Mesoplasmas and “second breakdown” in silicon junctions

Solid-State

Electronics

Pergamon Press 1963. Vol. 6, pp. 511-521.

MESOPLASMAS

Printed in Great Britain

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A. C. ENGLISH Institut fiir Hohere Elektrotechnik, Eidgenossische Technische Hochschule ,Ziirich, Switzerland, and Department of Electrical Engineering, University of California, Berkeley (Received

5 April

1963)

Abstract-Mesoplasma breakdown in silicon p-n junctions is analyzed on the basis of a stable microscopic molten sphere of semiconductor controlling the electrical properties of the breakdown region. The size of the melt ranges from a radius of a few p upwards for a power input above 1 W. An explanation of transistor “second breakdown” is given. RBsumB-La rupture du mesoplasma dans les jonctions de silicium p-n est analysee sur la base d’une sphere de semiconducteur fondue, stable et microscopique contralant les proprietes Clectriques de la region de rupture. Les dimensions de la fonte vont d’un rayon de quelques p et audessus pour une puissance d’entree superieure a 1 W. Une explication de la “seconde rupture” due transistor est donnee. Zusammenfassung-Der Mesoplasma-Durchbruch in Silizium p-n-tfbergangen wird analysiert unter Annahme einer stabilen mikroskopischen geschmolzenen Halbleiter-Kugel, die die elektrischen Eigenschaften der Durchbruchszone steuert. Die Schmelze hat bei einer Eingangsleistung von 1 W einen Radius, dessen Grossenbereich iiber wenigen p liegt. Es folgt eine Erkliinmg des “sweiten Durchbruchs” in Transistoren.

INTRODUCTION

IN 1958, THORNTON and SIMMONS(~) reported the existence of a form of breakdown in transistors which appears as a sudden transition to a high current, low voltage mode of operation in the collector junction, occurring at a collector current of value, Im, which is a decreasing function reverse bias voltage at the base and which increases as a function of frequency. In studying the physical effects of subjecting a device to one or more cycles (SB)@) condition in this “second breakdown” they found that (a) the effect may be stable and reproducible from the moment of its initiation, (b) the collector breakdown voltage may fall to a low, stable, value, which can occasionally be improved by etching or voltage cycling, (c) Im may diminish, with several breakdown cycles, to a final low, stable, value which they claim could not be improved, 511

(d) a short circuit may develop between emitter and collector, although they may still be rectifying relative to the base. They found, in case (d), cavities in the transistor, presumably due to a melting through between emitter and collector due to high local heating. For this destructive case alone they suggest that the recrystallized germanium melts, but they do not mention, as later authors do,(a) the stronger case for an alloy penetration, especially where there are indium contacts. Their mention of a possible ‘fnew family of thyratron devices” clearly implies that no melting effect is contemplated for stable, reversible, SB. SCHAFFT and FRENCHt2) pursued the investigation further, in an effort to resolve conflicting experimental evidence and several proposed mechanisms. They find that all transistors with adequate leads can be driven into the SB condition. Im is found to be dependent on base current,

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frequency, and series resistance and, in contradiction to TS,(rJ a function of temperature. Switching speed for the effect is not well defined and appears to be temperature sensitive. They introduce a new effect, a delay time, 7, observed when the collector current is suddenly switched on, to a value greater than the d.c. Im, when the transition to the SR condition is delayed for times as long as 150 msec. 7 is a decreasing function of current, temperature, and reverse base current. The earlier suggestion(r) that SB is due to a of the current in “pinch-in ” (334)or concentration the center of the transistor is rejected on the grounds that pinch-in can hardly be important for open-base or forward bias conditions. (It might be added that pinch-in should not be important in a highly elongated geometry). They reject surface effects as a primary cause, in view of the nondependence of SB on surface treatment, and the frequency of breakdown far from a surface. The pnpn model@ is opposed on the basis that the “on” dynamic resistance is negative and that the voltage drop is too high. The effect of base current on Im seems to be contrary to that expected from this model and the switching time is far too long. SF’s(s) principal contribution is their idea of an a critical energy, E,, which “energy concept”, must be dissipated in the device before SB will be initiated. They find that Em has a very similar dependence on temperature, base current and collector current to that of the delay time, as could have been predicted from the derivation of E, from current, voltage and 7. The argument is advanced to the point where the “energy seems to be in a sense exchangeable with temperature” and to the deduction that “some thermal effect may be significant to the mechanism involved in SB”. In the absence of a definite physical model, further progress was not to be expected. The fact is that the data reported for transistors are far too complex due to irrelevant side-effects, as an inspection of the oscilloscopic photographs in TS and SF will show. Localized breakdown at the collector results in a large majority carrier current in the adjacent base region, but there may at the time be a large minority carrier component injected by the emitter and collected over a much larger area of the collector. The influence of base current on, for example,@) Im, may

