4UTHOR 1 SUBJECT I NO. 

GENERAL@ ELECTRIC 

SCHENECTADY, NEW YORK - 

TECHNICA L INFORMATION 

SERIES 

Martzloff, FD 

nology Laboratories Rept. No. 64GL173. 

;El' WORDS PIV, overvoltages, transients, semiconductors, 

Reverse Transient Overvoltages 

ADDITIONAL COPIES AVAILABLE AT 

Distribution Unit, Bldg. 5, Room 345 

Research and Development Center 

P.O. Box 8, Schenectady, New York 12301 

breakdown 

semiconductors and 

transients 

d 

GOVT, CLASS - 

NO. PAGES 

57 

INFORMATION PREPARED FOR: 

67-C-066 

DATE 

March 1967 

nTLE Serniconduc tor Performance under 

SUMMARY Reverse transient voltage pulses were applied to five 

types of selected semicondutors until failure occurred. 

A number of test conditions were used, including 

fast and slow rise pulses, room temperature and high tern 

perature, steady-state current flowing or not flowing at 

- the time of surge application. 

Within each type of device, various groups were 

tested with different PIV ratings, or avalanche voltage 

class and aging or no aging. 

The results, covering two noncontrolled avalanche 

diodes and one controlled avalanche diode, plus two types 

of SCRs show the following: 

1. There is no correlation between the voltage 

rating (PIV) and the withstand capability under 

transient conditions. 

2. Aging does not affect conclusion 1, nor does it 

substantially decrease the withstand capability. 

This report was formerly issued as Advanced TechG.E. CLASS 
Intkoduct ion 

Conclus ions 

Tests on 4JAlO Rectifiers 

TABLE OF CONTENTS 

Tests on C20 Controlled Rectifiers 

Tests on A92 Controlled-Avalanche Rectifiers 

Tests on A90 Rectifieys 

Teets on 25260 Controlled Rectifiers 

Appendix 

References and Acknowledgements 

Page 

1 
Introduction 

The behaviour of semiconductors under reverse transient overvoltages, and 

especially their failure levels, has been investigated during the two previous 

years under the Transient Otrervoltage Pooled Program. 

One of the most interesting conclusions reached after testing several types 

of devices was that the PIV rating of the device was not a significant indication 

of the withstand capability of this device beyond its rating. In other words, 

an increased PIV was not a guarantee of a higher transient withstand capability. 

With hindsight, this may now appear obvious; however, questions were still raised 

on the general validity of this conclusion. 

Therefore, the test program reported here was undertaken for the purpose of 

verifying this lack of correlation on a greater number of types, and also to 

investigate if it would remain valid after ageing, such as that produced by high 

temperature storage. 

Test Results and Discus$ions 

Tests and discuesibns leading to specific conclusidns are grouped in five 

separate sections of this report, corresponding to the three types of rectifiers 

and two types of SCRs included in this program. General conclusions are sumnarized 

below. 

Conclusions 

1. The PLV rating or the voltage class of an avalanche device does nbt offer 

a valid indication on the transient withstand capability beyond the rating. 

2. This conclusion is likely to be valid for all semiconductors under the 

present system of ratings, and is not affected by factors such as ageing and 

temperature. 
Tests on 4JA10 Rectifiers 

Srmannry of Tests 

Test Samples: 200 volts and 400 volts PIV unite, aged and unaged 

Tests: Reverse transient with no bias, to failure 

Results: Neither PIV nbr ageing significantly affect the 

failure level 
TO 

CRO 

Notes: 5 ohm - 0.001 RC to reduce overshoot 

Connection of probe 18" from test piece requ1rt.d for hot 

oven, but checked against connection at test piece with 

open-door oven. 
Tests on 4JA10 Rectifiers 

1. Purpose of the Tests 

Limited tests (63G~133) on new samples of these diodes had shown that the 

"TIV" did not seem related to the 'TIV" of the diode. Therefore, a larger group 

of samples were obtained in order to conduct a planned experiment to show the 

effect of the PIV rating on new as well as on aged samples. 

2. Test Specimens 

Four groups of A10 rectifiers were obtained from the Rectifier Components 

Department. First, two groups were obtained by precise voltage grading, 200 and 

400 volts PIV. Then, half of each group was aged by storage at 200°c for 1000 

hours (with little effect on the characteristics), followed by 500 hours at 225O~ 

which produced a significant shift of the characteristic as tested on the 

production line. In this manner, a direct comparison is possible between aged 

and unaged devices of the same voltage class, as well as comparisons between 

voltage classes, aged or unaged. 

3. Test Procedure 

The reverse voltage pulse was supplied by a square pulse generator consisting 

of a coaxial cable discharged into a resistance equal to its surge impedance (see 

sketch). Since the highevoltage probe for measuring the voltage across the diode 

could not withstand the oven temperature, it was connected immediately outside of 

the oven, at the selector switch. However, comparison of the voltages there and 

directly across the diode (with a cold oven) showed no significant difference. 

The RC network at the thyratron was added to minimize the overshoot on the front 

of the square pulse due to the necessary cable length between the thyratron and 

the men. 

This diode does not exhibit any reverse avalanche phenomenon under reverse 

voltage. The voltage of the applied wave was raised after each pulse application 
by steps of 100 volts, and the voltage at which failure occurred recorded. 

Tests were made with 25Oc and 150'~ ambient on the four groups, and the 

results are tabulated below. Failure was on the surface of the junction. 

I Avg. 1 1800 

200 Volts, 1 400Volts 1 400Volts, 

Testing the eight averages of these test groups against each other, or 

grouping for instance all 400 volts units on one hand and all 200 volts units 

on the other hand or all aged versus all unaged show that the differences are 

significant at the 5% level and generally at the 2% level. 

