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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).

History

The first paper dealing with avalanche transistor was : the paper describes how to use alloy-junction transistor 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 transistor 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

M

, 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 a 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 . 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 . 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.

Basic theory

Static avalanche region characteristics

In this section, the

I_C-V_{CE}

static characteristic of an avalanche transistor is calculated. For the sake of simplicity, only an NPN device is considered: however, the same results are valid for PNP devices only changing signs to voltages and currents accordingly. The analysis closely follows that of William D. Roehr in . Since avalanche breakdown multiplication is present only across the collector-base junction, the first step of the calculation is to determine collector current as a sum of various component currents though the collector since only those fluxes of charge are subject to this phenomenon. Kirchhoff's current law applied to a bipolar junction transistor implies the following relation, always satisfied by the collector current

I_C

I_C=I_E-I_B,

while for the same device working in the active region, basic transistor theory gives the following relation

I_C=beta I_B+(beta+1)I_{CBO},

where

$I_B$ is the base current,
$I_{CBO}$ is the collector-base reverse leakage current,
$I_E$ is the emitter current,
$beta$ is the common emitter current gain of the transistor.

Equating the two formulas for $I_C$ gives the following result

I_E = (beta + 1)I_B + (beta + 1)I_{CBO},

and since $alpha = beta{(beta +1)^{-1}}$ is the common base current gain of the transistor, then

alpha I_E = beta I_B + beta I_{CBO} = I_C - I_{CBO} iff I_C = alpha I_E + I_{CBO}

When avalanche effects in a transistor collector are considered, the collector current $I_C$ is given by

I_C=M(alpha I_E +I_{CBO}),

where $M$ is Miller's avalanche multiplication coefficient. It is the most important parameter in avalanche mode operation: its expression is the following

M = {frac{1}{1-left(frac{V_{CB}}{BV_{CBO}}right)^{n}}},

where

$BV_{CBO}$ is the collector-base breakdown voltage,
$n$ is a constant depending on the semiconductor used for the construction of the transistor and doping profile of the collector-base junction,
$V_{CB}$ is the collector-base voltage.

Using again Kirchhoff's current law for the bipolar junction transistor and the given expression for $M$ , the resulting expression for $I_C$ is the following

I_C=frac{M}{1-alpha M}(I_{CBO} + alpha I_B)iff I_C =frac{I_{CBO} + alpha I_B}{1-alpha - left(frac{V_{CB}}{BV_{CBO}}right)^{!n} }

and remembering that $V_{CB} = V_{CE} - V_{BE}$ and $V_{BE} = V_{BE}(I_B)$ where $V_{BE}$ is the base-emitter voltage

I_C =frac{I_{CBO} + alpha I_B}{1-alpha - left(frac{V_{CE}-V_{BE}(I_B)}{BV_{CBO}}right)^{!n} }cong frac{I_{CBO} + alpha I_B}{1-alpha - left(frac{V_{CE}}{BV_{CBO}}right)^{!n} }

since $V_{CE}>>V_{BE}$ : this is the expression of the parametric family of the collector characteristics $I_C-V_{CE}$ with parameter $I_B$ . Note that $I_C$ increases without limit if

left(frac{V_{CE}}{BV_{CBO}}right)^{!n}= 1-alpha iff V_{CE}=BV_{CEO} = sqrt[n]{(1-alpha)}BV_{CBO}=frac{BV_{CBO}}{sqrt[n]{beta+1}}

where $BV_{CEO}$ is the collector-emitter breakdown voltage. Also, it is possible to express $V_{CE}$ as a function of $I_C$ , and obtain an analytical formula for the collector-emitter differential resistance by straightforward differentiation: however, the details are not given here.

Differential dynamical model

The differential dynamical mode described here, also called the small signal model, is the only intrinsic small signal model of the avalanche transistor. Stray elements due to the package enclosing the transistor are deliberately neglected, since their analysis wolud not add anything useful from the point of view of the working principles of the avalanche transistor. However, when realizing a electronic circuit, those parameters are of great importance: particularly stray inductances in series to collector and emitter leads have to be minimized to preserve the high speed performance of avalache transistor circuits. Also, this equivalent circuit is useful when describing the behavior of the avalanche transistor near its turn on time, where collector currents and voltages are still near their quiescent values: in the real circuit it permits the calculation of time constants and therefore rise and fall times of the $V_{CE}$ waveform. However, since avalanche transistor switching circuits are intrinsically large signal circuits, the only way to predict with reasonable accuracy their real behaviour is to do numerical simulations. Again, the analysis closely follows that of William D. Roehr in .

