4
734 IEEE TRANSACTIONSON ELECTRON DEVICES, VOL. ED-13, NO. 11, NOVEMBER 1966 SUMMARY Visual observations in silicon t,end two models attractive. It is necessary, the theories experimentally. tomakethe first however, to prove To adequately judge the merits of the third model we must wait until we learn more about the ionization coefficients in the Townsend equation. The last two models suggest possible mechanisms which can play a part in causing the voltage decrease of second breakdown when current crowding occurs. REFERENCES 111 A.. C. English and H. M. Power, ‘%Mesoplasma breakdown in silicon Junctions,” Proc. ZEEE (Correspondence), vol. 51, pp. 500-501, March 1963. A. C. English, ‘Wesoplasmas and ‘second breakdown’ in silicon junctions,” Solid-State Electronics, vol. 6, pp. 511-521, Septem- ber-October 1963. F. Weityh, “Zur Theorie des zweiten Durchbruchs bei Tran- slstoren, Arch. Elekt. Ubertragung, vol. 19, pp. 27-42, January 1965. H. Melchior and XI. J. 0. Strutt, “Secondary breakdown in transistors,” Proc. IEEE (Correspondence), vol. 52, pp. 439-440, April 1964. transistors,” Scientia Electrica (Switzerland), vol. 10, no. 4, pp. , “On the initia.tion of second breakdown in diodes and G. M. Ford, “Collector to emitter breakdown related to thermal runaway in inhomogeneous base germanium power transistors,” Solid State Design, vol. 4, pp. 29-36, June 1963. C. G. Thornton and C. D. Simmons, “A new high current mode of transistor operation,” IRE Trans. on Electron Devices, vol. ED-5, pp. 6-10, January 1958. F. Weitzsch, “Zum Einschniireffekt bei Transistoren, die im Durchbruchsgebiet betrieben werden,” Arch. Elekt. Ubertragung, vol. 16, pp. 1-8, January 1962. -- 139-141, 1964. Secondary Breakdown Thermal Characterization and Improvement of Semiconductor Devices B. REICK, SENIOR MEMBER, IEEE, AND E. B. HAKIM, MEMBER, IEEE Abstract-This paper indicates that the energy dependence of semiconductor devices with respect to secondary breakdown can be explained on thebasis of transient thermal resistance.The pro- cedure for determining the transient thermal resistance is described. Results indicatethat the thermaltimeconstants of transistors is much shorter than heretofore recognized. Similar results are presented for voltage regulator diodes. In addition, a simple technique is described which significantly in- creases the thermal time constant of these devices. Similar changes are proposed for other semiconductor devices. hTRODUCTION T IS COMMONLY accepted by investigators in the field that secondary breakdown in transistors occurs upon the application of a critical energy to the device.’,’ Therefore, t,o characterize a transistor for pulsed applications, it is necessary togeneratea series of curves commonly labeled as “safe energy” charac- teristics. These curves are plots of the maximum safe combination of voltage and current that can be simul- taneously applied for varying lengths of time. These curves are usually generated by functionally determining safe points of operation, resulting in a high incidence of device failure during the course of testing.Yetthe Manuscript received December 20, 1965. Theauthorsarewiththe U. S. Army Electronics Command, 1 R. Miller, “Dependence of power transistor failure on their * R. Greenburg, “Breakdown voltage in power transistors, Semi- Fort Monmouth, N. J. energy characteristics,” Ser/~iconductor Products, July 1962;, conductor Products, November 1961. generator of these curves does not yield data which can be related back to the usual device parameters. The importance of thisparametriccharacterization becomes evident in the light of some of the findings subsequently described. One of the purposes of this paper is to explain the energy dependence of secondary breakdown in terms of the more conventional device parameters. This should greatly clarify the time dependence of the secondary breakdown characteristics of the transistor and other semiconductor devices. DISCUSSION Transistor Thermal Resistance in the Avalanche Mode In order to more effectively characterize the transistor for secondary breakdown in terms of the moreconven- tional device parameters, a new method of thermal re- sistancemeasurement was developed. The new method yields a determination of the transistor thermal resistance while the device is in the open base avalanche mode and while the maximum voltage and current are applied. The method of thermal resistance measurement is made by pulsing the transistor in the circuit shown in Fig. 1. This circuit is recognized as the inductive sweep test. When the switch X is closed, thetransistor under test is driven insaturation,andduringthe process energy is stored in the inductance L. When the switch is opened, the inductor tends to discharge its energy through the reverse-biased transistor, resulting in a high voltage across