merely reflect a change in the component of lnr which is not strongly involved in the local breakdown. Thus the highly idealized Fig. 3 of TS would represent the reduction of almost 2 A of minority carrier current from the emitter as VHn is increased (reverse base current increasing) superimposed upon an undetected component, tens of milliamperes, active in the breakdown region. It is clear, then, that the observation@J of an apparently identical electrical phenomenon in the simpler configuration of the diode, with the concurrent microscopic observation of “giant micro-plasma” visible radiation, is of fundamental importance in the resolution of the SB problem. In this paper, experimental and theoretical “6) are given and the details on “mesoplasmas results are used to give a full accounting of the SB effect in transistors. EXPERIMENTAL

Silicon p-n junction experiments The phenomenon of mesoplasma breakdown is quite readily obtained at current levels of about 200 mA on fragments of 10 V silicon diffusedjunction Zener diodes.* The fragment is clamped between metal electrodes in an orientation suitable for microscopic inspection of the junction on a fractured face and current-voltage characteristics obtained either on an X-Y recorder under d.c. conditions or on an oscilloscope with any combination of a.c., d.c. and switching conditions. The simplest example of the phenomenon, from an X-Y plot, is shown in Fig. 1. A reproducible high current low voltage mode of operation appears at the rapid transition, A, and disappears if the current level is lowered to B. Under the microscope in the dark, typical microplasmas are visible(T) up to the point of switching. When the transition occurs, the microplasmas dim or go out, as would be expected in view of the drop in voltage, and a large red spot appears at the junction, visible to the naked eye. The mesoplasma glow is quite diffuse, but if the microscope is first focussed on neighboring microplasmas, a definite sharpening of the * Samples used were p-type, phosphorus-diffused, from which nickel-plate electrodes were removed with aqua regia. Diodes were fragmented with n sharp blow and mm-size pieces used.

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contour is obtained by refocussing nearer to the specimen, as though the glow originates a little below the surface. It is hard to say whether the optics of the system permit a firm decision on this point. With increasing current, the intensity and size of the red glow increase. While the effect is most readily obtained at fractured surfaces it is also observed on unbroken diodes and in at least one case was seen through the diffused layer rather than at the junction periphery. An insulated metal wire point brought into contact at the glowing spot displaces it to a new position, from which it may or may not return on removal of the wire.

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spots are observed under the microscope, with relative intensities qualitatively corresponding to the electrical duty cycles. This is the only circumstance in which more than one red spot can be seen at, apparently, the same time.

FIG. 2. An example of a two-position mesoplasma. A. Position 1. B. Positions 1 and 2 occupied at different times during the cycle. C. Position 2. Small a.c. voltage superimposed on d.c. voltage.

0

2 REVERSE

6

4 VOLTAGE

[V]

FIG. 1. Mesoplasma breakdown in a fragment of a 10 V Zener silicon diode. Points marked by 0 are obtained by subtracting the low-current mode line from the high-current mode.

It is often observed that the mesoplasma spontaneously moves as the current is increased. In one case the spot occupied five different reproducible positions along the line of the junction. Upon decreasing the current, the spot normally remains until switch-off in whatever position it has reached, and does not reverse the sequence of shifts. Figure 2 shows the electrical characteristics under a.c. conditions of a twoposition mesoplasma where each state has its own high-current-mode line. In Fig. 2(b), where both positions are occupied during the cycle, two red

