Лавинные транзисторы: принцип работы, история и современные исследования.

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, which is 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.

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 for better understanding transistor behavior in the avalanche breakdown region,
not only for use 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. The kind of bipolar junction transistor best suited for use in the avalanche breakdown region was studied. 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. The use of avalanche transistors 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, capable of switching currents of several tens of amperes even faster than "traditional" avalanche transistors.

When the collector current rises above the data sheet limit a new breakdown mechanism become important: the second breakdown. This phenomenon is caused by excessive heating of some points (hot spots) in the base emitter region of the bipolar junction transistor, which give rise to an exponentially increasing current through these points: this exponential rise of current in turn gives rise to even more overheating, originating a positive thermal feedback mechanism. While analyzing the static characteristic, the presence of this phenomenon is seen as a sharp collector voltage drop and a corresponding almost vertical rise of the collector current. At the present, it is not possible to produce a transistor without hot spots and thus without second breakdown, since their presence is related to the technology of refinement of silicon. During this process, very small but finite quantities of metals remain in localized portions of the wafer: these particles of metals became deep centers of recombination, i. e. centers where current exists in a preferred way. While this phenomenon is destructive for Bipolar junction transistors working in the usual way, it can be used to push up further the current and voltage limits of a device working in avalanche mode by limiting its time duration: also, the switching speed of the device is not negatively affected. A clear description of avalanche transistor circuits working in second breakdown regime together with some examples can be found in the paper .

Avalanche transistor circuits are intrinsically large signal circuits, so small signal models, when applied to such circuits, can only give a qualitative description. To obtain more accurate information about the behavior of time dependent voltages and currents in such circuits it is necessary to use numerical analysis. The "classical" approach, detailed in the paper which relies upon the book , consists in considering the circuits as a system of nonlinear ordinary differential equations and solve it by a numerical method implemented by a general purpose numerical simulation software: results obtained in this way are fairly accurate and simple to obtain. However, these methods rely on the use of analytical transistor models best suited for the analysis of the breakdown region: those models are not necessarily suited to describe the device working in all possible regions. A more modern approach is to use the common analog circuit simulator SPICE together with an advanced transistor model supporting avalanche breakdown simulations, which the basic SPICE transistor model does not. Examples of such models are described in the paper and in the paper : the latter is a description of the Mextram model, currently used by some semiconductor industries to characterize their bipolar junction transistors.

A graphical method for studying the behavior of an avalanche transistor was proposed in references and : the method was first derived in order to plot the static behavior of the device and then was applied also to solve problems concerning the dynamic behavior. The method bears the spirit of the graphical methods used to design tube and transistor circuits directly from the characteristic diagrams given in data sheets by producers.

Avalanche transistors are mainly used as fast pulse generators, having rise and fall times of less than a nanosecond and high output voltage and current. They are occasionally used as amplifiers in the microwave frequency range, even if this use is not mainstream: when used for this purpose, they are called "Controlled Avalanche Transit time Triodes" (CATTs).

. A paper proposing and describing the IMPISTOR, a semiconductor device similar to the CATT. A paper analyzing the volt amperometric characteristic of diodes and transistors using the computer algebra program Mathematica. A paper about the design of an avalanche transistor relaxation oscillator using the computer algebra program Mathematica A brief description of the basic physical principles of avalanche transistor circuits: instructive and interesting but "restricted access". A theoretical study of the stability of a transistor biased in the avalanche region (restricted access). available from the U. S. Department of Energy Office of Scientific & Technical Information. A report describing a transistor model capable of including avalanche effects in SPICE simulations. A paper describing the Mextram SPICE model from the point of view of avalanche behavior simulation. For a free copy found in the Mextram home page of NXP see here. A paper describing a transistor model for bipolar circuit simulation including avalanche effects (restricted access). Jochen Riks "Avalanche Transistor" (in German). A brief description of the working principles of the avalanche transistor, part of the course "Impulsschaltungen F Praktikum EXP 10", June 1996, Fachschaft Physik Uni Düsseldorf. A paper proposing a graphical method to plot the static characteristic of an avalanche transistor (restricted access). A paper pushing further the study of avalanche transistor by a graphical method proposed in the preceding work (restricted access). A paper analyzing the trigger delay time of avalanche transistors by means of numerical analysis (restricted access). A paper where an analytical model of the behavior of an avalanche transistor is derived after suitable approximations (restricted access).

. A paper describing a fast sweep generator for a streak camera constructed using series connected avalanche transistor circuits. A paper describing an application of avalanche transistors to the design of a sampling oscilloscope: available abstract, full paper is "restricted access". Available from the U. S. Department of Energy Office of Scientific & Technical Information. A report describing the design of a driver for Pockels cells Q switches. A project, inspired to Jim Williams' Linear Technology application notes AN72 and AN94, of an avalanche transistor astable multivibrator with schematics, waveforms and photos of the layout. Academic Dissertation presented with the assent of the Faculty of Technology. A doctoral dissertation describing a Laser TOF (Time Of Flight) Radar and its construction using an avalanche transistor pulser. (preprint version here). A paper describing an avalanche transistor pulser and its use as Laser driver in a laser radar. NXP Mextram home page A very rich repository of documents about the Mextram bipolar junction transistor SPICE model, capable of avalanche breakdown behavior simulation. "Operating the pulsed laser diode SPL LLxx", "Range finding using pulsed laser diodes" OSRAM Opto Semiconductors GmbH Application Notes, 2004 09 10. Two application notes from Osram Opto Semiconductors describing pulsed operation of a Laser diode, using avalanche transistors and other kind of drivers. A paper describing a method to enhance performances of banks of series connected avalanche transistor circuits. (for a PDF copy, see here). A detailed paper describing the construction and performance of an avalanche transistor pre trigger pulse generator to test the slew rate of very fast operational amplifiers. Also appeared under the title "Slew Rate Verification for Wideband Amplifiers The Taming of the Slew", application note AN94, Linear Technology, May 2003. See also, from the same author, Linear Technology application note AN47, High speed amplifier techniques", August 1991, where an astable circuit similar to that described by Holme is detailed in appendix D, pages 93–95.

R. Jacob Baker Academic Web Page at University of Nevada, Las Vegas. A contributor to the theory and applications of avalanche transistors. Владимир Павлович Дьяконов (Vladimir Pavlovich D'yakonov) (in Russian). Some biographical notes about one of the leading contributors to the theory and application of avalanche transistors. Ari Kilpelä Academic Web Page at the University of Oulu. A researcher working on theory and applications of avalanche transistor circuits.