AlllDQ NBS

NBSIR 81-2204

Power Transistor Switching

Characterization

D. L Blackburn

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

April 1981

Prepared for The Lewis Research Center

^QC«iMMwtional Aeronautics & Space Administration

200 ie*' NASA Order No. C-3281 8-D

.U56

81-2204

1981

C.2

NBSIR 81-2204

> m * POWER TRANSISTOR SWITCHING

CHARACTERIZATION

MATKMiAi;. nnnuu

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MAY i 8 1981

?

D. L. Blackburn

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

U S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

April 1981

Prepared for The Lewis Research Center

National Aeronautics & Space Administration

Under NASA Order No. C-32818-D

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Table of Contents

Page

Abstract 1

Introduction 1

Accomplishments 2

Acknowledgment 14

Dissemination of Results 14

References 14

Appendix A - An Experimental Study of Reverse-Bias Second Breakdown . 16

Appendix B - Use of Vacuum Tubes in Test Instrumentation for Measuring Characteristics of Fast High-Voltage Semiconductor

Devices 21

List of Figures

1. Typical dopant profile as determined frcmi spreading resistance

of a high-voltage n"*‘-p-n“-n'*' power transistor 3

2. The measured voltage at which RBSB occurred, Vgg, for a collector current of 10 A for various values of reverse-bias current,

Ibr/ for three devices. A, B, and C 4

3. Collector voltage and current waveforms for a device which reaches

prior to RBSB 7

4. Reverse base current waveform for variable current 8

5. Dopant profile for a device with a step-like transition between

collector and substrate 10

6. Collector voltage waveforms during RBSB 13
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Power Transistor Switching Characterization

David L. Blackburn

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

ABSTRACT

The results of the first year of an experimental investigation of

the switching characteristics of power transistors are discussed.

The devices studied were housed in TO-3 cases and were of an vertical dopant structure. The effects of the magnitude of

the reverse base current and temperature on the reverse-bias second

breakdown characteristics are discussed. Brief discussions of

device degradation due to second breakdown and of a constant voltage turn-off circuit are included. A description of a vacuum tube

voltage clamp circuit which reduces clamped collector voltage overshoot is given.

INTRODUCTION

This report covers activities during the first year of the National Bureau of

Standards (NBS) Power Transistor Switching Characterization project being

sponsored by The Lewis Research Center, National Aeronautics and Space Administration. The purpose of this work is to develop improved methods for characterizing the switching properties, particularly the reverse-bias safeoperating-area limits, of power transistors. Ideally, the methods would be

circuit-independent

The tasks to be undertaken the first year were:

Task 1 - Complete the study of the effect of base current magnitude on the

switching characteristics of power transistors.

A. Study variable base current during turnoff.

B. Study the influence of device construction on the base current

variation of second breakdown voltage.

Task 2 - Begin study of the influence of device temperature on switching

characteristics

A. Design and construct a heat sink with low electrical capacitance

and select and purchase an electrical controller for the test

apparatus

B. Initiate study to determine the effect of case temperature on switching characteristics and reverse- bias second breakdown.

1
Task 3 - Initiate study of degradation mechanisms of second breakdown.

A. Incorporate "controlled delay" into second breakdown protection

circuitry.

B. Study the effect on device parameters of permitting device to

to "stay" in second breakdown for varying lengths of time.

Task 4 - Develop constant-voltage base turn-off circuitry to aid in developing a circuit-independent method for establishing the reverse-bias

safe-op^erating-area limits.

ACCOMPLISHMENTS

The power transistors investigated in this study have an n'^-p-n~-n'^ vertical

dopant density profile and are housed in TO-3 packages. An example of the

dopant profile as determined by spreading resistance [1,2] for an actual

device is shown in figure 1. Transistors of this structure are used in applications that require the device to be switched on and off rapidly, to

dissipate substantial energy, and to support high voltages.

Most of the measurements described have been made using the nondestructive

reverse-bias safe-operating-area test system developed previously at NBS [3] . Some of the important characteristics of this system are listed in table 1

The most important feature is that all power can be removed from the transistor under test within 40 ns of the occurrence of reverse-bias second breakdown (RBSB). Because of this, the devices that have been tested in this work

(TO-3 housed) have experienced RBSB many times (in most instances, over 100

times) with no measurable degradation of their electrical properties.* Most

such devices exhibited no change in either their open base, collector-emitter

leakage current, IceO' their open emitter, collector-base leakage

current, IcbO' after experiencing RBSB. In prior work, before the power

removal circuit was optimized, some devices were degraded during RBSB testing. The degradation was first evident as an increase in leakage currents.

Task 1

An extensive study of the effect of reverse base current, IgR, on the

switching characteristics and RBSB behavior of power transistors was performed. Measurements for both constant and variable Igj^ were made.

Examples of the RBSB behavior as a function of Igj^ for three different

devices are shown in figure 2. Each data point represents the measured voltage at which second breakdown occurred, Vgg, for a constant IgR, with a

*The TO-3 housed devices usually have a chip area of less than 0.4 cm^.

Larger area devices ( ~3 cm^) housed in "stud-mounted" packages have degraded

electrically after experiencing RBSB in the test circuit. As with the TO-3

encased devices, all power was removed within 40 ns of RBSB. The degradation may occur because as the device area is increased, the collector capacitance is also increased. When RBSB occurs, the energy internally discharged increases as the collector capacitance is increased. The protection

circuit cannot reduce this discharge of internal energy. If larger area devices are to be studied, this problem must be investigated further.

2
Figure 1 . Typical dopant profile as determined from spreading

resistance of a high-voltage power transistor.

Table 1

Parameter Limits

Device Type

Collector Voltage (Vce)

Collector Current (Ic)

Forward Base Current (Ibf)

Reverse Base Current (Ibr)

Reverse Base-Emitter Voltage (V3gj^)

On Time (ton)

Protection Time (tp)

npn

0 to 1100 V (clamped)

0 to 30 A

10 mA to 15 A

50 mA to 15 A

2 to 15 V (cleunped)

1 ps to 0 . 1 s <40 ns after RBSB

3
Figure 2. The measured voltage at which RBSB occurred, VgB/

a collector current of 10 A for various values of reverse-bias current, Ibr/ for three devices. A, B, and C.

4
maximum collector current, Iq, of 10 A. In Appendix A, it is shown that

the variation of VgB with depends upon the behavior during turnoff of

the reverse-bias sustaining voltage, also that the variation can be qualitatively explained by the theory of avalanche injection [4] . The mechanism of avalanche injection is commonly thought to be the dominant initiating

mechanism of RBSB [5,6]

.