Using terminology from the transistor papers,(lls) Im, the “trigger” current at the point of mesoplasma switch-on, increases with frequency for a.c. conditions. I, and the corresponding V, are decreasing functions of the ambient temperature. (It must be recognized that the specimen temperature is highly inhomogeneous, very sensitive to the heat transfer conditions to the electrodes, and on small samples difficult to measure. A typical measured temperature is over 200°C). Figures 3 and 4 show the variation in Im and V, below room temperature. The delay time(a), 7, can be demonstrated in the diode case by switching to a d.c. current level higher than the d.c. value of Im. 7 is a decreasing function of the current and the ambient temperature, (Figs 5 and 6). The delay time also varies with the intensity and direction of a magnetic field but no clear cut relationship has yet been established. In addition, superficial experiments were performed at various reduced pressures down to 0.1 TVbut since temperature was not controlled and since the experiments were rather long and apparently not reversible, little can be said about the observed variations in the delay time with ambient gas pressure. The on- and off-transients (A and B in Fig. 1) are approximately exponential with time and, independent of the parameters to which the

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delay time is sensitive, have time constants of about 60 psec. Another important time constant is the response time of the mesoplasma to a small change in

electrical conditions, to be distinguished from the case of establishing the mesoplasma. For this purpose, current and voltage transients may be displayed oscilloscopically upon switching small changes in the circuit series resistance, (Figs 7 and 8). It will be observed that after a transient

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temperature dependence current.

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TEMPERATURE FIG. 4. Ambient

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Variation of delay time \vith ambient temperature . . . Swtched d.c. current, 36-t rn.4

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change that is controlled only by circuit constants, current and voltage change exponentially, in opposite directions, with a time constant of about 1 psec. Again, this time constant is not very sensitive to environmental conditions.

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A kind of artificial mesoplasma has been obtained by bringing into contact with the diffused layer of the junction a wire point at the potential of the base. The properties of this mesoplasma have not yet been studied but it is clear that the wire point produces the high current density which is pre-requisite to the mesoplasma.

ANALYSIS

FIG. 7. Voltage and current transients for a mesoplasma upon reducing (A, B) or increasing (C, D) the series resistance. Numbers correspond to Fig. 8.

VOLTAGE

FIG. 8. Schematic representation of transients in Fig. 7, to which numbers correspond. (With acknowledgments to H. M. POWER.)

It is necessary first to dispose of the more intractable parts of the problem before attempting to fit a physical model to the experimental results. The pre-breakdown region and Im are especially difficult to handle in anything but a qualitative way since there is no way of estimating what fraction of the current passes through the mesoplasma region and what through the rest of the junction. That the latter component is not negligible may be argued from the fact that after a mesoplasma has ceased to appear due to aging effects, the steep breakdown region of the junction characteristic does not change drastically except to extrapolate beyond Im. The situation is quite different for the mesoplasma condition, where at a significantly lower voltage most of the current must pass through the mesoplasma. The “time constant” for switching the mesoplasma on or off, 60 psec, is probably thoroughly confused by time-varying power input, as the current at the mesoplasma increases from tens of milliamperes at Im to hundreds when fully on, and by the most violent changes in local physical conditions. The time constant for small changes in the mesoplasma is presumably more amenable to analysis. The observation of a strongly glowing spot with current magnitude 1000 times greater than a typical microplasma demands a specific mechanism for the production of radiation. While avalanche or Zener breakdown with some modified form of microplasma radiation is conceivable, the total phenomenon is enough orders of magnitude different in power dissipation, light intensity and time constants that a completely different mechanism is indicated. Of the two possible alternatives to hot electrons in the solid, radiation from a high temperature solid or radiation from a gaseous arc at an outer surface, the latter is immediately eliminated by the fact that mesoplasmas can

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persist, though not inevitably, at pressures as low as 0.1 cc. The physical model which fully explains the mesoplasma behavior happens to be derived from a related case in selenium,@.s) in spite of the intrinsic differences between the two semiconductors. It is based upon the concept that under appropriate breakdown conditions a thermal “runaway” occurs, which, either naturally for selenium, or through circuit resistance for silicon and germanends at a “stabilized” condition. This ium, “stabilized breakdown” (S@(s) or “second breakdown” (SB)(a) involves a microscopic melt that is for selenium 10” times more resistive than the hot solid semiconductor and for silicon 10s times more conductive than a well-doped crystal. The dfodel and its application Specifically, it is claimed that the mesoplasma consists of a microscopic melt of semiconductor, of very low resistance, through which a very high current density flows to a form of spreading resistance in the bulk semiconductor. The original space-charge configuration of the junction in the region is, of course, irrelevant in view of the high temperature. Silicon maintains its semiconductor character up to the melting point (1420°C) and on melting converts to a liquid metal.(rs~ll) The conductivity makes an abrupt transition from about 300 Q-1

; j

--lie \

FIG.