Nevertheless, the differences in the averages are small. 

I have no explanation to suggest for the increase in withstand voltage 

when the test temperature is increased from 25Oc to 150~~. In order to show a 

possible effect, 2 of each group of diodes were baked for 30 minutes at 150°c 

(approximately the time required to insure stabilization of the oven and junction 

temperatures), then brought back to 25Oc and tested. The failure levels for each 

group was not significantly different from the level found at 25Oc without prior 

heating. 

- 6- 
Comments from the Rectifier Components Department suggest that this may 

be due to higher bulk leakage and avalanche, although contrary to steady-state 

surface quality-temperature relationships which they have observed. 

Another way to present the results of these tests is to express the 

averages in percent of each other. This yields the following comparisons: 

1. Effect of PIV Rating 

New Units 

At 25Oc 

At 150°C 

Aged Units 

At 25O~ 

At 150°C 

2. Effect of Aging 

200 Volts Units 

At 2s0c 

At 150°C 

400 Volts Units 

At 25Os 

At 150 C 

200 Volts PIV 

Withstand Level (index) 

Unaged Units Withstand 

Level (index) 

3. Effect of Temperature 

Withstand Level at 25Oc 

New Units (index) 

200 volts 100% 

400 volts 100% 

Aged Units 

200 volts 190% 

400 voltso 100% 

400 Volts PIV 

Withstand Level 

400 Volts PIV 

Withstand Level 

Withstand Level at 150~
SumrY 

1. Increasing th8 PIV from 200 V to 400 V increases the withstand level 

by no more than 4%, aged or unaged, at 25 or 150°c ambient. 

2. Ageing by storage at 225O~ for 500 hours decreases the withstand level 

by no more than 6% 25 or 150°c, 200 volts or 400 volts. 

3. The withstand level at 150°c ambient is 170% of the level at 25O~ 

ambient, 200 volts or 400 volts, aged or unaged. 
Tests on C20 sCR'~ 

Stlnmrary of Tests 

Test Samplee: 100 qnd 400 volts PIV, aged and unaged. 

Tests: Reverse ttanrrient with and without bias. 

Results t The device. exhibits avalanche characteristics 

but fails at the surface. Neither PIV, ageing 

or bias affect the failure level. 
TO 

CRQ 

Notes: S ohm - OLIO1 RC to reduce overshoot (see un1odc.d gemtrator 

oscL'Llogram #1 next page) 

Connection of probe 18" from teat piece required for hot 

oven, but checked against connection at tent piece wfth 

open-door oven. 
Tests on 020 SCRs 

1. Purpose of the Tests 

Following earlier tests (THS 63GL144) on this device, it appeared worthwhile to also investigate for this device the possible effect of ageing. Ian 

addition, no tests had been performed on the SCR during conduction; in view of 

the substantial reduction of the withstand capabilities when conducting observed 

on some diodes, this also seemed to warrant investigation. 

2. Test Specimen 

Four groups of C20 SCRs wkre supplied by the Rectifier Components Department. 

First, two groups were obtained by precise voltage grading, C20A and C20D. Then, 

half of each group of A's and b's were "aged" by storage at 200% for one week, 

in order to produce a significant shift in the device characteristics as tested 

at the production line. In this manner, a direct comparison is possible between 

aged and unaged devices of thci same voltage class, as well as comparison between 

voltage classes, aged or unaged. 

3. No-Load Tests 

A first test series was performed by applying reverse voltage pulses to the 

SCR without any steady-state voltage or current. 

The pulse was delivered from a square pulse generator consisting of a coaxial 

cable discharged into a resistance equal to its surge impedance (see sketch). 

In contrast to the capacitor discharge test wave wad previously, (4) this delivers 

a pulse with a flat top following a finite rise time instead of the exponential 

decay in the tail of the capacitor discharge. At voltages close to the anticipated 

knee of the reverse characteristic (which earlier tests indicated)(4) this should 

cause a more constant current flow, holding the voltage at one point of the curve. 

First, single pulses were applied, with increasing voltage amplitude, until 

failure of the device was observed, as shown by a "chop" of the voltage wave. 
Output of unloaded generator 

1600V x 6 )IS pulse 

Vertical: 500~/div. 

Horizontal: 1 ps/div. 

(same settings for all osicllograms 

on this page) 

Failure of a C20A SCR in 3.8 ps, 

while voltage was clamped at 

1050 volts. 

Failure of a C20D SCR in 2 ps, 

while voltage was clamped it 

1000 volts. 

Failure of a C20D SCR in O.lps, 

before avalanche could produce 

any voltage clamping. 

Failure of a C20A SCR (aged by high 

temperature storage) in 3 ps, while 

voltage was clamped at 1200 volts. 
With this test procedure, the onset of avalanche is quite apparent as the 

voltage recorded across the device stops increasing from shot to shot while the 

generator voltage is still increased. This approach normally produces the failure 

level corresponding to the minimum voltage and maximum time to failure. 

A second tbt procedure consisted in pre-setting the output of the pulse 

generator at values above the minimum determined in the first series. Presumably, 

this could produce failures at higher levels corresponding to shorter times to 

failure. 

Both of these test methods were applied on all four groups of test pieces. 

Furthermore, two series of tests were made with a 25'~ ambient and a 100°C ambient, 

The failure level and time to failure are tabulated below. Typical test waves 

are also shown in the test oscillograms, corresponding to a preset voltage of 1600 

volts, a duration of 6 ps, and a rise time of 0.1 ~s on all oscillograms shown. 