However, the theory of current focusing to the center of the emitter fingers

[5,6] , in conjunction with the theory of avalanche injection, fails to accu- rately predict the values of VgB for a given Ibr and Iq during sustaining operation. The theory of avalanche injection predicts that RBSB should occur whenever the electric field in the collector is large enough for significant carrier multiplication to occur and;

jg(0) > q • Vj^ • , ( 1

)

where

je(0) = peak emitter current density (A/cm^),

q = electronic charge (1.6 x 10”^^ C),

vjj^ = scattering limited drift velocity (~10^ cm/s), and

Nq = collector doping density (cm"3).

The value of jgCO) can be computed from the theory of current focusing for

any values of Iq and Ibr for a particular device. According to this theory,

the value of j^Cx), the emitter current density at any point x along the

emitter width, is given by [5,6]

sech2 (x/Lg) , (2)

where

X = position along emitter width (cm) and

je(x) = emitter current density at any point x along the width of the

f inger ( A/cm^ )

.

The expression for Lb is;

2 «G

1/2

where

Rsb = base sheet resistance under the emitter (J2) G “ ^c/^BR»

Vt

i • = kT/q,

k = Boltzmann's constant (1.38 x 10"23 j/k) and

T = temperature (K).

(3)

The total emitter current is given by;

Ie = Z • L, tanh (Lg/Lg) (4)
where

:

Ig = emitter current (A)

,

Z = total device emitter perimeter (cm), and

Lg = half of the emitter width (cm).

The value of jg(0) can be found by solving eqs (3) and (4) simultaneously.

The collector voltage and current waveforms for a device whose voltage

reaches VQgx(SUS) prior to Vgg are shown in figure 3. Second breakdown oc- curs at Iq = 1.5 A and ^CE - 650 V. The value of Igg was 0.10 A. In table

2 are listed the values of the various parameters required to compute je(0)

for this device as well as how those pareuneters were determined. Also listed

is the value of N^, which is required to compute the right side of eq (1).

For sustaining operation, the equality sign in eq (1) holds. The value of

jg(0) computed is 13 A/cm^, whereas the value of the right side of eq (1)

is 128 A/cm^. Thus, the requirement as stated by eq (1) that je(0)

least as large as the product of q-v^^N^ for RBSB to occur is not satisfied. This failure in agreement has been found to be the case for nearly all

devices during sustaining operation.

For the devices studied, the qualitative RBSB behavior as a function of

Ibr/ case temperature (discussed later), and energy dissipation (discussed in Appendix A) is in agreement with the theory of avalanche injection.

It is thought at this time that the failure of eq ( 1 ) to accurately predict

the RBSB behavior during sustaining is a result of a failure of the theory of

current focusing to accurately predict the peak current density. It is speculated that either a mechanism similar to thermal instability [7] or the

inability of the transistor to turn off \iniformly spatially due to its emitter or base structure may cause the current density to be greater than predicted by the theory of current focusing. These ideas are being actively

investigated at this time.

Measurements were also made for a variable reverse base current in order to

simulate circuit operating conditions for which Igg cannot be held constant. The reverse base current waveform for these measurements is shown in

figure 4. If the second step is applied at any time during the traditional

current storage time (and often any time before the collector voltage has

reached 100 V or so), Vgg depends only upon the magnitude of Igg at the end

of the turnoff (Ibr2) upon the magnitude of the initial Igg

(Ibri)* Some examples of the effect of such a variable Igg on Vgg are shown in table 3. The first column lists the constant values of Igg used

and the second column, the values of Vgg for these Ibr* Th® third and fourth

columns list the values of VSB for which the initial value of Igg was set

equal to 1 and 5 A, respectively, but the second step was adjusted so that

the final Igg value was the same as listed in column 1 . The second step was arbitrarily applied when the collector voltage reached about 10 V. In each

instance, the value of Vgg depends only upon the final value of Ibr*

These results are consistent with the theory of avalanche injection in that

it is assumed that the instantaneous value of Igg determines the magnitude

of jg(0) which determines the magnitude of Vgg.

6
time (2 ys/div)

Figure 3. Collector voltage and current waveforms for a device which reaches Vcgx(SUS) prior to RBSB. The occurrence of RBSB is noted on the voltage waveform.

Table 2

.

Parameter Value Method

Base Sheet Resistance, Rgg 798 Spreading Resistance

Emitter Perimeter, Z 14.1 cm Metallurgical Microscope

Emitter Half Width, Lg 0.014 cm Metallurgical Microscope

Collector Current, 1.5 A Oscilloscope

Base Current, Igg 0.1 A Controlled by Circuit

Collector Doping, N(-. 8 X 10^^ cm”

3

Spreading Resistance

7
time (500 ns/div)

Figure 4. Reverse base current waveform for variable

current.

Table 3.

^BR2

SB

(I = I BR1 BR2

= 5 A

BR1

0.05 490 490 490

0.1 480 480 480

i 0.2 460 460 460

i 0.5 450 450 450

1.0 450 450 430

2.0 430 410

5.0 300 300

' 10.0 >900

8
No correlation has been observed between the RBSB behavior as a function of

IgR and the construction of the transistors studied. As noted in Appendix A, devices of identical construction (same part number, manufacturer, and

date code) have exhibited a variety of different behaviors as a function of

Iqj^. The major difference in construction between the devices studied is

in the dopant profile at the transition region between the collector and

substrate. For some devices, the transition is smooth; in others, it is

step-like. An example of the step-like collector profile as determined by

spreading resistance measurements is shown in figure 5. This is in contrast

to the smooth transition shown in figure 1. No significant variation in the

RBSB behavior as a function of Ibr has been observed which can be attributed to this difference. Different collector doping profiles and small

differences in the base width or base doping density may have an effect on the value of VgB for a given Irr/ but no significant differences have been

observed in how VgB varies as Ibr is varied as these parameters are changed.

At this time, it is difficult to study the effects of device construction on RBSB behavior. It has been demonstrated that apparently identical devices

display a variety of different behaviors. It appears that obvious differences in construction may not be as important as the more subtle, as yet

unknown, differences. An understanding of these differences apparently must

await a more in-depth understanding of RBSB itself.

The conclusions drawn from this phase of the study are:

( 1 ) The RBSB behavior of a transistor in sustaining operation as a function of Irr depends upon the variation of Vqbx(SUS) as the

device is turning off. The voltage at which RBSB may occur either

increases or decreases as Irr is increased, depending upon how

VcEX (

behaves

.

(2) The theory of avalanche injection qualitatively explains the

VsB behavior with varying Irr*

(3) The theory of current focusing in conjunction with the theory of

avalanche injection does not accurately describe the RBSB behavior

during sustaining. Because avalanche injection describes qualitatively the behavior as a function of Irr as well as a function of

temperature and energy, it is concluded that the theory of current

focusing may be in error.

(4) It is the magnitude of Irr at the end of turnoff that is important for determining the value of VgR and not the value early in

the turnoff.