CURRENT FLOW LINES ‘\

9.

Schematic

I j

2x10- cm SPACE cti~m AT4 VOLTS /

diagram of a mesoplasma for an n-p+ junction.

cm-r in the solid to 104 n-1 cm-1 in the liquid, a factor of about 30. As will be shown later, the melt size is somewhat less than 10 p in radius, which leads to the approximate situation that for a current of 200 mA, all of which flows through the melt, the current density is 5 x 104 A cm-a, the field 5 V cm-l, and the voltage drop across the melt only 10 mV. Clearly the melt is not the primary region of power dissipation, although lying in the middle of such a region, but acts as a short-circuit path for the current which is then constrained to a spreading resistance type of geometry. Figure 9 shows a schematic representation of temperature and current geometry, excluding the effect of the specific initial source of the mesoplasma, but including, qualitatively, a displacement of the melt from the space-charge center of the junction towards the high resistivity side. If the low resistivity side is a thin diffused surface layer, or if surface channels are influential, the geometry will be still further distorted. The initiation of the mesoplasma can only be discussed in qualitative terms. The onset of intrinsic conduction leads to an exponential rise of conductivity with temperature. Taking pFLn= 4.0 x 10s T-2.6 cm2 VI set-I,(‘“) for a 10 1 diode with n z 1.5 x 1017 cm-a(133 the bulk semiconductor goes intrinsic at about 800°K where u z 3 (2-r cm-l. This gives an approximate value for the temperature required to initiate the mesoplasma. To pursue the model in more detail, simple as it is in concept, requires several simplifying assumptions since most of the functions of interest, current density, J, field, E, resistivity, p, temperature, ‘YK, and thermal conductivity, K, are obscure functions of position. The first assumptions are that temperature is radially symmetrical around a spherical melt of radius rs, that k takes on some average value and that the electrical power is effectively dissipated in the melt. The general expression(14) P TX-+ To (1) 4rkr where P is the electrical power and To the temperature of the bulk semiconductor, defines the temperature distribution outside the melt as a function of the spherical coordinate, r. The thermal conductivity of silicon is 0.84 J set-l

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cm-1 “K-l at 293”K(15) and varie&s) as T-1. Taking an average temperature of 1273°K for the active region to define an average value for K of 0.2, the radius of the melt, rs, for appropriate values of T (1693”K), To (493°K) and P(1 J sec- 1 at 250 mA and 4 V) in equation (l), is 3.5 x 10-a cm. Similarly, the radius at the 800°K contour is 1.4 x 10-3 cm. (The width of the space-charge region for 4 V is about 2 x lo-4 cm). For our particular specimens the environmental geometry of the mesoplasmas observed under the microscope is essentially that of a quarter of an infinite sphere, that is the mesoplasma is very close to two outer surfaces making a right-angled corner. This must be taken into account in quantitative analysis. For “corner geometry” it may be argued that heat transfer in the direction of the two outer surfaces is very poor, and incidentally that the local surfaces become very hot, so that to some degree the temperature distribution should be calculated for a quarter-spherical model by writing T=;+To

(4

upon which rs becomes 1.4 x 10-s cm and the radius at 800”K, 5.6 x 10-3 cm. Returning to the initiation of the mesoplasma, it is plausible to propose the onset of instability when a region of radius no greater than 5 x 10e4 cm reaches a temperature greater than 800°K. From equation (2), using 6 V and K = 0.5, the current required through the mesoplasma itself is less than 40 mA, which is of the right order of magnitude. If the temperature dependence of k is introduced in the form 293

k=o*84xTthen from the differential form of the heat flow equation (l/4-spherical geometry) p= we obtain

-k,+‘~

(4)

