effects in diodes, or just a single junction in a transistor do not exhibit this effect.
Jim Williams describes a practical circuit in Appendix B of Linear's
AN-72 (p40-41). Whilst you can do better with tunnel diodes and step recovery diodes, both are far more exotic than a 2N3904 as used
here. The latter article also has a number of very useful links. David Jones (eevblog) has also done at least one video describing this circuit.
The circuit presented in this thread uses breakdown of the base-emitter, rather than collector-base junction. It also lacks any base bias. The practical upshot of this is that the breakdown occurs at a lower voltage, and the rise time is not optimised (for brevity). Both factors are probably advantageous in this circuit.
