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Avalanche transistor
An avalanche transistor is a bipolar junction transistor designed for operation in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector-to-emitter breakdown voltage, called ''avalanche breakdown region''. This region is characterized by avalanche breakdown, a phenomenon similar to Townsend discharge for gases, and negative differential resistance. Operation in the avalanche breakdown region is called avalanche-mode operation: it gives avalanche transistors the ability to switch very high currents with less than a nanosecond rise and fall times (transition times). Transistors not specifically designed for the purpose can have reasonably consistent avalanche properties; for example 82% of samples of the 15V high-speed switch 2N2369, manufactured over a 12-year period, were capable of generating avalanche breakdown pulses with rise time of 350 ps or less, using a 90V power supply as Jim Williams writes.〔("Linear Technology AN47" ), High-speed amplifier techniques, 1991, Appendix D: Measuring probe-oscilloscope response.〕〔("Linear Technology AN94" ), Slew Rate Verification for Wideband Amplifiers The Taming of the Slew"〕
==History==
The first paper dealing with avalanche transistors was : the paper describes how to use alloy-junction transistors in the avalanche breakdown region, in order to overcome speed and breakdown voltage limitations which affected the first models of such kind of transistor when used in earlier computer digital circuits. Therefore the very first applications of avalanche transistors were in switching circuits and multivibrators. The introduction of the avalanche transistor served also as an application of Miller's empirical formula for the avalanche multiplication coefficient , first introduced in the paper : the need of better understanding transistor behavior in the avalanche breakdown region,
not only for using them in avalanche mode, gave rise to an extensive research on impact ionization in semiconductors (see ). From the beginning of the 1960s to the first half of the 1970s, several avalanche-transistor circuits were proposed, and also it was studied what kind of bipolar junction transistor is best suited for the use in the avalanche breakdown region: a complete reference, which includes also the contributions of scientists from ex-USSR and COMECON countries, is the book by . The first application of the avalanche transistor as a linear amplifier, named ''Controlled Avalanche Transit Time Triode'', (CATT) was described in : a similar device, named ''IMPISTOR'' was described more or less in the same period in the paper of . Linear applications of this class of devices started later since there are some requirements to fulfill, as described below: also, the use of avalanche transistor in those applications is not mainstream since the devices require high collector to emitter voltages in order to work properly. Nowadays, there is still active research on avalanche devices (transistors or other) made of compound semiconductors, being capable of switching currents of several tens of amperes even faster than "traditional" avalanche transistors.
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