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Using the same values as before, TOis 1.0 x 1O-3 cm and the 800°K contour is at r = 2.7 x 10-3 cm. The results just obtained may be used to determine whether the idea of a spreading resistance has merit. Taking the region outside the 1100°K contour at r = 1.5 x 10-3 cm and p = 0.1 Q cm, from

one obtains I’ = 2.7 V or, if the model approaches &spherical geometry, as high as 5.4 V. The correct analysis is probably a combination of hemispherical geometry close to the melt and quarter-spherical geometry at larger distances. The contribution from the higher temperature region is about 0.3 V and some voltage drop occurs on the highly doped side. It is clear that the theory is very close to observed voltages of 4 or 5 V for the type of diode used. The “active” mesoplasma lends itself to a somewhat more detailed theory aimed at interpreting the current-voltage characteristics (Fig. 1) and the transient time constant of Fig. 7. One of the best defined aspects of Fig. 1 is the approach to essentially constant voltage as current is increased in the high-current mode. The simplest explanation of this fact, based on the model, is obtained by solving for rs from equation (5), substituting in equation (6), upon which, setting P equal to IV, voltage may be derived as a function independent of current I’ = [123p ln(T/Ts)]l/s

(7)

or for the $ sphere, I’ = [246 p In (T/To)]1’2

(8)

Assuming, as has been shown, that most of the voltage drop occurs outside the region of highest temperature, p is about 0.1 Q cm and the two equations give 3.9 and 5 *5 V respectively. An attempt to introduce the temperature dependence of ~(10~11)and the radial dependence of power dissipation derived from p into a complete theoretical analysis results in formidable mathematics. However it is not difficult to use “piecewise” methods to develop an approximate analysis. The principal effect of redistributing the power dissipation away from the center of the melt is to

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make the analysis near the melt poorest-the melt should be smaller than previously calculatedand to affect the analysis less and less for larger r. The theory as presented appears to be adequate for present purposes. The process which coincides with the electrical changes in Fig. 7 is a readjustment of the radial temperature distribution after a change in electrical conditions. Since the circuit-time constant is much shorter than 1 psec, switching the load to a new value results in an initial electrical adjustment which is over before there are significant physical changes in the mesoplasma. Thus, during the fast transients of Fig. 7, the mesoplasma resistance is constant, a fact which can be determined from the data. The transient changes may be neatly summarized on a current-voltage diagram (Fig. 8) showing two load-lines crossing the negative resistance characteristic. The sequence of events is then readily analyzed by comparing the two Figs. The transitions 1-2 and 3-4 correspond to the fast circuit transients, during which the mesoplasma maintains a constant resistance and the rapid change on Fig. 8 follows the constant resistance line of the diode. Transitions 2-3 and 4-l are slower, follow the load lines, and represent changes in the mesoplasma resistance controlled by a relatively slow physical process. On the basis of the melt model the parameter of interest is the radius of the melt, which is essentially proportional to current in the region where voltage is almost constant (equation 5). Thus, whatever else happens, a change in size of the melt requires the transfer of the latent heat of fusion to or from the environment. Taking, for example, a power input of 1 J set-1, an ultimate reduction in current level of 10 per cent leading to a similar reduction in rs, and taking, with the benefit of hindsight, a “reasonable” value for rs of 7 x lo-4 cm, then the mass of silicon which crystallizes is approximately 9 x lo-10 g, and since the latent heat of crystallization(r5) is 1,400 J g-r, the heat liberated is about 1.3 x 10-a J. The steady power level indicates that the mesoplasma is capable of delivering this amount of heat to the environment in a time of the order of 1 psec even if the temperature distribution remains constant, but, of course, the driving force resulting in crystallization is a small drop in temperature due to the approximately 10 per cent reduction in power input upon switching the