(5) Although device construction may influence the relative magnitude

of VgR for various Irr, the variability of behavior for seemingly

identical devices makes it difficult at this time to attribute to

differences in construction any variation in the functional

dependence of VgR on Irr.

9
13 10 I I I 1 L_

0 ?0 40 6Q 80 100 DEPTH INTO TRANSISTOR |jum|

Figure 5. Dopant profile for a device with a step-like

transition between collector and substrate.

10
Task 2

The influence of the temperature of the transistor on its RBSB behavior was

studied. The results of this study are discussed in Appendix A. It was found that as temperature increases, the voltage at which RBSB occurs also

increases. These results are in agreement with the avalanche injection theory of RBSB; i.e., the value of Vg3 increases as T is increased.

In order to make these measurements, a temperature-controlled heat sink was designed and built. The heat sink is an adaptation of the commercial test

fixture used in previous studies. The temperature is sensed with a Type K

thermocouple inserted in the heat sink directly beneath the transistor being

tested and insulated from the device by a thin layer of Thermal Film 1 (Thermaloy Corp.).* The heat sink is heated by four strands of coiled nichrome

resistance wire inserted in four holes drilled through the heat sink. Cooling is achieved with forced air. Both the power supplied to the heating

resistor and the volume of forced air are under the control of a temperature

controller. The temperature range of the heat sink is 25®C to 100®C. The

heat sink has very low thermal capacitance and therefore has short heating

and cooling response times. More importantly, it features low electrical

capacitance and thus the switching times of the transistor under test are not

adversely affected.

Task 3

A study of the degradation of transistors due to RBSB was begun. To do this,

the ability to introduce a controlled delay into the RBSB protection circuit

has been implemented. The delay can be varied between 10 ns and 1 ys in 10- ns increments. Thus, transistors can be "taken into" RBSB and be allowed to

remain in the low-voltage (<200 V) high-current density state associated with

RBSB for up to 1 ys. Because of the length of time they can remain in this

state, the electrical properties of the devices can be degraded.

The device parameters that seem to be most sensitive to numerous or extended

RBSB excursions are the leakage currents, either Iceo ^CBO* found

that for the lowest values of I33 (0.05 A in this work), some devices may

"stay in" RBSB for the entire 1-ys limit and experience no apparent degradation. This occurs when the device reaches its reverse-bias sustaining voltage, V03x(SUS), prior to RBSB and the collector current subsequently decays

to a small value ( »1 A) before RBSB occurs. For larger values of collector

current, device degradation usually begins after only a few (only one for

large currents) extended excursions into RBSB.

The collector voltage has been observed to experience two distinct "collapses" after the onset of RBSB. The first collapse occurs within the first

10 ns after RBSB is initiated. In the devices studied, the voltage collapses

to about 200 V. The voltage then decays to a value near 0 V (^0 V) in

Certain commercial equipment, instruments, or materials may be identified in

order to adequately specify or describe the subject matter of this report.

In no case does such identification imply recommendation or endorsement by

the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

11
about another 30 to 40 ns. These events can be observed because the firing

of the protection circuit, which is normally activated some time during the

initial collapse, can be delayed as discussed above. The reason for the two

periods of collapse is not known. Oscillographs of these events for two

different temperatures, 25® and 100®C, are shown in figure 6. The initial

collapse is slower at the higher temperature, but the second collapse is

faster at the higher temperature. The reasons for this behavior is also

unknown

.

Task 4

A constant reverse base voltage circuit was developed which permits the device being tested to be turned off with a constant voltage of 0.05 V to 5 V

through a resistance of 0.1 to 50 ^2. The power supply of the constant

reverse base current circuitry was altered to supply a constant voltage.

This modification will be used in the study to compare the effects of the

constant current and constant voltage turn-off conditions on transistor turnoff and second breakdown.

Other Accomplishments

A new collector voltage clamp circuit has been developed, which, when added

to the measurement system, reduces or eliminates the voltage overshoot for

extremely fast-rising collector voltage waveforms.

When a transistor which has an inductive load at its collector terminal begins to turn off, the voltage at the collector rises as the inductor attempts

to maintain a constant current. For high-voltage, fast-switching transistors, the voltage can rise rapidly, perhaps as much as 500 V in 100 ns. In

most applications for these devices, the maximum voltage rise is limited by a voltage clamping circuit connected to the collector. Usually, the voltage

clamp consists of a solid-state diode in series with a voltage source (clamp

voltage) arranged in such a manner that the bias on the diode is equal to the

difference between the collector voltage and clamp voltage. When the collector voltage is lower than the clamp voltage, the diode is reverse biased.

When the collector voltage rises slightly above the clamp voltage, the device

is forward biased, the current is shunted from the transistor, and the collector voltage is held constant at the clamp voltage minus the voltage drop

across the diode.

The diode in the clamp circuit cannot instantaneously go from a reverse-bias

condition to a forward-bias condition. Even the fastest solid-state diodes

must have time for the depletion region to collapse, which delays the forward

biasing of the diode. This results in an overshoot of the collector voltage

above the clamp voltage for a short period of time (~50 ns) for fast-rising

collector voltages. The overshoot has been observed to be over 100 V for

some very fast bipolar transistors tested on the nondestructive NBS test

circuit. Because of the overshoot, the devices appear to experience second

breakdown at lower clamp voltages than they would if no overshoot occurred.

There is no overshoot for slower rising voltages.

To reduce the overshoot on the NBS circuit, a vacuum tube diode clamp circuit

has been constructed which can replace the solid-state diode. The vacuum

12
a. Case temperature = 25®C.

b. Case temperature = 100®C.

Figure 6. Collector voltage waveforms during RBSB. The

time base has been delayed to eliminate most of the portion of the waveform prior to RBSB.

13
tube has no reverse recovery time emd thus has a shorter delay time in assum- ing its conducting state than does the solid-state diode. The result has

been to eliminate overshoot in some instances and to substantially reduce it

in others.

Two disadvantages of the vacuum tube clamp are that the vacuum tube can have

a substantial voltage drop, cibout 45 V for 10 A of current, and that it introduces an increased parasitic capacitance at the collector of the device

being tested. The first disadvantage is overcome by merely accounting for

the extra voltage when recording the clamp voltage. The second does present

a problem in that the collector current can droop substantially during the

early stages of the collector voltage rise as the extra clamp capacitance is

charged. Measurements have indicated that this may have an effect on the

second-breakdown characteristics of the device being tested. The vacuum tube

clamp is only used when voltage overshoots are a problem, euid at other times

the solid-state diode clamp is used. The effect of the current droop on the

second-breakdown characteristics is being investigated. A paper describing

the new clamp circuit as well as some of the other "vacuum tube" features of

the circuit is included as Appendix B.

Acknowledgment

The spreading resistance measurements were made by Dr. James R. Ehrstein.