4. No-Eoad Test gesults 

C20A C20D 

Test Sample C20A C20D Aged Aged 

volts and time to failure in ps 

Test Condition 

25'~ Increasing pulse 1100 6 1000 5 1000 6 900 6 

1100 5 1000 6 900 5 1100 6 

1000 4 1100 4 1000 5 1000 5 

1110 5 1100 5 1100 4 1100 5 

Preset 1600 V - - - .l* 1000 5.5* 1100 4* 1000 -5 

6 YS 1000 2 - - - . l* 1000 .2 1000 3.5 

1000 4* 1000 3 - - - . l* - - - .1 

100~~ Preset 1400 V no failure no failure no failure --- .1 

(3 ps long only) 

Preset 1600 V (1) 1050 3.8 (2) 1200 2.5 1000 3.8 1100 5 

6 Ys 1100 2.5 1000 2 (3) 1000 4.5 1200 3 (5) 

1000 3 --- .1 (4) 1200 3 1100 2.5 

Preset 3200 V --- .1 1100 .8 - - - .1 *-- .1 

6 1.r~ 1000 .g 1100 .9 1100 .2 --- .1 

--- Indicates failure before onset of avalanche. 

* These samples were disdected after failure--see discussion below. 

(1)-(5) indicate oscillograms of failures on opposite page. 
I I 

I I 

I I 

I I 

1 I I I 

t 

P ti 

time to breakdown 

Ii Ip 

reverse current 
5. Discussipn of the Results 

The avalanche effect in the SCR, although neither deliberate nor advertized 

as such is effective in clamping the voltage appearing across the device and 

will reduce the risk of flashover, or breakdown of the surface. However, it is 

apparent that the time to failure, at the voltage corresponding to the avalanche, 

can be quite short. Thus, the effectiveness of this avalanche is rather limited, 

On the other hand, it makes it rather difficult to select a quantitative criterion 

of the ability of the SCR to withstand reverse transients in the failure range. 

In order to investigate the possibility that the very short time to failure 

observed or some samples could be the result of a different breakdown mechanism, 

six cells were carefully examined by RCD. These units are marked (*) in the 

test result table, and were selected in each of three groups to include a 0.1 ps 

time to failure aM a time to failure over 4 ps/ However, no correlation was 

found between the time to breakdown and the type, degree or location of the 

failure. All werQ due to surface arcing. 

Inspection od the test results reveals some significant points. 

1. The increasing pulse approach produces the longest time to failure, 

with voltages somewhat lower than the preset pulse approach. 

This is not surprising as this method, although bringing the device 

into thd avalanche state, does it with the minimum current. This 

means that the voltage is somewhat lower on the Zener characteristic 

than it trould be for the higher preset condition. Thus, breakdown 

of the surface dielectric corresponds to the right part of its volttime characteristic. 

This is sho~ inthe sketch on the opposite page. A hypothetical 

curve for the breakdown of the surface insulation and a zener curve 

for the reverse avalanche are drawn. Consider two currents I and Ii, P 
t ime to breakdown reverse current 
respectively for preset and increasing pulse methods, producing the 

corresponding avalanche voltages Vp and V These voltages will in i' 

turn cause breakdown at the times t and ti, with ti longer for the P 

lower Ii than t for the higher I 

P P ' 

As a result of this, low voltage preset pulses of short duration 

may not cause failure of the device. 

2. There is no significant difference in the time to breakdown observed 

here for the A's or D's or for the aged or unaged SCRs. Even if there 

were, as reported in 63GL144, the practical value of this difference 

would be cancelled by the fact that short time failure (0.1 ks) can 

and does happen in any group, so that the average time to breakdown 

is meaningless. 

3. The avalanche voltages observed at 100°c tend to be higher than those 

at 25'~ (not on the same devices). This is due to the fact that the 

thermal vibration of the semiconductor lattice increases with temperature 

and acts as a brake on the carriers, so that a higher accelerating field, 

i.e., the avalanche voltage, is required to maintain the avalanche state. 

4. Increasing the preset voltage value tends to shorten the time to failure, 

which means that as the corresponding point climbs on the zener 

characteristic to slowly increase the avalanche voltage, the failure 

point of the surface dielectric moves towards the left on its ascending 

volt-time characteris tic. 

6. Conclusions From No Load Tests 

For reverse transients such that the avalanche voltage of the device can be 

reached or even exceeded, there is very little if any difference in the performance 

of the four groups of SCRs. Specifically, this means that the PRV rating of these 

devices does not reflect a difference in the ability to withstand surges beyond 

their ratings, and that this conclusion is not affected by ageing of the device. 
Timing of Reverse Transient 

Top Trace: voltage across SCR 

with transient applied during 

conduction 

Bottom Trace: Current in SCR 

with current reversal but no 

turn off 

Sweep: 2 ms/div. 

Same as above except 1 ps/div. 

with sweep triggered by reverse 

voltage transient and zero lines 

added 
7. Tests With Steady-State 60 cps Voltage Applied 

Earlier tests (62GL191) have shown the substantial reduction of the withstand capabilities of diodes when the reverse transient is applied during 

conduction. Similar results are also reported by Chowdhuri. (5 

7.1 Tests with Capacitor Discharge 

In order to apply this type of transient to the SCR, the test circuit 

sketched on the opposite page was first used. It is similar to the circuit 

suggested in AIEE standard #59 for the measurement of reverse recovery 

time. (8) A steady-state 60 cps voltage is applied across the SCR, which 

is turned on soon after the start of the forward voltage when the gate is 

fired from the 100-10 K divider. A capacitor charged by a DC supply is 

discharged into the combination of the output capacitor, resistor and test 

piece, when the thyratron is turned on by a synchronous firing circuit. 