load resistance. As in the case of the selenium phenomenon,@) the mesoplasma can function rather like a thermo-regulator. After the transition 3-4 (Fig. 8) during which the power level drops, the mesoplasma reacts in the direction to raise the power, by raising the voltage more than the current falls, in the course of returning to a steady state. For a rough analysis of the switch-on time, we may use ROSE’S analysis(l7) that in delivering energy to a spherical region, of diameter 2r, the time taken to reach 50 per cent of the final temperature is 0.3C(2r)z/K where C is the specific heat. Taking the region of interest as Y = 3 x 10-s cm, within which most of the power is delivered, the time calculated is 23 psec, using a value for C of 1.8 J/cms”C.(r5) Since the power input to the mesoplasma starts at a much lower level than the final steady state, the time taken to make the switch-on transition is plausibly about 60 psec. The negative dynamic resistance and switch-off region of Fig. 1 submit to qualitative arguments. At high current the voltage is almost constant and the assumption is that the fraction of the current passing through the melt dominates the characteristics. As the current is lowered and the melt contracts, a higher proportion of current must bypass the melt through hot semiconductor surrounding the melt, a condition which must, then, more and more approach the conditions existing before the melt formed. In other words, the current-voltage characteristic must move back towards the low-current-mode line as the melt contracts, giving the negative resistance as a byproduct. The observed negative resistance is, however, unlikely to be that of the mesoplasma, since the low-current-mode component of the junction current contributes a substantial positive dynamic resistance. Subtraction of the low-mode current from the high-mode current gives a characteristic line which is probably much closer to the true mesoplasma characteristic. This line extrapolates to less than 10 mA at the voltage where the off-transition (B) rejoins the low-current-mode line, (Fig. 1). The conditions for switch-off are then shown to be fairly uninteresting, although still meriting further study. The negative and positive dynamic resistance components combine to give, with decreasing current a negative resistance of

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increasing magnitude. When this magnitude reaches that of the circuit series resistance the system is unstable, since a further increase in the magnitude of the negative resistance requires a rise in applied voltage, and switching occurs to the low-current-mode line. Temperature dependences of I, and V, are in the expected direction. For a temperature change from 20 to -40°C (Figs 3 and 4) Vm increases by 2.5 per cent while Im increases by 50 per cent. As in the transistor case, only a part of Im is at the mesoplasma. Since the activation of the mesoplasma must be critically dependent on achieving a minimum temperature, the increased current needed at lower temperatures will be composed of a component which heats the mesoplasma directly and the remainder which heats other regions of the junction, the latter component accompanying the necessary small increase in voltage. It has not yet been possible to analyze quantitatively this situation. Multiposition mesoplasmas are an interesting phenomenon. The drop in voltage which accompanies the appearance of a mesoplasma must have the effect of inhibiting any other mesoplasma regions with the exception of those close enough to experience a high temperature. It is a fact that the only transitions observed under the microscope with increasing current have been to closely adjacent points of the junction, never to a distant region. The fact that two mesoplasmas are never seen in a stable state together follows from the logic that the most susceptible region at moderate temperatures switches on first, and if under high temperature conditions a neighboring region becomes more favorable, the slightly lower voltage which is characteristic of a more favorable mesoplasma switches off the first one. DISCUSSION

There seems little doubt that the mesoplasma phenomenon has been often observed in one form or another in both transistors and diodes, including point-contact diodes.(l*-21) SF’s statement(a) that all transistors with adequate leads can be driven into SB cannot yet be extended to all diodes but the hypothesis seems tenable unless some other breakdown process (for example simple heating and melting of an indium contact) interposes. However, the conditions under which a meso-

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plasma appears are obscured by at least two secondary effects. SF (Fig. 3) show values of Im in the region of 4.5 A. This is far too much current for that required to initiate and sustain the mesoplasma and it seems completely clear that most of this current is the normal minority carrier current from emitter to collector, controlled by circuit conditions. The mesoplasma current is completely obscured, although the collector voltage is controlled by the mesoplasma. Since the collector is just as effective in collecting minority carriers at the reduced SB voltage, the large change in voltage does not in itself induce a change in the main current. All that is necessary is the appropriate redistribution of majority and minority carrier currents to satisfy circuit conditions and the physical conditions of a mesoplasma in parallel with the collector junction. The second obscuring effect, possibly more pertinent to diodes, is the presence of local breakdown currents in regions of the junction distant from the mesoplasma. The components of the breakdown current cannot, at present, be distinguished in the pre-fired condition but in the on-condition in diodes the mesoplasma current predominates. Similar arguments apply to the analysis of the delay time, which is partly a function of the component of current passing through the mesoplasma region and partly of currents elsewhere which affect the diode temperature. Unfortunately, delay time is not simply the time taken to heat the local region at constant, high current. Initial current is comparatively low and increases with time as the region and environment heat up. Similarly, the time for the mesoplasma to switch on has no simple dependence on parameters. It also involves large changes in current, and in the diode the transfer of most of the current through the rest of the junction to the mesoplasma region. In the transistor, the redistribution of current during switch-on is still more complicated and so long as large minority carrier currents are present it does not seem possible to analyze the switch-on time. TS(1) find no temperature variation of switch-on time, SF(a) find changes. Figure l(b) in SF shows a delay in the middle of the on-transition which may represent a shift in the mesoplasma position such as we have found (Fig. 2). The

