Avalanche transistor
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 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.
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 Дьяконов (Dyakonov) (1973).
A similar device, named IMPISTOR was described more or less in the same period in the paper of Carrol & Winstanley (1974).
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.
while for the same device working in the active region, basic transistor theory gives the following relation where Equating the two formulas for
Particularly, stray inductances in series with collector and emitter leads have to be minimized to preserve the high speed performance of avalanche transistor circuits.
An avalanche transistor operated by a common bias network is shown in the adjacent picture:
The intrinsic time constant of the basic equivalent small signal circuit has the following value where The two parameters are both negative.
This means that if the collector load const of an ideal current source, the circuit is unstable.
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.
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 Baker (1991).
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 Дьяконов (Dyakonov) (2004b) which relies upon the book Дьяконов (Dyakonov) (1973), 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.
Examples of such models are described in the paper Keshavarz, Raney & Campbell (1993) and in the paper Kloosterman & De Graaff (1989): the latter is a description of the Mextram[1] model, currently used by some semiconductor industries to characterize their bipolar junction transistors.
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).
is set to zero ohm (obviously not both) in order to have a single output, can be found in reference Millman & Taub (1965).
It has usually a high resistance to limit the static collector current, so the recharging process is slow.
Sometimes this resistor is replaced by an electronic circuit which is capable of charging faster the energy storage components.
In practical designs, an adjustable impedance like a two terminal Zobel network (or simply a trimmer capacitor) is placed from the collector of the avalanche transistor to ground, giving the transmission line pulser the ability to reduce ringing and other undesired behavior on the output voltages.
A practical, easily realised, and inexpensive application is the generation of fast-rising pulses for checking equipment rise time.
Consequently, These two requirements imply that a device used for amplification need a physical structure different from that of a typical avalanche transistor.
The current amplification mechanism is the same of the avalanche transistor, i.e. carrier generation by impact ionization, but there is also a transit-time effect as in IMPATT and TRAPATT diodes, where a high-field region travels along the avalanching junction, precisely in along the intrinsic region.
The device structure and choice of bias point imply that The theory for this kind of avalanche transistor is described completely in the paper Eshbach, Se Puan & Tantraporn (1976), which also shows that this semiconductor device structure is well suited for microwave power amplification.
A similar device structure, proposed more or less in the same period in the paper Carrol & Winstanley (1974), was the IMPISTOR, being a transistor with IMPATT collector-base junction.