Dissemination of Results

Published - An Experimental Study of Reverse-Bias Second Breakdown, D. L.

Blackburn and D. W. Berning, Tecihnioai Digest, 1980 Intevnationdt

Eleotvon Devices Meeting, Washington, D.C. (December 1980).

Associated talk - An Experimental Study of Reverse-Bias Second Breakdown, D.

L. Blackburn and D. W. Berning, 1980 International Electron Devices

Meeting.

Submitted for publication - Use of Vacuum Tubes in Test Instrumentation for

Measuring Characteristics of Fast High-Voltage Semiconductor Devices,

D. w. Berning, submitted to IEEE Trans* on Measurement and Instrumentation.

Tour - Tour of laboratory conducted in December 1979 for attendees of 1979

International Electron Devices Meeting (''^80 participants).

References

1. Ehrstein, J. R. , Ed., Semiconductor Measurement Technology: Spreading

Resistance Symposium, NBS Special Publication 400-10 (December 1974).

2. Dickey, D. H., and Ehrstein, J. R. , Semiconductor Measurement Technology: Spreading Resistance Analysis for Silicon Layers with Nonuniform

Resistivity, NBS Special Publication 400-48 (May 1979).

14
3. Berning, D. w. , Semiaonduator Measurement Technology: A Reverse-Bias

Safe Operating Area Transistor Tester, NBS Special Publication 400-54

(March 1979).

4. Hower, P. L., and Reddi, V. G. K., Avalanche Injection and Second

Breakdovm in Transistors, IEEE Trans* Electron Devices ED- 1

7

, 320-

335 (April 1970).

5. Krishna, S., and Hower, P. L., Second Breakdown of Transistors During

Inductive Turn-Off, Froc* IEEE 61, 393-394 (March 1973).

6. Beatty, B. A., Krishna, S., and Adler, M. A., Second Breakdown in Power

Transistors Due to Avalanche Injection, IEEE Trans* Electron Devices ^

23, 851-857 (August 1976).

7. Hower, P. L., Blackburn, D. L., Oettinger, F. F., and Rubin, S., Stable

Hot Spots and Second Breakdown in Power Transistors, PESO '76 Record,

Proc* 1976 Power Electronics Specialists Conf*, pp. 234-246, June

1976.

15
Reprinted from INTERNATIONAL ELECTRON

DEVICES MEETING, December 1980 APPENDIX A

AN EXPERIMENTAL STUDY OF REVERSE-BIAS

SECOND BREAKDOWN*

D. L. Blackburn and D. W. Berning

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

ABSTRACT

Experimental results showing the influence of

reverse-base current, case temperature, collector

inductance, and peak collector current on the

reverse-bias second breakdown (RBSB) behavior of high-voltage power transistors are presented. The results are in qualitative agreement with the theory that avalanche injection initiates RBSB. The inductance and peak collector

current results are in conflict with the theory

that RBSB is initiated at a critical temperature.

It is concluded that for these devices for the con- dition studied, RBSB is not initiated at a critical

temperature. It is shown that the theory of cur- rent focusing, in conjunction with the theory of

avalanche injection, does not accurately predict

the RBSB conditions during device sustaining. It

is proposed that other mechanisms in addition to current focusing contribute to the nonuniformity of current during transistor turnoff.

INTRODUCTICW

The reverse-bias (turn-off) characteristics of high

voltage, fast switching power transistors have become more important as these devices are used in increasing nvimbers as high speed switches in power

conditioning applications. Research to improve the device and circuit operating characteristics [1-3]

and to better model and understand their operation

[4,5] is continuing. Perhaps the most severe impediment to this research is the continued lack of documented experimental results on which to base

device design changes and with which to compare and

improve the prediction of models. The scarcity of

experimental data results primarily because of the difficulty in obtaining repeatable, accurate measurements in the presence of the large voltage and current transients that occur during transistor

turnoff and because of the destructive nature of reverse-bias second breakdown (RBSB). The phenomenon of RBSB determines the reverse-bias limits of

safe operation, but the physical mechanisms of RBSB

are only partially understood.

•This work was done as part of the Semiconductor

Technology Program at the National Bureau of Standards and was supported by Lewis Research Center,

National Aeronautics & Space Administration,

Cleveland, Ohio under NASA Order No. C-32818-D.

The purpose of this paper is to present experimental results of the RBSB behavior of high voltage,

fast switching power transistors obtained using the

nondestructive reverse-bias safe operating area test circuit developed at the National Bureau of Standards (NBS) [6]. Earlier results have previously been reported [7,8]. This paper reports re- sults of measurements of the RBSB behavior of power transistors as a function of the reverse-base current, case temperature, collector load inductance, and peak collector current.

It is anticipated that the new results will be valuable to device and circuit designers and modelers

for testing and iir5>roving their designs and models.

In addition to helping to develop a Isetter understanding of the switching and RBSB characteristics

of power transistors, the objective of the NBS work

is to develop a basis for the improved characterization of these devices.

THEORETICAL BACKGROUND

It is generally laelieved that the mechanism of avalanche inj ection is the dominant initiating mechanism of RBSB in high-voltage power transistors [4,10]. This theory assumes that the

collector current density becomes large enough lo- cally that the charge density in some region of the

collector is con^jrised primarily of the free,

current-carrying charges. The net effect of this

is that the peak electric field in the collector occurs at the collector-substrate (n~-n^) interface rather than at the base-collector (p-n”)

junction. If the field is peaked at the collectorsubstrate interface and is simultaneously large

enough for significant carrier multiplication by

impact ionization to occur, avalanche injection

will be initiated [9]. This forces the device to

operate in a negative resistance region (current

density increases as voltage decreases ) which is inherently unstable [11]. The instability manifests itself as RBSB. The electric field peaks at the collector-substrate interface whenever

3c

>

where

j_ = collector current density (A/cm^),

w. —IQ q

= electronic charge (1.6 x 10 ” C),

vj^ = scattering- limited drift velocity

('vIO^ cm/s), and

Nq = collector dopant density (cm~^).

The large current density required for avalanche

injection to occur is thought to be achieved as a 16
result of focusing of the emitter current to the centers of the emitter fingers [4,10]. When the

transistor is turned off from a saturated operating

condition, the charges that have been stored in the

base and collector regions are extracted via the base terminal. Because the active base beneath the

emitter acts as a resistive component from and

through which most of the stored charge is extracted, a voltage gradient is created along the width of the base-emitter junction. The gradient

is such that the center of the emitter is more strongly forward biased than the edges and thus injects a larger current density. Expressions which

have been developed for calculating the current

density as a function of position along the emitter

width [4, 10] predict that the current density at the center of the finger, jg(0), increases as emitter current, Ig/ or reverse-base current,

IgR* is increased.