Forward current is limited by the I00 ohm resistor, and the pulse is 

blocked from the 60 cps supply by the 0.3 mH choke. 

The first oscillogram shows timing of the reverse transient to 

coincide with the peak of the forward current (the trace brightener of a 

Tektronix 535 was used to recotd the otherwise invisible spike at this slow 

sweep rate, even with repetitive shots). 

The second oscillogram shows at a high sweep rate the applied reverse 

voltage transient and the resulting reverse current in the SCR, with 

forward current resuming as soon as the reverse transient has vanished. 

In these two oscillograms the voltage amplitude was held below the 

avalanche region, so that negligible current flows after the initial 

reverse recovery period. 

The effect of timing on the device behaviour is illustrated by the 

oscillograms on the next page. There were all recorded with a voltage 

output of the surge generator sufficient to drive the SCR into avalanche. 

- 19- 
All Oscillograms: 

Top Trace: Voltage across SCR 

500 ~/div. 

Bottom Trace : Current through SCR 

20 A/div. 

Sweep: 0.1 psldiv (repetitive) 

No steady state 60 cps voltage 

No forward current (gate open) Forward current flowing at start of surge 

Surge applied at peak of forward 60 cps voltage 

No forward current (gate open) 

-9nForward current flowing during alternate 

4 cycle 

Y V 

Surge applied at peak of reverse 60 cps voltage 
Oscillograms 1, 2, 3, 5 corresponding to surge application at 

various times but not during forward conduction all exhibit the same 

general appearance. The voltage trace starts above, at, or below zero 

depending upon the valde of the 60 cps voltage applied at the time of the 

impulse, but the current is not significantly affected. The apparent 

oscillations of the current and voltage traces are most likely due to the 

combination of natural frequencies in the circuit with the swinging of the 

current and voltage on the Zener characteristic. 

Oscillogram 4 corresponds to impulse application at the peak of 

forward current (conditidn shown on previous page, note the current trace 

starting above zero). It differs significantly in that the initial current 

rise is steeper, the current is larger and the large oscillations absent. 

The Zener effect is quite apparent between .1 and .4 ps where the current 

changes 2:l with only a 10% change in voltage. 

These oscillograms show that while there is a difference in the 

response of the device, the ultimate performance under transient conditions, 

within the limitation of rise times longer than 0.1 ps, will still exhibit 

the voltage clamping effect of the avalanche characteristics of the device. 

In order to illustrate further the initial response of the device, 

the three oscillograms on the next page were recorded during conduction, 

below, at the start, and well after the start of the avalanche. 
Top Trace: Voltage across SCR 

200 ~/div. 

Bottom Trace: Current in SCR 

10 A/div. 

Sweep: 0.1 ps/div. 

Top Trace: Voltage across SCR 

500 ~/div. 

Bottom Trace: Current in SCR 

20 A/div. 

Sweep: 0.1 ps/div. 

Top Trace: Voltage across SCR 

500 V/div. 

Bottom Trace : Current in SCR 

40 ~/div. 

Sweep: 0.1 ps/div. 

-22- 
Oscillogram 1, with only 500 volts, shows a current flowing during 

the voltage rise, with a peak at 15 amperes, corresponding to the "charging" 

current of the junction capacitance, which is due mostly to minority carrier 

storage. 

Oscillogram 2, at the onset of the avalanche above 1000 volts shows the 

same initial current with a peak at $8 amperes followed by the avalanche 

current. 

Oscillogram 3, well above the start of the avalanche, shows the initial 

"charging" current soon swamped out by the avalanche current which reaches 

80 amperes for 1300 volts. Note that this voltage is significantly higher 

than those reported as causing failure in the no-load tests. There are 

probably two reasons for this: one is the higher steady-state junction 

temperature due to the flow of forward current (the test was made with 25O~ 

ambient but after steady-state current flowed for several minutes). For this 

particular specimen, the volt time characteristic of the junction insulation 

surface was then such that no breakdown would occur for this short pulse 

duration. Raising the generator voltage effectively increases the time for 

which the SCR is held in the avalanche state as the portion of the exponential 

decay above 1000 volts will then increase. This increased duration above 1000 

volts then makes possible for a number of devices to fail by breakdown of the 

surface, as shown by the failure voltages and times tabulated below. 

C2 OA C20D . Test Sample C20A C20D An ed An ed 

Test Condition Numbers show volts at failure and time to failure 

Junction at Y 80°c 1100 .5 -1100 .2 1100 1.5 1000 .4 

(due to fw'd current) 1200 .4 1200 .4 1000 .5 1100 .9 

1100 .9 1100 .5 1000 .7 1000 .4 

Pulse applied by 

increasing stepe wPth 

.1 x 5 bs waveshape, 

applied at peak of 

forward current 

Applied reverse voltage (no-load 

generator output) was in the 

1100-2000 volts range for these 

tests. 
Output of unloaded generator 

(500 volts/div, .I ps/div.) 

Voltage across SCR with no 

steady state 60 cps 

(500 volts/div, .l ps/div.) 

Voltage across SCR with 60 cps 

forward current 

(500 volts/div, 1 ps/div.) 

Failure of CZOA SCR with forward 

current in 3.5 ps 

Top: 500 volts/div. 

Bottom: 40 amp/div. 

Sweep : 1 ps/cm 

Available pulse length 6 ps 
7.2 Tests with Square Pulse Generator 

In order to eliminate the effect of the decaying waveshape obtained 

from the capacitor discharge circuit, a few more samples were subjected to 

the same type of test, i.e., impdlse during forward conduction, but the 

impulse was delivered from the cable discharge generator. 