An avalanche transistor operated by a common bias network is shown in the picture on the right: $V_{BB}$ can be zero or positive value, while $R_E$ can be short circuited. In every avalanche transistor circuit, the output signal is taken from the collector or the emitter: therefore the small-signal differential model of an avalanche transistor working in the avalanche region is always seen from the collector-emitter output pins, and consist of a parallel $RC$ circuit as shown in the picture on the right, which includes only bias components The magnitude and sign of both those parameters are controlled by the base current $I_B$ : since both Base-Collector and Base-Emitter junctions are inversely biased in the quiescent state, the equivalent circuit of the Base input is simply a current generator shunted by Base-Emitter and Base-Collector junction capacitances and is therefore not analyzed in what follows. The intrinsic time constant of the basic equivalent small signal circuit has the following value

tau_{Ace}=r_{Ace}C_{Ace},

where

$r_{Ace}$ is the collector-emitter avalanche differential resistance and, as stated above, can be obtained by differentiation of the collector-emitter voltage $V_{CE}$ respect to the collector current $I_C$ , for a constant base current $I_B$

r_{Ace}=frac{partial{V_{CE}}}{partial{I_C}}Bigg|_{I_B=const.}

$C_{Ace}$ is the collector-emitter avalanche differential capacitance and has the following expression

C_{Ace}=-left(frac{1}{r_{Ace}omega_beta}-C_{ob}right)

where

omega_beta=2pi f_beta

is the current gain angular cutoff frequency

C_{ob}

is the common base output capacitance

The two parameters are both negative: this means that if the collector load const of an ideal current source, the circuit is unstable. This is the theoretical justification of the astable multivibrator behavior of the circuit when the

V_{CC}

voltage is raised over some critical level.

Second breakdown avalanche mode

When the collector current rises above the data sheet limit

I_{CMAX}

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 this points: this exponential rise of current in turn gives rise to even more overheating, originating a positive thermal feedback mechanism. While analyzing the

I_C-V_{CE}

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 .

Numerical simulations

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 this 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 consist in using the common analog circuit simulator SPICE together with an advanced transistor model supporting avalanche breakdown simulations, since 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

A graphical method for studying the behavior or 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.

Applications

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).

Avalanche mode switching circuits

Avalanche mode switching relies on avalanche multiplication of current flowing through the collector-base junction as a result of impact ionization of the atoms in the semiconductor crystal lattice. Avalanche breakdown in semiconductors and has found application in switching circuits for two basic reasons

it can provide very high switching speeds, since current builds-up in very small times, in the picosecond range, due to avalanche multiplication.
It can provide very high output currents, since large currents can be controlled by very small ones, again due to avalanche multiplication.

The two circuits considered in this section are the simplest examples of avalanche transistor circuits for switching purposes: both the examples detailed are monostable multivibrators. It is possible to find several more complex circuits in the literature, for example in the books and . First, it is worth noting that the largest part of circuits employing an avalanche transistor is activated by the following two different kind of inputs:

Collector triggering input circuit: the input trigger signal is fed to the collector via a fast switching diode $D_S$ , possibly after being shaped by a pulse shaping network. This way of driving an avalanche transistor was extensively employed in first generation circuits since the collector node has a high impedance and also collector capacitance $C_{ob}$ behaves quite linearly under large signal regime. As a consequence of this, the delay time from input to output is very small and approximately independent of the value of control voltage. However, this trigger circuit requires a diode capable of resist to high reverse voltages and switch very fast, characteristics that are very difficult to realize in the same diode, therefore it is rarely seen in modern avalanche transistor circuits.
Base triggering input circuit: the input trigger signal is fed directly to the base via a fast switching diode $D_S$ , possibly after being shaped by a pulse shaping network . This way of driving an avalanche transistor was relatively less employed in first generation circuits because the base node has a relatively low impedance and an input capacitance $C_{ib}$ which is highly nonlinear (as a matter of fact, it is exponential) under the large signal regime: this causes a fairly large, input voltage dependent, delay time, which was analyzed in detail in the paper . However, the required inverse voltage for the feed diode is far lower respect diodes to be used in collectior trigger input circuits, and since ultra fast Schottky diodes are easily and cheaply found, this is the driver circuit employed in most modern avalanche transistor circuit. This is also the reason why the diode $D_S$ in the following applicative circuits is symbolized as a Schottky diode.

Avalanche transistor can also be triggered by lowering the emitter voltage $V_E$ , but this configuration is rarely seen in the literature and in practical circuits.: in reference , paragraph 3.2.4 "Trigger circuits" it is described one of such configuration where the avalanche transistor is used itself as a part of the trigger circuit of a complex pulser, while in reference a balanced level discriminator where a common bipolar junction transistor is emitter-coupled to an avalanche transistor is briefly described.

The two avalanche pulser described below are both base triggered and have two outputs. Since the device used is a npn transistor, $V_{out1}$ is a positive going output while $V_{out2}$ is a negative going output: using a pnp transistor reverses the polarities of outputs. The descripion of their simplified versions, where resistor $R_E$ or $R_L$ is set to zero ohm (obviously not both) in order to have a single output, can be found in reference . Resistor $R_C$ recharges the capacitor $C_T$ or the transmission line $scriptstyle TL_{t_f}$ (i.e. the energy storage components) after commutation: it has usually a high resistance in order 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: however this kind of circuits usually are patented so they are rarely found in mainstream application circuits.