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734 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-13, NO. 11, NOVEMBER 1966

SUMMARY Visual observations in silicon t,end

two models attractive. It is necessary, the theories experimentally.

to make the first however, to prove

To adequately judge the merits of the third model we must wait until we learn more about the ionization coefficients in the Townsend equation.

The last two models suggest possible mechanisms which can play a part in causing the voltage decrease of second breakdown when current crowding occurs.

REFERENCES 111 A.. C. English and H. M. Power, ‘%Mesoplasma breakdown in

silicon Junctions,” Proc. ZEEE (Correspondence), vol. 51, pp. 500-501, March 1963.

A. C. English, ‘Wesoplasmas and ‘second breakdown’ in silicon junctions,” Solid-State Electronics, vol. 6, pp. 511-521, Septem- ber-October 1963. F. W e i t y h , “Zur Theorie des zweiten Durchbruchs bei Tran- slstoren, Arch. Elekt. Ubertragung, vol. 19, pp. 27-42, January 1965. H. Melchior and XI. J. 0. Strutt, “Secondary breakdown in transistors,” Proc. IEEE (Correspondence), vol. 52, pp. 439-440, April 1964.

transistors,” Scientia Electrica (Switzerland), vol. 10, no. 4, pp. , “On the initia.tion of second breakdown in diodes and

G. M. Ford, “Collector to emitter breakdown related to thermal runaway in inhomogeneous base germanium power transistors,” Solid State Design, vol. 4, pp. 29-36, June 1963. C. G. Thornton and C. D. Simmons, “A new high current mode of transistor operation,” IRE Trans. on Electron Devices, vol. ED-5, pp. 6-10, January 1958. F. Weitzsch, “Zum Einschniireffekt bei Transistoren, die im Durchbruchsgebiet betrieben werden,” Arch. Elekt. Ubertragung, vol. 16, pp. 1-8, January 1962.

--

139-141, 1964.

Secondary Breakdown Thermal Characterization and Improvement of Semiconductor Devices

B. REICK, SENIOR MEMBER, IEEE, AND E. B. HAKIM, MEMBER, IEEE

Abstract-This paper indicates that the energy dependence of semiconductor devices with respect to secondary breakdown can be explained on the basis of transient thermal resistance. The pro- cedure for determining the transient thermal resistance is described. Results indicate that the thermal time constants of transistors is much shorter than heretofore recognized.

Similar results are presented for voltage regulator diodes. In addition, a simple technique is described which significantly in- creases the thermal time constant of these devices. Similar changes are proposed for other semiconductor devices.

h T R O D U C T I O N

T IS COMMONLY accepted by investigators in the field that secondary breakdown in transistors occurs upon the application of a critical energy to the

device.’,’ Therefore, t,o characterize a transistor for pulsed applications, it is necessary to generate a series of curves commonly labeled as “safe energy” charac- teristics. These curves are plots of the maximum safe combination of voltage and current that can be simul- taneously applied for varying lengths of time. These curves are usually generated by functionally determining safe points of operation, resulting in a high incidence of device failure during the course of testing. Yet the

Manuscript received December 20, 1965. The authors are with the U. S. Army Electronics Command,

1 R. Miller, “Dependence of power transistor failure on their

* R. Greenburg, “Breakdown voltage in power transistors, Semi-

Fort Monmouth, N. J.

energy characteristics,” Ser/~iconductor Products, July 1962;,

conductor Products, November 1961.

generator of these curves does not yield data which can be related back t o the usual device parameters. The importance of this parametric characterization becomes evident in the light of some of the findings subsequently described. One of the purposes of this paper is to explain the energy dependence of secondary breakdown in terms of the more conventional device parameters. This should greatly clarify the time dependence of the secondary breakdown characteristics of the transistor and other semiconductor devices.