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temperature and voltage sensitivities(“) of the switch-on time in transistors are probably due to changes in the primary driving force, the current at the mesoplasma, as external conditions vary. For diodes, the driving current is essentially the total current and, of course, has no dependence on a base-emitter voltage, so that fairly constant switch-on times are observed. The sensitivity of Iln and delay time to the base current is merely a reflection of the influence of base-emitter voltage on the fraction of the collector current due to minority carriers from the emitter. Temperature sensitivity reflects the expected variation in the required power or energy input to bring the mesoplasma region up to the instability temperature. The frequency dependence of I, is an intimate mixture of a time varying power input and a delay time, but the central requirement is again that a critical temperature must be attained. SF’s “energy concept” may now be better defined. Enough energy must be provided to bring the mesoplasma region to a critical state, allowing for heat transfer to the mesoplasma environment. Large currents flowing in the rest of the junction contribute energy to the system but with very low efficiency so far as a critical SB energy is concerned. The “energy dissipated in the transistor dependent before SB is initiated “(2) is strongly on how the energy is distributed in space and time, as, for example, in SF (Fig. 6), where at higher current levels the mesoplasma region heats up in a shorter time (delay time is shorter) and heat lost to the mass of the semiconductor is reduced. There may be somewhat more obscure interactions between a minority carrier current and a breakdown region in view of GOETZBERGER’S~~“) observation that junction breakdown is radically modified by emitter current, but the two situations are not necessarily related since the mesoplasma is essentially a thermal breakdown while microplasmas are not. The immediate effects of SB, as listed by TS,(l) seem to he readily explained. The junction hreakdown voltage may or may not he adversely affected depending on the changes effected by the high temperatures. If the high temperature region extends to an alloy contact, the melted alloy may dissolve the semiconductor and penetrate the resulting in the reported cavities.(Q) device, Even if an alloy attack does not occur, a meso-

plamsa melt in a narrow base region could result in emitter-collector short-circuits. Damage that can be repaired bv etching must have occured at the surface. _ Failures or “burn-outs” arc not normally observed with our diodes during SB, probably because of the thick base region and the absence of alloying metals near the mesoplasma. It is an interesting fact that the operating condition of a power transistor, \vith high current through a reverse-biased collector, is hcttcr suited to observing SB than the normal operating conditions for power rectifiers. In the latter case high rcvcrse currents are abnormal and circuit conditions arc frequently such as to permit very high, destructive, currents in a breakdown condition. Even in a circuit limited by series resistance, if the total diode current is very high before the mcsoplasma switches on, since practically all of the current transfers to the mesoplasma as the voltage drops, destruction follows. The transistor has some protection from this danger since a high total current may be principally minorit>carrier current which need not transfer to the mcsoplasma when the collector voltage drops, and if there is a high breakdown component before s\vitching occurs this may largely convert to minority carrier current to satisfy the circuit conditions. The aging of a mesoplasma, the variation of its properties with time, has not yet been studied. The melted region, with its redistribution of impurities is probably not vitally involved in aging but the surrounding hot semiconductor, in the presence of fairly high fields,(“s) undoubtcdlg experiences significant changes >lS impurity diffusion (and in our case, surface oxidation) goes on, apparently in such a direction as to “heal” the breakdown region since mesoplasmas seem to have a finite life. This is reminiscent of the equivalent process in selenium(“) lvhere hot spot aging is the fundamental process in electrical forming, hut the parallel does not extend far into the details of the mechanism. It would seem to be possible to devise manufacturing processes which would reduce the susceptibility of transistors to SB, based on a “forming effect” in the mesoplasma region. The specific imperfections that lead to a mesoplasma are not known. A pnpn mechanism is almost certainly not of importance and probably