The test circuit is shown oh the opposite page. The only difference 

from the capacitor discharge test is that a cable is used in a manner 

similar to no-load tests described in section 3. 

The oscillograms on the opposite page, as well as others in this report, 

exhibit some minor oscillations at the start of the traces, between time 

zero and 0.1 ps. These are more likely to be caused by measurement problems 

than by device phenomena, so that the reader should be cautioned against 

attempting to draw conclusions from these initial oscillations. Several 

examples of these spurious signals are discussed in the appendix. 

Oscillogram 1 shows the output of the test circuit with no test piece 

connected, while oscillograms 2 and 3 correspond to the same setting, with 

the test piece connected. Oscillogram 2 was recorded with no forward 

current in the SCR while oscillogram 3 was recorded with forward current. 

Compared to the unloaded output, the rise time of the voltage appearing 

across the SCR with forward current is substantially longer, as the result 

of the larger junction capacitance. The initial overshoot of the generator 

output is also removed by the ldading effect of the test sample, which 

results from this minority carrier storage due to the forward current. 

Oscillogram 4 shows, with a slower sweep rate, a typical failure 

oscillogram. 
Test results with this procedure on the four groups of C20 SCRs are 

tabulated below: 

C2 0A C20D 

Test Sample C20A C2 OD Aged Aged 

Test Condition Numbers show volts at failure and time to failure 

Junction at = 80°C 

5 ps square pulse applied 1000 .4 1300 3 1200 .4 1200 .6 

by increasing steps at 1100 4 1000 6 1000 .9 1100 1 

peak of forward current 

Same except pre-set pulse 1100 35 

for higher voltage with 1300 1 

high avalanche current 1200 2 

7.3 Discussion of the tests with forward current and comparison with 

no-load tests 

The immediate effect of the forward current is to increase the 

effective junction capacitance due to minority carrier storage, compared 

to the lower diffusion or barrier capacitance existing when there is no 

forward current. 

The consequence of this capacitance is that the rise time of the 

voltage is increased so that the avalanche effect can take place without 

the overshoot in voltage which was observed in the no-load tests, and 

which sametimes caused (see page ) flashover of the device before the 

voltage clamping effect could take place. 

Therefore, the flow of a forward current at the time of the reverse 

transient application seems to reduce the risk of early flashover. The 

device behaviour is then controlled exclusively by the avalanche voltage 

and the dielectric capability of the anode junction surface. 

The avalanche voltage of this device, as previously stated, is not 

deliberately controlled, so that sample variations may be expected. 

Furthermore, this voltage will depend on the junction temperature so that 

the test values of the avalanche voltage with forward current reported 
hbre, i.e., with junction at about 80°C can be expected to be somewhat 

higher than the values with no load. 

Failure of the device is caused by surface flashover. This flashover 

voftage should exhibit for one sample the typical volt-time characteristic 

of insulation breakdown, for which of course only one point per sample be 

obtained here since each flashover is destructive. However, considering 

all the samples, there is a trend towards shorter time to flashover with 

increasing avalanche voltage. Thus, if a higher transient current is 

available and can drive the voltage higher on the avalanche characteristic, 

failure is likely to occur for a shorter pulse duration; however, the data 

shows this increase in bulk avalanche voltage to be rather small. 

8. Conclusions on the C20 Tests 

Failure occurs by surface arcing while the voltage is held in the 

1000 volts range by the avalanche effect of the anode junction. 

There is no significant difference between the performance of a 200 

or 400 volt SCR, and ageing does not change this lack of correlation. 

Ageing of the SCR by high temperature storage does not significantly 

affect the transient behaviour of the device. 

Timing of the transient during forward current does not reduce the 

withstand capability of the device; actually, this capability may be 

enhanced as the increased junction capacitance will reduce the initial 

rate of voltage rise, before avalanche can occur. 

While the transient behaviour is dominated by the avalanche phenomenon, 

this should not be construed as a suggestion that this can be used to 

advantage, as the present withstand capability of the junction surface 

is such that the SCR will fail in the range of this avalanche voltage. 
Tests on A92 Rectifiers 

Summary of Tests 

Test Samples: 500, 900, 1100 volts units 

Tests: Avalanche characteristics 

Reverse transients st slow and fast rate of rise, 

no bias 

Results: Bulk failure will occur for low (relatively) current 

of long duration 

Surfate failure will occur for very high currents, 

even bf short duration 

The avalanche voltage class is not a valid 

indication of the transient voltage withstand 

capability. Energy levels are eseentially 

the same for all. 
TO 

CRO 

I \\O AC I OVEN I 

I_______--_ ------' 
Tests on A92 Controlled Avalanche Rectifiers 

1. Purpose of tye Tests 

All tests reported so far in this program were conducted on non-controlledavalanche diodes. It is then appropriate to investigate the behaviour of 

controlled-avalanche rectifiers under reverse transients in excess of their ratings. 

- 2. Test Specimens 

Three groups of A92 rectifiers were supplied by the Rectifier Components 

Department, graded 1100, 900 and 500 volt avalanche. The three groups were 

obtained from two diffusion processes. 

3. Test Procedure 

Reverse voltage was applied to the diode without other steady-state voltage. 

The avalanche of the rectifier would lower the impedance of the device to the 

point where the available surge generators had too high internal impedance to 

deliver sufficient energy to the test piece. An LC line was then built for 

storing the energy, instead of the cable used for smaller devices. The test 

circuit is shown on the opposite page, with a typical oscillogram of the voltage 

and current delivered to a fixed resistor instead of the non-linear test piece. 