EXPERIMENTAL RESULTS

All of the results to be discussed were generated

for the transistor being turned off with an inductive load at its collector terminal. Unless otherwise noted, the magnitude of the reverse-base current, IgR» was held constant during turnoff.

The test circuit and conditions were the same as have been described previously [6-8]

Measurements were typically made for both clamped

and undamped conditions. The clamped condition is one for which a circuit external to the device does not allow the collector voltage to rise above a specified value, and the undamped condition is one

for which the voltage is allowed to rise with no clamping external to the transistor. Because the

only significant difference observed in the RBSB behavior between clamped and undamped conditions

is in the magnitude of the collector voltage at which RBSB occurs, Vgg, and not in the functional dependence of this voltage on other parameters, only undamped results are presented.

The occurrence of RBSB manifests itself as a sudden

collapse of the collector voltage of the transistor

from the maximum voltage achieved to about 200 V. The collapse occurs in less than 10 ns, and in this

work the voltage is reduced to zero within an additional 40 ns by the protection circuit [6]. Be- cause of these short transition times, RBSB is observed on most of the oscilloscope traces presented

in this work as an abrupt halt of the collector

voltage waveform with an apparent discontinuous

transition to zero voltage.

The results presented in this paper represent a sampling of results of numerous measurements on numerous devices. Each of the oscilloscope tracings presented shows the results of several measurements made on a single device. Typically, be- tween 10 and 100 similar measurements have been

performed on each of these devices. The repeatability of these measurements is within the resolution of the oscilloscope presentation. The large

number of RBSB measurements is made possible by the

protection circuit that is used [6] . For the device types discussed in this paper, hundreds of

RBSB measurements can be made on a single device

with no apparent degradation of the device's electrical parameters.

Temperature

The temperature of the transistor may have a strong

influence on its RBSB behavior. The collector current and voltage waveforms for a device for the case temperature Tj, equal to 25®, 50®, 75®, and

100®C are shown in figure 1. It has been observed

that in almost every instance, for all operating

conditions, as T^ is increased, the voltage at which RBSB occurs, Vgg, is increased. This behavior is consistent with the avalanche injection

theory of RBSB. Because the ionization coefficients for electron and holes decrease as the tem- perature is increased, the critical electric field

required for significant impact ionization to occur

is increased. This reqpaires that the collector

voltage at which the critical field is reached is

also increased, l.e., Vgg is increased.

Reverse Base Current - Ipp

The magnitude of Igg has a strong but varied

effect on the RBSB behavior of high-voltage transistors. The measured Vgg is shown as a function of IgR for three devices in figure 2. The

variation of Vgg with Igg can be partially

explained with the aid of collector voltage and current waveforms for these same devices , which are shown in figure 3. For some values of IgR» the

collector voltage reaches the reverse-bias

sustaining voltage, prior to Vgg.

The sustaining condition is evident in figure 3 as the voltage reaching a plateau and the current decaying at a relatively slow rate and approximately

linearly with time. Because of the time scales of

figure 3, it is not obvious for devices A and B at Ibr = 0.2 A and device C at IgR = 1 A that

the voltage reaches V(,gjjjgygj. By expanding the time scale for these waveforms, as done for figure

4 for device C, it can be seen that the devices do

reach the sustaining condition for a brie^ time

prior to RBSB.

Sustaining Conditions - If the device voltage

reaches Vf;EX(SU5) RBSB, as Igg is increased,

VgB may either Increase or decrease. For devices

such as device A which remain in the sustaining

condition for only a short time prior to RBSB and

for which '^CEX(SUS) constant in time (does not change as Iq decays), Vgg will usually Increase as Ibr is increased for sustaining conditions.

Most often, though, '^cex(SUS) constant with

time, but tends to Increase as Iq decays during

the sustaining condition. Devices of this type

tend to remain in the sustaining condition for a rather long time prior to RBSB for the lowest values of Irr* For these devices, exan^les of which are devices B and C, as IgR is Increased

for sustaining operation, Vgg usually decreases.

The Vgg of device C experiences both types of behavior, first decreasing and then increasing as Ibr is Increased. The reasons for the differ- ences in the sustaining behavior of these devices are not known. Devices A and B are identical de-

•vices made by the same manufacturer with the same

17
> Ibr (a) = 2.0 1.0 0.5

Figure 1. The collector voltage waveform for a device for three values of reverse-base current and

the case temperature at 25°, 50°, 75°, and 100°C. For each I^p(, increasing T is from left to right. RBSB occurs at the peak of each voltage

waveform for which the waveform abruptly halts.

Figure 2. The measured voltage at which RBSB occurred, ^SB' for a collector current of 10 A for various values of reverse-base current,

Ibr' three devices. A, B, and C.

Irr (a) =

a. Device A. b. Device B. c. Device C.

Figure 3. The collector voltage and current waveform for the same devices as in figure 2. RBSB occurs when

the voltage waveform appears to halt and discontinuously goes to zero.

Figure 4. The collector voltage waveform at an

expanded time scale for device C, Igp ~ J A. The voltage collapses to about 200 V after RBSB. The voltage waveform after RBSB is determined both

by the device and the protection circuit.

Figure 5. The collector voltage and current wave- form for I(^^ A, IQp 0.1 A, and L — 0.5 mH and 1.0 mH.

18
date code. Device C is of similar construction to

A and B, but was made by a different manufacturer.

For sustaining conditions, the variation in Vgg

with is not a predictable function.

By using the expression developed for calculating

jg(0) [4,10], it is possible to test the quantitative predictions of the theories of current focusing and avalanche injection for sustaining con- ditions. For small Igg compared to I^. (Ig = Iq)

and for sustaining conditions, eq (1), the criteria

for RBSB to occur, becomes

jg(0) = q-V^-N^ . (2)

The device physical parameters required for computing jg(0) can be determined from spreading resistance measurements and emitter dimension measurements. Typically, for the smallest values of

Ibr 3nd for sustaining operation, it has been

found that the computed values of jg(0) for the

observed second-breakdown conditions, such as shown

in .figure 3, are less than 10 percent of those

necessary to satisfy eq (2). Also, for the devices

and conditions studied, the theory of current focusing predicts that during sustaining, as is decreasing, jg(0) is also decreasing.* Thus, the

theories of current focusing and avalanche injection predict that if the device does not experience

RBSB before sustaining begins, RBSB will not occur and the device will safely turn off. The results

in figure 3 show this not to be the case. One rea- son for these discrepancies may be that other me- chanisms in addition to current focusing can cause

the current to constrict to a locally high density. More will be said about this later. Also, because

the device may be dissipating a significant amount

of energy during the sustaining condition, the possibility was investigated that a thermal mechanism

rather than avalanche injection might initiate

RBSB. These measurements will also be discussed

later.