DISCUSSION

Transistor Thermal Resistance in the Avalanche Mode In order to more effectively characterize the transistor

for secondary breakdown in terms of the more conven- tional device parameters, a new method of thermal re- sistance measurement was developed. The new method yields a determination of the transistor thermal resistance while the device is in the open base avalanche mode and while the maximum voltage and current are applied. The method of thermal resistance measurement is made by pulsing the transistor in the circuit shown in Fig. 1. This circuit is recognized as the inductive sweep test. When the switch X is closed, the transistor under test is driven in saturation, and during the process energy is stored in the inductance L. When the switch is opened, the inductor tends to discharge its energy through the reverse-biased transistor, resulting in a high voltage across

1966 REICH AND NAKIM : SECONDARY BREAKDOWN CHARACTERIZATION AND IMPROVEMENT 735

+VERTICAL-CRO

R ~ = O . I ~

- - p C C

4 HORIZONTAL-CRO

Fig. 1. Circuit used for obtaining characteristic curves.

the device. The transistors examined during this study with the aid of the circuit were primarily silicon planar epitaxial and triple diffused types.

For the purpose of describing the thermal measurement and results, the characteristics of a silicon planar epitaxial transistor will be used. The pulsed characteristic curve for such a device is illustrat,ed in Fig. 2. At low value of collector current I,, the curve indicates the normal negative resistance characteristic leading to the collector- emitter sustaining voltage. Of prime interest to this analysis is the region of the curve where the collector- emitter voltage again rises to a second peak V, followed by a second negative resistance region. It is assumed that this rise in collector-emitter voltage is attributed to a variation in resistivity of the original silicon epitaxial material possibly at a hot spot somewhere within the device. The second peak, therefore, reflects the variation of series resistance in the collect,or following the resis- tivity-temperature variation of the silicon material. The curve in Fig. 2 is so drawn that any increase in the supply voltage V,, (Fig. 1) which would carry the curve beyond the point Va, I. would result in the initiation of secondary breakdown.

Referring to the assumption relating to the second rise of collector-emitter voltage V,, at a large value of col- lector current, it is possible to relat,e the second peak point of the characteristic curve V,, I,, to a specific hot spot temperature Ti,. This may be accomplished by referring to resistivity-temperature curves for the starting epitaxial materials. The starting material used in the transistor under examination was n-type having a re- sistivity of 3 to 5 ohm-cm at 25°C. Referring to resis- tivity-temperature curves developed by Gai~tner,~ the peak of the resistivity-temperature curve for n-type silicon occurs approximately at 250°C. Therefore, the tempera- ture at the point V,, I,, is 250°C. From this the transient thermal resistance e, is determined to be

where

el = transient thermal resistance

Ti, = junction temperature a t point V, - ID, and

T , = case temperature.

Princeton, N. J.: Van Nostrand, 1960. 8 W. W. Gartner, Transistors: Principles, Design and Application.

I C I

vs VP 'CE

Fig. 2. Characteristic curve for a silicon planar epitaxial transistor.

For the specimen under consideration

when the measurements are made at a case t,emperature of 25°C.

Utilizing this technique, a series of thermal resistance measurements were made as a function of pulse width. Pulse width for the purpose of this series of measurements is the time the collector-emitter voltage remains at its maximum value.

Once the thermal resistance measurement i s obtained, it is then possible to calculate the "hot spot" temperature Ti. prior to the onset of secondary breakdown. The spot temperature is

T ~ , = e,(v, x I,) + T,. (2)

Therefore, this method of thermal resistance measure- ment yields the value of transient thermal resistance and spot temperature just prior to the onset of secondary breakdown.