The rise time is relatively long, but since the tests produced failures in 

long times, as will be seen below, it would seem that this low rate of rise is 

not objectionable. 

Tests were perfdnned with 2S0c and 150°C initial ambient temperatures on 

several diodes. In addition, the avalanche characteristics of one diode below 

the failure level were obtained with a range of initial ambient temperatures in 

order to correlate with the rise in avalanche voltage observed during the length 

of the test pulse. 

Avalanche Characteristics 

Starting with 500 volts for the reverse pulse, the setting of the generator 

was progressively increased, while the voltage across the diode and the resulting 

current flowing through it were simuataneously recorded. 
Figure 1 

Avalanche characteristics 

of 500, 900, & 1100 volts 

A92 test samples 

voltage across diode - volts 

voltage across diode - volts voltage across diode - volts 

Figure 2 Figure 3 

Avalanche characteristics of a Junction temperature as a function 

500 volt diode, measured at of the voltage measured across the 

start of pulse, with junction diode for the constant current 

at the initial temperature shown shown 
The test points for one device fell on an approximately straight line with 

linear volt and ampere scale (see Figure 1). Furthermore, the lines for a group 

of devices graded in the same voltage class lie within a 100 volt band and run 

parallel; only one has been drawn in Figure 1 in the case of the 500 volt 

characteristic which corresponds to three test samples. The slope of the 500 

volt rectifiers on one hand and 900 and 1100 on the other hand are different, 

corresponding to constant dynamic resistance of 1.6 and 2.6 ohms respectively. 

With increasing current levels, an increase in the voltage across the diode 

between the start and the end of the pulse became apparent, as seen on the 

oscillogram on the opposite page. In order to correlate this voltage rise with the 

change of avalanche voltage (or, more accurately, voltage across the device) due to 

tepperature, a series of recordings were also made on one 500 volt diode with 

several values of ambient temperature, assumed to be the initial junction temperature. Thus, from the family of curves shown in Figure 2, the instantaneous temperature increase at various times of the pulse duration at constant current can be 

calculated. This is shown in Figure 3, in a plot of avalanche voltage vs. average 

junction temperatures. 

For two specific current levels, the_ instantaneous volt amperes dissipated in 

the device were calculated; these two values of energy input into the semiconductor 

were compared to the temperature increase derived from Figure 3 and the apparent 

voltage change. Excellent correlation was found, i.e., for energy inputs with a 

2:l ratio, the calculated temperature increase was also 2:l. 

Therefore, with the high reverse currents involved in this investigation (well 

above the rated values) the pulee duration has the effect of increasing the voltage 

across the device for long pulses. 

The question then is to know which will increase faatest. The temperature of 

the semiconductor will increase until the melting temperature is reached at a hot 

spot (temperatures shown in Figure 3 are average temperatures.) On the other hand, 

the voltage also increases with this temperature, so that flashover voltage may be 
Failure of 1100 volts unit at 

1900 volts, arc across varnish 

Test at 30°c ambient 

Top trace: Voltage across diode 

500~/div. 

Bottom trace: Current in diode 

100A/div. 

Sweep : 10 ps/div. 

Failure of 500 volts unit at 

1400 volts, arc at upper PP' 

junction due to field pinch 

Test at 150~~ ambient 

Top trace : Voltage across diode 

500~/div. 

Bottom trace : Current in diode 

100A/div. 

Sweep : 10 ps/div. 

Bulk failure vs. Arc failure 

Note how breakdown occurred as voltage had begun to decrease shortly 

before end of applied pulse, in contrast with arc failure which occurred 

during voltage rise. 
reached towards the end of a pulse which could not produce at the start a high 

enough voltage to cause immediate flashover. 

It may also be possible to derive from these tests some information on the 

hot spot effect in the junction. When a "bulk" failure is observed, (i.e., not a 

surface breakdown), it may be assumed that the melting temperature of the material 

wag reached, or exceeded at one hot spot. As discussed above, the increase in 

voltage observed between the initial voltage at the start of the pulse crest and 

theqvoltage reached just before failure yields an estimate of the average temperature rise during this interval. A comparison of the difference between the 

initial temperature plus this estimated average increase, and the temperature at 

which local thermal failure occurs should yield an index of the importance of 

hot spot effects. The small number of tests as well as the general scope of the 

present investigation, however, would not justify a thorough evaluation of this 

idea. 

Tests to Failure 

Typical oscillograms recorded during the tests are reproduced on the opposite 

page. Failure is apparent as the voltage across the diode is abruptly chopped 

while the current on the diode jumps from the Zener current to the maximum current 

available from the pulse forming network. 

Results on individual diodes are tabulated below, listing ambient temperature, 

current, and voltage at failure and nature of the failure: 

Voltage Grade 

500 30°c 

30OC 

150~~ 

150% 

900 30°c 

30°C 

1100 30% 

Diffusion I 

450 amp 1700 volts, surface 

1500 volts, surface 

160 amp 1450 volts, bulk 

120 amp. 1250 volts, bulk 

200 amp 1750 volts, surface 

300 amp 1950 volts, surface 

190 amp 1900 volts, surface 

Diffusion I1 

30'~ 300 amp 1100 volts, surface 

30% 1150 volts, surface 

150°C 160 amp 2400, surface arc 

30% 280 amp 2000 volts, surface 

300C 420 amp 2200 volts, surface 
I I I I I I I I less than 10 ns. 