Nonsustaininq Conditions - For values of IgR Such

that the voltage of the device does not reach

VcEx(sus)' behavior of Vgg with varying IgR

is as predicted by the theory of current focusing

and avalanche injection. That is, as Igjj is increased, for a given emitter current, jg(0)

should increase. This causes RBSB to occur at a decreased voltage. Eventually, though, as the mag- nitude of IgR approaches that of the naxlmum

collector current, Icm» the magnitude of the

maximum emitter current, approaches zero

because:

As Igfij Because the emitter

effectively is turned off and there is no current

focusing, the breakdown voltage increases and can approach V^gQ, the open emitter, collector-base

breakdown voltage. This occurs for Ig^ = 10 A for all three devices of figures 2 and 3.

*A correction for variation in the effective base

sheet resistance due to changing injection levels

was made in performing these calculations.

Energy Dissipation

There is ample evidence that for devices of different structures and for different operating conditions than those studied here, RBSB may be initiated by a thermal mechanism [12]. When this mechanism dominates, RBSB should be initiated when the device locally reaches a critical junction temperature. Because the energy dissipated may be significant, measurements have been made to determine if

a thermal mechanism is responsible for the RBSB behavior for high-voltage devices during the sustaining condition.

Inductance - If RBSB is initiated at a critical

junction temperature, if only the collector load

inductance L is varied, the total energy dissipated

by the transistor prior to RBSB should not change.

This energy is given by:

where tg is the time that turnoff begins and

tsb is the time RBSB occurs. The collector

voltage and current waveform for L = 0.5 and 1.0 mH

are shown in figure 5. From these waveforms, the calculated energy dissipated prior to RBSB is about

twice as great for L = 1.0 mH as for L = 0.5 mH. This indicates that the temperature at which RBSB occurs is significantly greater at L = 1.0 mH than at L = 0.5 mH. Although this is not in agreement

with the concept of RBSB being initiated at a critical temperature, the results are in qualititative

agreement with the prediction of the theory of avalanche injection. That is, L should have almost no effect on the magnitude of Vgg. The slight in- crease in Vgg for Ii = 1 mH in figure 5 is probably due to the higher temperature at which RBSB occurred.

Peak Current - Another test of the critical temper- ature theory is the RBSB behavior as I^j^ is

varied, with all other parmeters held constant. The energy dissipated prior to RBSB for different IcM should be about the same if RBSB occurs at

a critical temperature. The collector current and

voltage waveforms for several values of I(.j^ are shown in figure 6. As I^jj is increased, the

energy dissipated prior to RBSB also increases until Iqj^ = 15 A for which the device no longer

reaches the sustaining condition, but experiences

RBSB first. Neither the varying energy dissipation

with Ij.jjj nor the sudden change in behavior at

15 A can be explained by the critical temperature

theory of RBSB.

The results of figure 6 can be explained by the

theory of avalanche injection if it is assumed that

some other mechanism ^ addition to current focus - ing contributes to the generation of a high-current

density. The discontinuity in behavior can occur

as Iqih is increased to 'v>15 A if the value of jg(0) at the instant the device reaches the sus19
'CH

Figure 6. The collector current and voltage wave- form for various values of and = 0.1 A, all other parameters held constant. When = 15 A, the device no longer sustains prior to RBSB. taining condition satisfies eq ( 1 ) with the equality sign. If Iqiij is increased further, the inequality in eq (1) is satisfied and RBSB occurs prior to sustaining. Below ^CM ''5 v^en

the device reaches the sustaining condition jgCO)

does not satisfy eq ( 1 ) . As the sustaining condition continues, other mechanisms in addition to current focusing may cause the current to begin to

localize within the transistor. Perhaps a phenomenon similar to the thermal instability [13] that is known to occur for forward-bias operation causes

the current to begin to localize, or perhaps some regions of the transistor do not "turn off" as readily as others because of their location with

respect to the base or emitter leads or because of the device geometry. If such mechanisms do occur, then the effective active area of the transistor

may begin to decrease and the current density in- crease even though the total emitter current is de- creasing during sustaining. This can cause jg(0) to increase and eventually become large enough to satisfy eq ( 1 )

.

CONCLUSIONS

Measurements showing the influence of reverse-base

current, case temperature, collector inductance,

and peak collector current on the reverse-bias

second-breakdown (RBSB) behavior of n*-p-n~-n*

high-voltage power transistors have been presented.

The inductance and peak collector current results

are in conflict with the concept that RBSB may be initiated at a critical temperature. It is concluded, for these devices and for the operating

conditions studied, that the theory that RBSB is initiated at a critical temperature is incorrect.

All of the results are in qualitative agreement

with the theory of avalanche injection as the initiating mechanism of RBSB. The quantitative pre- dictions of the theory of current focusing during

sustaining are shown not to be accurate in predicting the RBSB behavior. It is speculated that other

mechanisms contribute to the nonuniformity of the current during this type of operation.

REFERENCES

(1) W.R. Skanadore, "A New Bipolar High Frequency

Power Switching Technology Eliminates LoadLine Shaping," Proc. Powercon 7, 7th National

Solid State Power Conversion Conf, pp. D2.1-d2.14, Mar. 1980.

(2) B. Jackson and D. Chen, "Effects of Emitter

Open Switching on the Turn-Off Characteristics of High-Voltage Transistors," PESC '80 Record, 1980 IEEE Power Electronics Specialists Conf., pp. 147-154, June 1980.

(3) K. Owyang and P. Shafer, "A New Power Transistor Structure for Improved Switching Per- formances," 1978 lEDM Technical Digest, 1978 IEEE Int. Electron Devices Meeting, pp. 667-670, Dec. 1978.

(4) B.A. Beatty, S. Krisna, and M.A. Adler,

"Second Breakdown in Power Transistors Due to Avalanche Injection," IEEE Trans. Electron

Devices ED-23 , pp. 851-857, Aug. 1976.

(5) P.L. Hower, "Collector Charge Dynamics and

Second Breakdown Energy of Power Transistors," PESC '74 Record, 1974 IEEE Power Electronics Specialists Conf., pp. 144-153, June

1974.

(6) D.W. Berning, "Semiconductor Measurement

Technology: A Reverse-Bias Safe Operating

Area Transistor Tester," NBS Special Publication 400-54, April 1979.

(7) D.L. Blackburn and D.W. Berning, "Some Effects of Base Current on Transistor Switching

and Reverse-Bias Second Breakdovm," 1978 lEDM

Tech. Digest, 1978 IEEE Int. Electron Devices

Meeting, pp. 671-675, Dec. 1978.

(8) D.L. Blackburn and D.W. Berning, "ReverseBias Second Breakdown in Power Transistors,"

Electrical Overstress/Electrostatic Discharge

Synp. Proc., pp. 116-121, Sept. 1979.

