From my days in EE class (1973~1979) Motorola and RCA referred to avalanche mode as "breakdown that collapses to (near) zero." Much like a 4 layer Shockly diode.

A zener or normal BE/BC breakdown stabilizes at the breakdown voltage, where "avalanche" (as used by Motorola way back when) refers to a switch like action.

The 7nS pulse generator I gave a link to uses a transistor in what I was taught was avalanche mode.

The 90V charges a capacitor through a resistor, at some voltage the C/E junctions avalanche and produce an unholy narrow pulse. I built the circuit using a NE555 driving a complimentary pair driving an 8 ohm to 1K followed by a voltage quad or quint (5) multplier. The transistor type, and I think (if I remember correctly) manufacturer were important. I know not all Motorolas worked, most did and I think no RCA worked.

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