0 

(Power pulse ends at 50 ns, but 

cable reflections hold the voltage 

up for a longer duration) 
Discussion of the Test Results 

The relatively slow rise of the voltage, made necessary by the design of 

the line, might appear a limitation of the tests. Work sponsored by ATL at the 

Laboratory Operation of the Power Transmission Division includes an evaluation 

of semiconductor performance under fast transient conditions(l). Instrumentation 

as well as short pulse generator techniques were developed in this work, with the 

capability of producing and accurately recording pulses of several kilovolts with 

rise times of less than 10 nanoseconds. A sample of the A92 diodes involved in 

the present test series was also subjected in Philadelphia to this extremely 

steep voltage pulse. The oscillograms on the opposite page show the output of 

the unloaded pulse generator and the voltage measured across the diodes. 

These show that the avalanche voltage can be reached in less than 10 nanoseconds, 

and that the device will then hold the voltage at that level without any disturbance. 

The tests made with the slow rise time have shown that breakdown will occur onIy 

after a sufficiently high voltage is reached on the dynamic characteristic of the 

diode to cause flashover. Since the avalanche voltage is reached without other 

overshoot or excursion in 10 nanoseconds, the failures produced with the slow rise 

circuit, during avalanche, represent valid tests which are not restricted by the 

slow rise time. 

The failure levels for the Diffusion I group, due to surface breakdown, are 

all in the 1500 -1950 volts range, without any correlation with the voltage class 

of the diode. Failures in the bulk occurred at lower avalanche voltages, but 

these are not related to voltage but rather to initial temperature and total 

energy input in the device. 

The failure levels of the Diffusion I1 group are all lower. This fact was 

previously established by the Rectifier Components Department and led to abandonment 

of this experimental process. 

(1) This work will be reported by the Philadelphia Llaboratory Operation in the 

near future; the distribution of the report will include all sponsors of the 

present Pooled Program. 
r'0r.c lusion of the Tests on the A92 Rectifiers 

These rectifiers exhibit a substantial capacity for dissipating reverse 

surge epergy. Unless the initial junction temperature is already high and the 

pulse duration long enough to raise the temperature of the junction hotest spot 

ta'its melting point, the actual failure may still be caused by arcing over the 

junction surtace. The avalanche characteristic is such, however, that it 

requires extremely high reverse current to drive the voltage up, in fact - well 

above the rated current. 

On the other hgnd, a lower current is required to bring a high avalanche 

voltage diode to the danger point (see Figure 1) so that a high voltage diode 

is more sgnsitive to a current source type OF surge. This is pointed out in - 

the specification sheets and application notes. 

Failqre by surface breakdown can occur with high currents, while bulk 

failure Ls more likely with lower but longer current valves. 

In the case of surface breakdown, when it occurs, the rating of the diode, 

agaiq, has no bearing on the actual failure level; however, surface breakdown 

is less likely to occur on lower voltage diodes for a given enera pulse, ad 

in that sense the rating of the diode is significant. 
TBsts on A90 Rectifiers 

Sumnary of Tests 

Test Samples: Small number of 200 ad 1200 PfV units 

Tests: Rderse transient with no bias 

Results : Neqther PIV of the unit nor energy of the transient 

afdect the failure level 
1200 volts PIV unit test at 

30°c ambient 

Failure at 2000 volts in 12 ps 

Arc across varnish 

200 volts PIV unit test at 

30°C ambient 

Failure at 2000 volts in 40 ws 

Arc across varnish 

s PIV unit t 

150° ambient 

1200 volt 

Failure at 1800 volts in 12 ps 

Arc at upper PP+ junction 

200 volts PIV unit test at 

150~~ ambient 

Failure at 2000 volts in 4 ps 

Arc across varnish 

Oscillograms showing failure of A90 rectifiers at 30°c and 150~~ 

All oscillograms: top trace: voltage across rectifier - 1000 volts/div. 

bottom trace: current in rectifier - 100 A/div. 

(except first oscillogram, 10 A/div.) 

sweep: 10 ps/div. 

applied pulse width: 60 ps 
.a. Tests on A90 Diodes 

A limited number of A90 diodes were subjected to tests similar to those 

applied to the A92 diode in order to illustrate the difference in the performance 

between a controlled-avalanche rectifier (A92) and a non-avalanche rectifier 

(A90) . 

Six units were obtained from the Rectifier Components Department, three 

were 200 volts and three were 1200 volts units. 

Four units were subjected to the long, slow rise pulse generated by the LC 

line described for the A92 tests. One of each voltage class was tested at 30°c 

ambient and at 150~~ ambient. This provides direct comparison with the A92 

tests. 

All of these three test series were made with no steady-state bias on the 

diodes. 

In the case of the slow rise pulse, the voltage of the generator was 

.progressively increased, with a constant pulse duration of 60 (rs, applied by 

single shots, until the failure occurred, as recorded in oscillogramo 1 to 4 on 

the opposite page. The step appearing in the pulse top is due to a partial arcover in the thyratron of the pulse generator, reducing the thyratron drop and 

thereby increasing the voltage applied to the diode. Failure, of course, occurs 

when the voltage trace is abruptly chopped while the current trace jumps from 

zero to the val;e of current available from the pulse generator. 

Although the time to breakdown of the 200 volt diode at 150'~ was shotter 

compared to the time at 30°c the difference is well within what might be expeated 

from a gap breakdown at the minimum voltage which can caure breakdown. 

Dissection of he four unite revealed surface breakdown. In one case, 

removal of the tracked-over varnieh allowed the diode to recover satisfactory 

characteristics, However, had 60 cps power-follow current been available, the 

flashover triggered by the pul$e would most likely have caused permanent damage. 
No-load output of generator 

(2000 volt setting) 

Failure of 200 volts PIV unit 

same generator setting as above, 

with oscillations caused by 

interaction of the diode capacitance. 