(9) P.L. Hower and V.G.K. Reddi, "Avalanche In- jection and Second Breakdown in Transistors,"

IEEE Trans. Electron Devices ED- 17 , pp. 320-335, Apr. 1970.

(10) S. Krishna and P.L. Hower, "Second Breakdown

of Transistors During Inductive Turn-Off,"

Proc. IEEE pp. 393-394, Mar. 1973.

(11) M.W. Muller and H. Guckel, "Negative Resistance and Filamentary Currents in Avalanching

Silicon £’*’-4~n‘*‘ Junctions," IEEE Trans. Electron Devices, ED-15 , pp. 560-568, Aug.

1968. (12) H.A. Schafft, "Second Breakdown - A Comprehensive Review," Proc. IEEE 55 , pp. 1272-

1288, Aug. 1967.

(13) P.L. Hower, D.L. Blac)cburn, F.F. Oettinger,

and S. Rubin, "Stable Hot Spots and Second

Breakdown in Power Transistors," PESC '76 Record, Proc. 1976 Power Electronics Specialists Conf., pp. 234-246, June 1976.

20
APPENDIX B

USE OF VACUUM TUBES IN TEST INSTRUMENTATION FOR MEASURING

CHARACTERISTICS OF FAST HIGH-VOLTAGE SEMICONDUCTOR DEVICES*

David Berning

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

Circuit techniques are described that permit measurement of fast

events occurring in power semiconductors. These techniques were developed for the dynamic characterization of transistors used in

inductive load switching applications. Fast voltage clamping using

vacuum diodes is discussed, and reference is made to a unique

circuit that was built for performing nondestructive reverse-bias

second breakdown tests on transistors.

INTRODUCTION

Recently, there has been a large increase in the demand for fast switching,

high-voltage power transistors. Along with the increased use of these

transistors has come the need for accurate measurement of the limits of safe

operation for these devices. Reverse-bias second breakdown xisually

determines these limits for the transistors operating in a switching mode.

Because of the large currents and voltages and fast transients involved,

there are numerous instrumentation problems in making controlled and

repeatable measurements of the limits.

The devices being studied are used primarily for various types of power

conditioning equipment including switching power supplies, motor controls, TV

deflection circuits, and automotive ignition systems. In these applications,

the transistor is usually required to turn off from a saturated condition

with an inductive load in the collector circuit. The inductance causes the

collector voltage to rise rapidly to a high value upon transistor turnoff.

The voltage that the collector reaches is usually limited by a clamp diode

that is intended to prevent breakdown of the transistor switch. In testing

these transistors, it is desirable to develop circuitry that will approximate

the circuitry in which these devices are normally used, but to maintain

additional control over various parameters such that accurate measurements

can be made.

BACKGROUND

A circuit built for measuring the reverse-bias safe operating area (RBSOA)

for high-voltage bipolar switching transistors is described in detail

elsewhere [B-1]. This circuit was developed using some ideas from one built

by Jahns [B-2] . The circuits perform second breakdown tests by turning off

the transistor under test (TUT) that has been in a saturated conducting

state. The TUT has a load inductor in the collector circuit, and upon

* This work was supported in part by the NASA-Lewis Research Center,

Cleveland, Ohio under NASA Order No. C-32818-D.

21
turnoff, the collector voltage rises rapidly until either the voltage is

limited by a voltage clamp, the device reaches its reverse-bias sustaining

voltage, or the device breaks down causing the voltage to rapidly fall to a low value. Both the circuits of Berning and of Jahns include a feature that

allows second breakdown measurements to be made, usually without destroying

the TUT. The collector voltage collapse at breakdown is sensed and a shunt

protection circuit removes the remaining energy stored in the inductor so that the energy absorbed by the TUT is minimized. If this energy is not

diverted quickly, the transistor will be destroyed. The effectiveness with

which the circuit can test and save devices is related to how fast the energy

can be diverted from the TUT after it breaks down.

One of the major differences between the two circuits is the capability of

the protection circuit. The earlier circuit by Jahns uses high-voltage

transistors to shunt the inductive energy away from the TUT, and a 350-ns

energy removal time was reported. The later circuit uses pentode tubes and

has an energy removal time of 40 ns. Vacuum tubes are faster than

transistors for high-voltage applications because of the time involved in

storing the charge required for conduction in transistors. The pentode tubes

used for the protection circuit are type 6LF6 , which is a high perveance type

intended for horizontal deflection in television receivers. In the

protection circuit 16 of these tubes are driven by low voltage, high speed

transistors biased class A. In this configuration the circuit can handle

1000 V and 30 A, with a slew rate of 5 x 10“^ V/ys. The circuit of Berning

consistently permits hundreds of second breakdown measurements to be made on

a single transistor without destroying the device.

Numerous results using this later circuit have been published [B-3,B-4]. The

entire safe operating area has been generated and the effects of different

base currents on the second breakdown susceptibility has been shown, using a single device for all the measurements.

CIRCUIT DETAILS

A simplified diagram for the circuit described in reference [B-1] is given in

figure B-1. Before a test is initiated on a transistor, a clcimp voltage,

which can be set anywhere between 0 and 1000 V, is applied to the shunt protection circuit to isolate the TUT and the load inductor, L3 , from the protection circuit by the reverse-biased clan5> diodes, CR7 to CR9. This minimizes stray capacitance at the collector of the TUT. Three fast-switching

diodes are used in series for the clamping function to obtain the recjuired

voltage capability. The test for reverse-bias second breakdown is initiated

by applying a base current to the TUT to turn it on for a period of time

during which energy is stored in the load inductor. The TUT then receives a reverse-base current pulse to turn off the device, and the load inductor

forces the collector voltage on the TUT to rise. The limits of the safeoperating area are determined by measuring how much voltage the device can withstand before it breaks down. Since the safe-operating-area measurements

are generally done for many different conditions of base drive on the same device, it is desirable to save the device from destruction with each

breakdown test.

22
Figure B-1. Simplified diagram for a circuit used to test transistors for reverse-bias second breakdovm.

23

I
A capacitor. Cl, activates the shunt protection circuit when the collector

voltage of the TUT collapses upon the onset of second breakdown. This

capacitor consists of a short length of wire near the base of a sensing

transistor internal to the shunt protection circuit. This very small

capacitance acts as a differentiator to detect only the very rapid fall in

collector voltage associated with second breakdown as this voltage transition

typically occurs in 10 ns. The collector voltage decrease associated with

the collapsing inductor field after nearly all of the stored inductive energy

has been dissipated is a slower voltage transition (occurring over about 1 ys or more) and is ignored by the shunt protection circuit.