Measurement Results Utilizing the previously described technique of thermal

measurement, a series of measurements was made as a function of pulse width on a silicon planar epitaxial transistor. The device examined had a specified gain- bandwidth product f, of 180 mc/s, steady-state thermal resistance Bo of 30"C/W, and a thermal time constant T of 20 ms. These device data were supplied by the manu- facturer.

The results of the measurements made are plotted in Fig. 3. From Fig. 3 it is evident that the transistor thermal resistance varies with pulse width, and in a short period of time approaches its steady-state value. The plot illustrates that a pulse width in excess of ap- proximately 750 p s can be considered as steady state.

As a first approximation to the peak pulse power capability of the transistor, the following equation could be applied

736 IEEE TRANSACTIONS ON ELECTRON DEVICES NOVEMBER

I I I 100 500 1000 0000

PULSE WIDTH, n8

Fig. 3. Thermal resistance vs. pulse width.

where

P, = peak pulse power

ATjmax = maximum allowable junction temperature rise

Bo = steady-state thermal resistance, and

t / r = ratio of pulse width to the thermal time constant on start.

The utilization of the value of T quoted by the manu- facturer and applied to (3) could lead to large errors in the peak pulse power capability of the device. Indeed, in actual practice, applying (3) has been virtually aban- doned for this very reason. As shown in Fig. 3, the thermal time constant quoted by the manufacturer was too large by a factor in excess of 25.

Finally, for the device examined it was found by applying (2) that the value of "hot spot" temperature a t point Va, I , was between 525°C and 55OoC, irrespective of pulse width. While this temperature was quite uniform for many devices in this lot, it was not representat'ive of all devices examined in this study. Depending on the manufacturer's type examined, the temperatures ob- tained varied from 350°C to 1000°C. In all cases lot temperature uniformity was noted. The reasons for dif- ferently manufactured devices having different spot tem- peratures is not understood at this time.

Zener Diode Transient Characteristics The results of the investigation on the transistor were

directly applied to the characterization of a zener diode used as a transient limiter. For the purpose of this dis- cussion, a particular type of zener diode will be discussed, a nominal 36 volt, 50 watt, stud-mounted device. The general results, however, should be applicable to all other zener types as well. I n order to ascertain the thermal characteristics of the zener devices, transient thermal resistance measurements were made in the circuit shown in Fig. 4. The thermal resistance measurement employed

Fig. 4. Thermal resistance test circuit.

t

1.0 10 100 PULSE WIDTH (MILLISECONDS)

Fig. 5. Thermal response of a zener diode.

utilized the forward voltage drop Vf of the device as the temperature sensitive parameter and it was assumed that the forward voltage changed a t -2 mV/"C.

In the circuit of Fig. 4. the pulse generator and load resistance RL must be capable of supplying the peak reverse avalanche current required. I n addition, this capability is required over a range of pulse widths in order to obtain the required data. The pulse rise and fall time should be kept as small as possible, consistent with the pulse direction. Diodes D , and D, block the pulse current when the forward voltage of the diode under test is measured. These diodes must be capable of carrying the peak pulse current and have a recovery time short enough so that this does not disturb the measurement waveform. Diodes D, and D , serve to clamp the scope against the large voltage across the unit under test during the pulse interval. These diodes must also have a negligible recovery time. The change in forward voltage is measured between the steady-state forward voltage and at the instant reverse current ceases.

A series of measurements employing the above tech- niques were made on several manufacturers' types of 36 volt, 50 watt zener diodes. A typical plot of thermal resistance vs. pulse width €or a given manufacturer's type is shown in Fig. 5. From Fig. 5 it appears that the thermal resistance does not vary with junction temperature in the transistor region, i.e., prior to approaching the steady- state value. The curves also indicate that when the steady- state value of thermal resistance is reached, it increases with increasing junction temperature. This can be ex- plained primarily by considering the variation of the thermal conductivity of silicon with temperature.

The manufacturer, in his thermal rating of this device,