Capacitor discharge avalanche pulser: a trigger signal applied to the base lead of the avalanche transistor cause the avalanche breakdown between the collector and emitter lead. The capacitor $C_T$ starts to be discharged by a current flowing through the resistors $R_E$ and $R_L$ : the voltages across those resistors are the output voltages. The current waveform is not a simple RC discharge current but has a complex behavior which depends on the avalanche mechanism: however it has a very fast rise time, of the order of fractions of nanosecond. Peak current depends on the size of the capacitor $C_T$ : when its value is raised over a few hundred picofarads, transistor goes in to second breakdown avalanche mode, and peak currents reach values of several amperes if not tens of amperes.
Transmission line avalanche pulser: a trigger signal applied to the base lead of the avalanche transistor cause the avalanche breakdown between the collector and emitter lead. The fast rise time of the collector current generates a current shock wave of approximatively the same amplitude, which propagates along the transmission line: the wave reaches the open circuited end of the line after the characteristic delay time $t_f$ of the line has elapsed, and then is reflected backward. If the characteristic impedance of the transmission line is equal to the resistances $R_E$ and $R_L$ , the backward reflected shock wave reaches the beginning of the line and stops. As a consequence of this traveling wave behavior, the current flowing through the avalanche transistor has a rectangular shape of duration

t=2t_f,

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 to the tramission line pulser the ability of reduce ringing and other undesidered behavior on the output voltages.

It is possible to turn those circuits into astable multivibrators by removing their trigger input circuits and

raising their power supply voltage $V_{CC}$ until a relaxation oscillation begins, or
connecting the base resistor $R_B$ to a positive base bias voltage $V_{BB}$ and thus forcibly starting avalanche breakdown and associated relaxation oscillation

A well detailed example of the first procedure is described in reference . It is also possible to realize avalanche mode bistable multivibrators, but their use is not as common as other tipes described of multivibrators, one important reason being that they require two avalanche transistors, one working continuously in avalanche breakdown regime, and this can give serious problems from the point of wiev of power dissipation and device operating life.

The Controlled Avalanche Transit-time Triode (CATT)

Avalanche mode amplification relies on avalanche multiplication as avalanche mode switching. However, for this mode of operation, it is necessary that Miller's avalanche multiplication coefficient

M

be kept almost constant for large output voltage swings: if this condition is not fulfilled, significant amplitude distortion arises on the output signal. This implies that

avalanche transistors used for application in switching circuits cannot be used since Miller's coefficient varies widely with the collector to emitter voltage
the operating point of the device cannot be in the negative resistance of the avalanche breakdown region for the same reason

These two requirements imply that a device used for amplification need a physical structure different from that of a typical avalanche transistor. The Controlled Avalanche Transit-time Triode (CATT), designed for microwave amplification, has a quite large lightly doped region between the base and the collector regions: this implies that the device has fairly high collector-emitter breakdown voltage $BV_{CEO}$ respect to bipolar transistors having the same geometry. 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

Miller's avalanche multiplication coefficient $M$ is limited to about 10.
The transit-time effect keep this coefficient almost constant and independent of the collector to emitter voltage.

A complete description of the theory for this kind of avalanche transistor is available in the paper : in this paper it is also showh that this semiconductor device structure is well suited for microwave power amplification. It can deliver several watts of radio frequency power at a frequency of several gigahertz and it also features a control terminal (the base). However its use is not mainstream as already said, since it requires high voltages (greather than 200 volts) to work properly, while nowadays gallium arsenide or other compound semiconductors FETs deliver a similar performance while being easier to work with. A similar device structure, proposed more or less in the same period in the paper , was the IMPISTOR, being a transistor with IMPATT collector-base junction.

References

. A clear description of avalanche transistor circuits working in the second breakdown region.
- . The first article describing the working principles and potential applications of the CATT.
  - . Sections 3.1.5 "Avalanche transistors", 3.2 and 3.4 "Trigger circuits containing avalanche transistors".
    - . Mainly sections 6.9, 6.10, 12.10, 13,16, 13.17.
      - . Chapter 9 "Avalanche mode switching".
        The ZTX413 Avalanche Transistor Zetex Semiconductor Design Note 24, October 1995.
        
        The ZTX415 Avalanche Mode Transistor Zetex Semiconductors Application Note 8, January 1996.
        Bibliography
        . The first paper analyzing the use of bipolar junction transistors in the avalanche region.
        . A paper containing an accurate analysis of the avalanche breakdown phenomenon in planar pn-junctions, as those found in almost all modern transistors.
        . The paper where the above formula for the avalanche multiplication coefficient $M$ first appeared.
        . (Zipped djvu format) A very scarce book worth a look, especially for the Russian reader: in-dept coverage of the theory of avalanche transistor, analysis of practical circuits, and a rich bibliography of 125 titles.
        External links
        
        Theory
        . 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 cicuit 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).
        Applications
        . 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 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, University of Oulu, ISBN 951-42-7261-7. 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 TOF (Time Of Flight) 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 seies-connected avalanche transistor circuits.
        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.
        Varia
        Russel Jacob Baker Academic Web Page at Boise State University. 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.
        
        Paolo Spirito Academic Web Page at University of Naples Federico II. One of the leading contributor to the theory of avalanche transistors and avalanche process in semiconductors.