When the TUT experiences second breakdown, the high-speed vacuum tube

protection circuit removes the stored energy remaining in the inductive load

by shunting the clamp supply to the -140 V power supply. Additionally, when

the protection circuit is activated, the clamp supply and the Vqq supply

are turned off. An additional diode-resistor-inductor network, which

consists of CRi to CR3, R1 , and L2, allows the clamp voltage to go negative

for a short period of time after second breakdown has occurred and then decay

to 0 V to overcome the inductance in the wire that connects the protection

circuit to the TUT. Five power Schottky diodes, CRIO to CRi 4, are in series

with the collector of the TUT to effectively open the collector lead when the

clartp voltage is driven negative.

CIRCUIT IMPROVEMENT

In using the test circuit described above, it has been noted that some of the newer bipolar transistors measured have a very fast collector voltage rise

time when large reverse base currents are used. Voltage rise rates of 10^

V/ys have been observed. The new power MOSFET devices also produce similar

slew rates. The solid-state clamp diodes used for limiting the collector

voltage are unable to turn on fast enough to prevent the voltage at the

collector of these very fast transistors from going far beyond the desired

clamp value during the test. Over 100 V of overshoot has been observed.

Because it is very difficult to characterize devices accurately under such

conditions, a vacuum diode clan^ was introduced into the circuit between the

junction of L3 and CRIO and the clcunp supply. This new clamp greatly reduces

the overshoot. Figure B-2 is a photograph showing the collector voltage

waveforms for both the solid state and tube clamp. Most of the overshoot

remaining in the tube claitp is caused by the inductance in the wires leading

to the clamp. These wires could have been considerably shorter had this new added clamp been designed into the original circuit. The tube diodes used

for the fast clamp are type 6CG3, which are designed for damper diode use in

television receivers. Six were wired in parallel to handle a clamping

current of 10 A.

There are some disadvantages in using the tube clamp. One disadvantage is

the internal resistance which causes a voltage drop of about 45 V for 10 A

through this six-diode clamp. This voltage drop decreases to approximately

zero as the current decreases to zero, and the clamped voltage changes

somewhat as the current through the clamp changes. This is not a serious

problem because the data can be corrected for this effect if necessary. A

second disadvantage is the added capacitance of the tube clamp. The six vacuum diodes and associated wiring add 150 pF from the collector circuit of

24
Figure B-2. Examples of voltage overshoot using solid-state diode

and vacuum tube diode. The overshoot is 100 V for solid-state

diode and 50 V for the tube diode. The vertical scale is 100 V

per division and the horizontal scale is 50 ns per division.

Figure B-3. Collector current for conditions of momentary clamping

and subsequent second breakdown. The top trace is the collector

current when the solid-state diode clamp is used, and the bottom

trace is the collector current when the tube diode clamp is used.

The scale factors are 5 A per small division on the vertical and 50 ns per small division on the horizontal.

25
the TUT to ground. Measurements have been made to determine the effect of

150 pF of additional snubbing to the collector circuit for the fastest

transistors turning off 10 A. The rate of voltage rise is not significantly

reduced, but there is a reduction in the collector current during the time in

which the collector voltage is rising since the stray tube capacitance must

be charged.

Often, the collector voltage of the TUT is clamped momentarily, but the

device experiences second breakdown before all of the collector current is

diverted through the clamp. When the device breaks down under these

conditions, there is a very large reverse recovery current from the

solid-state clamp diodes before the protection circuit can take over. This

surge of current does not occur with the tube clan^) as there is no equivalent

recovery time. The superior behavior of the tube clamp under these

conditions permits the collector current to be measured more accurately

during second breakdown with less stress than with the solid-state clamp.

Figure B-3 is a photograph that shows the difference in collector current at

breakdown between the solid-state clamp euid the tube clamp. It can be seen that the current appears to reach peadc values of 30 A and -20 A with the

solid-state clamp. Since the bandwidth of the current probe used is only

about 50 MHz, it is likely that the peak current is even larger than

indicated in this figure as these transitions are very fast. With the tube

clamp, only one large peak in the current is observed. This peak is caused

by the discharge of the parasitic capacitance associated with the collector

of the TUT. The other large current spikes are not present.

CONCLUSION

It has been found that some difficult measurements of high-voltage

fast-switching transistors can be improved by the use of vacuum tubes in

critical parts of the test circuits. The fast response of the tubes and

freedom from recovery phenomena reduce voltage overshoot in voltage clcimping

and reduce uncontrolled current spikes normally generated in solid-state

clamping diodes when measuring transistor second breakdown.

REFERENCES

B-1. Berning, D. w. , Semiaonduotov Meaeurement Teehnology : a Reverse-Bias

Safe Operating Area Transistor Tester, NBS Special Publication 400-54

(April 1979).

B-2. Jahns, T. M. , Investigation of Reverse-Bias Second Breakdown in Power

Transistors, Massachusetts Institute of Technology MS Thesis, Dept, of

Electrical Engineering, May 1974.

B-3. Blackburn, D. L. , and Berning, D. W. , Some Effects of Base Current on Transistor Switching and Reverse-Bias Second Breakdown, Teah» Digest,

1978 Intermationat Eteatvon Deviaes Meeting, Washington, D.C., December

4-6, 1978, pp. 671-675.

B-4. Blackburn, D. L. , and Berning, D. W. , Reverse-Bias Second Breakdown in

Power Transistors, Pvoe* Eleatviaal Ovevstvese/Eleotvostatio Discharge

Symp., Denver, Colorado, September 24-27, 1979, pp. 116-121.

26
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 81-2204

2. Gov* t Accession No. Z* No.

4. TITLE AND SUBTITLE

Power Transistor Switching Characterization

5. Publication Date

$. Performing Organization Code

7. AUTHOR(S)

David L . Blackburn

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

Mr Project/Task/Work Unit No.

11. Contract/Grant No. NASA Order No.

C-32818-D

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreer. city. State. ZIP)

Lewis Research Center

National Aeronautics and Space Administration

Cleveland, Ohio

13. Type of Report & Period Covered Annual, July 1, 1979

to June 301 1980 '

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.) The results of the first year of an experimental investigation of the switching

characteristics of power transistors are discussed. The devices studied were housed

in TO-3 cases and were of an rt-p-rT-rt vertical dopant structure. The effects of

the magnitude of the reverse-base current and temperature on the reverse-bias second

breakdown characteristics are discussed. Brief discussions of device degradation

due to second breakdown and of a constant voltage turn-off circuit are included. A

description of a vacuum tube voltage clamp circuit which reduces clamped collector

voltage overshoot is given.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first tetter of the first key word unless a proper name;

separated by semicolons)

Base current, reverse; degradation; reverse bias; second breakdown; switching;

temperature; transistors, power.

18. AVAILABILITY Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF

PRINTED PAGES

For Official Distribution. Do Hot Release to NTIS UNCLASSIFIED

29

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22. Price

1 1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161 UNCLASSIFIED

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