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BREAKDOWN”

the “pinch-in” effect is merely a contributory factor in influencing which local region first goes into SB. Possibly some major imperfection, an oxide or metal inclusion,(sJJ5) an irregularity in the junction geometry, or a local high density of microplasma breakdown centers, is necessary to initiate a mesoplasma. The phenomenon seems to be encouraged by a freshly fractured surface across the junction. It is probable that still more direct physical evidence of the melt could be obtained by the tedious procedure of lapping the junction in steps of 1 p and staining, since the recrystallized silicon should exhibit a change in impurity distribution. Confirmation of the approximate size and shape of the melt is highly desirable. Finally it should be obvious that THORNTON and SIMMONS’hope(l) for a “new family of thyratron devices” is unlikely of fulfilment. Acknowledgments-The work was supported by Contract Nonr-222(57), Office of Naval Research. I am indebted to Mr. H. M. POWER for experimental work and somewhat over-stimulating discussions. Thanks are due to Professor M. J. 0. STRUTT in whose friendly and active Institute part of the work was performed while a visiting professor. Experimental samples were obtained from International Rectifier Corporation.

REFERENCES 1. C. G. THORNTON and C. D. SIMMONS, I.R.E. Trans. on Electron Devices ED-5, 6 (1958). 2. H. A. SCHAFFT and J. C. FRENCH, I.R.E. Trans. on Electron Devices ED-9, 129 (1962). 3. J. THIRE, Colloque International SW les dispositifs g Semiconducteurs, Vol. 1, Production, Editions Chiron, Paris (1961).

pp. 277-293.

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4. F. WEITZSCH, Arch. Elek. tf., 16, 1 (1962). 5. H. C. LIN, A. R. HLAVECEK and B. H. WHITE, I.R.E. Trans. on Electron Devices ED-7,174 (1960). 6. A. C. ENGLISH and H. M. POWER, Proc. Inst. Elec. Elect. Engrs. 51, 500 (1963). 7. R. NEWMAN, Phys. Rev. 100,700 (1955). 8. A. C. ENGLISH and W. H. TOBIN, Commun. Electron. 79, 9 (1960). 9. A. C. ENGLISH and R. W. THOMAS, Solid-State Electron. 1, 245 (1960). 10. A. F. IOFFE and A.‘R. REGEL, in Progress in Semiconductors. Vol. 4. D. 237. Wilev. New York (1957). ’ 11. N.‘P. MOHROVSKII and A. R. REGEL, J. Tech. Phys. Moscow 23, 779 (1953). 12. F. J. MORIN and J. P. MAITA, Phys. Rev. 96, 28 (1954). 13. S. L. MILLER, Phys. Rev. 105, 1246 (1957). 14. H. MARGENAU and G. M. MURPHY, The Mathematics of Physics and Chemistry, p. 35. Van Nostrand, Princeton, N.J. (1956). 15. E. M. CONWELI , Proc. Inst. Radio Engrs. 40, 1327 (1952). (A review article on the properties of silicon and germanium). 16. C. KITTEL, Introduction to Solid State Physics, p. 82. Wiley, New York (1953); D. S. BEERS, G. D. CODY and B. ABELES, in The Physics of Semiconductors, Conference, Exeter, p. 41. Inst. of Phys. London (1962). 17. D. J. ROSE, Phys. Rev. 105, 413 (1957). 18. S. BENZER, J. Appl. Phys. 20, 804 (1949). 19. E. BILLIG, Phys. Rev. 87, 1060 (1952). 20. A. C. ENGLISH and R. R. HAKE, orally reported but unpublished work on capacitor discharge breakdown of germanium alloy diodes. I.R.E. Conference on Semiconductor Device Research (1955). 21. S. H. TULLY, BROM~NBOVERI, Baden, Switzerland, Private communication. 22. A. GOETZBERGER,J. Appl. Phys. 31,226O (1960). 23. E. M. PELL, J. Appl. Phys. 31, 291 (1960). 24. W. SHOCKLEY, Solid-State Electron. 2, 35 (1961). 25. M. KIKUCHI and K. TACHIKAWA, J. Phys. Sot. Japan 15, 835 (1960).

Note added in proof Thanks to a comment by H. A. Schafft I was reminded of a paper (J. TAUC and A. ABRAHAM, Phys. Rev. 108, 936 (1957)) which specifically identifies the glowing red spot connected with the type of breakdown under investigation. However, the interpretation and analysis given differ considerably from those presented here, and the melt mechanism is not invoked.