Avalanche transistors have probably been known since the earliest days of silicon transistors but I have never heard of them being implemented with germanium devices, though some readers may know otherwise.

One important use for them was in creating extremely fast, narrow pulses to drive the sampling gate in a sampling oscilloscope.

Such oscilloscopes provided, in the late 1950s, the then incredible bandwidth of 2 GHz, at a time when other oscilloscopes were struggling, with distributed amplifiers and special cathode ray tubes, to make a bandwidth of 85 MHz.

Admittedly those early sampling oscilloscopes were plagued by possible aliased responses and, inconveniently, needed a separate external trigger, but they were steadily developed over the years, providing, by the 1970s, a bandwidth of 10–14 GHz.

The latest digital sampling oscilloscopes provide bandwidths of up to 50 GHz, although like their analog predecessors they are limited to displaying repetitive waveforms, making them inappropriate for some of the more difficult oscilloscope applications, such as glitch capture.

The basic avalanche transistor circuit is very simple, and a version published in the late 1970s (Ref. 1) apparently produced a 1 Mpulse/sec pulse train with a peak amplitude of 11 V, a halfamplitude pulse width of 250 ps and a risetime of 130 ps.

This with a 2N2369, an unremarkable switching transistor with a 500 MHz ft and a Cobo of 4 pF. The waveform, reproduced in the article, was naturally captured on a sampling oscilloscope.

Interest in avalanche circuits seems to have flagged a little after the 1970s, or perhaps it is that the limited number of specialised uses for which they are appropriate resulted in the spotlight always resting elsewhere.

Another problem is the absence of transistor types specifically designed and characterised for this application. But this situation has recently changed, due to the interest in high-power laser diodes capable of producing extremely narrow pulses for ranging and other purposes, in Pockel cell drivers, and in streak cameras, etc.

Two transistors specifically characterised for avalanche pulse operation, types ZTX413 and ZTX415 (Ref. 2), have recently appeared, together with an application note (Ref. 3) for the latter.

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