Failure at 1800 average volts in 

250 ns 

Arc across varnish 

Failure of 1200 volts PIV unit 

No-load generator setting 2200 volts 

Failure at 1900 average volts in 

200 ns 

Arc across varnish 

(trace indicated by scratches for 

reproduction) 

Tests on A90 rectifiers at 30'~ 

with capacitor-discharge generator 

All oscillograms: 500 volts/div. - voltage across rectifier 

0.1 ps/div. - sweep 
The remaining two units were subjected to a fast rise pulse from the 

capacitor discharge generator, in order to evaluate the response to a fast rise 

surge, which did not produce early failure in the case of the A92 diode. 

Here again, the voltage was progressively raised for each single shot, 

until faclure occurred as recorded in the two oscillograms shown on the opposite 

page. Interaction between the diode capacitance and the generator caused the 

oscillations shown on the two oscillograms. By coincidence, the chop due to the 

flashover across the varnish occurred at a minimum of the oscillation; for 

evaluation purposes we chose to take the average value of the oscillation as 

the failure level. 

By comparison with the tests made with slow rise waveshape, failure occurred 

very early, i.e., in less than 0.3 ys. The long tail of the wave (see no-load 

osciIlogram) could have produced failures with longer time to breakdown at 

slightly lower voltage, but this did not occur. 

2. Dfscussion of the Tests on A90 Rectifiers 

The failure levels with slow rising wave and with the fast rising wave are 

not significantly different, nor is these any difference in the levels or time 

to failure of the 200 volts units compared to the 1200 volts units. All failed 

between I800 and 2000 volts. 

The long time to failure observed in one case is probably the result of a 

slightly increasing voltage in the top of the "flat top" pulse. In all the 

other slow rise cases, failure occurred within less than a microsecond as the 

voltage was still rising. In the case of fast rise wave, failure occurred in 

less than 0.3 ys. 

Furthermore, as the rectifier does not draw an appreciable reverse current, 

it cannot contribute to dissipating the surge energy, so that a low energy 

pulse (capacitor discharge) will produce failure at voltage levels comparable 

to a high energy pulse (line discharge). 
3. Conclusions 

The failure level of the A90 rectifier is primarily determined by the 

dielectric strength of the junction surface. yeither temperature, energy of 

the pulse or PIV rating of the diode affect t& fhilara level. Energy levels 

at failure are substantially less than for controlled avalanche (A92) rectifiers 

of same current rating. 
Tests on ZJ 260 SCR1s 

Summary of Tests 

Test Samplest Units with 200 to 1000 volts class 

Tests: Reverse transient with no bias 

Results : No clear'iadications - in any event voltage class 

does not correlate with failure level 
500 volts unit - preset test 

Failure at 1900 volts in short time 

(200 A/div.) 

Failures of 25260 SCRs 

All oscillograms: Top Trace: Voltage across SCR - 500 ~/div. -46- Bottom Trace: current in SCR - 100 or 200 A/div. as shown 

Sweep: 5 psldiv. 
1. Tests on 25260 SCRs 

A number of 25260 SCRs wbtfi'valtage grades ranging from 200 to 1000 volts 

were obtained from the Rectifier Components Department. Reverse voltage pulses 

delivered from the LC circuit described in the A92 tests were applied to these 

rectifiers in two manners. 

One series of tests wad made by applying single shot pulses, with increasing 

voltage, until a failure was observed. This generally occurred after more than 

5 or 10 microseconds, as the voltage across the device was raised by heating of 

the junction, in a manner similar to that discussed for the A92 rectifiers. 

A second series of tests was made by setting*the generator to the maximum 

available voltage, in ordef. to cause breakdm.at the first pulse, presumably 

very early in the pulse dukation rather than after "prolonged" heating. 

Both test series were made without bias on the SCR, in the first approach. 

Attempts were made to alsd perform these tests with forward or reverse bias, 

but instrumentation probldms prevented doing this within the available time. 

Typical oscillograme recoMed during the test series are reproduced on the 

opposite page. The voltage across the SCR is abruptly chopped as failure occurs, 

while the current jumps &am its avalanche level to that of the maximum available 

from the pulse-forming ndtwork. 

The firs't three oscillograma correspond to the increasing voltage steps 

test procedure, while the fourth was recorded with a preset voltage pulse. 

Three of these samples were dissected by the Rectifier Components Department 

with the following rerults : all three units failed in the bulk. They were not 

surface limited. The failure occurred by overheating at some weak spot in the 

bulk, such as a diffusion spike. 
200 

lboo 

. lboo 1700 1800 \900 2000 &(OO 

Voltage across SCR - volts 

DISTRIBUTION OF FAILURE POINTS - VOLTAGE VS WRRBKT FOR VOLTAGE CLASS SHOWN 

-48- 
The voltage and current at the time of failure, and the time to failure are 

tabulated below for the specimens which were subjected to the two test methods. 

Sample 

Number 

1 

2 

3 

4 

5 

6 

7 

8 

9 

1 a 

11 

12 

13 

14 

15 

16 

17 

18 

Voltage 

Class 

Volts 

200 

200 

400 

500 

500 

7 00 

700 

800 

800 

9 00 

1000 

1 

Type of Test 

increasing steps 

preset 

increasing steps 

preset 

preset 

increasing steps 

preset 

preset 

increasing steps 

increasing steps 

increasing steps 

Voltage 

At Failure 

Volts 

Current 

At Failure 

Amp 

50 

5 0 

6 0 

100 

100 

40 

100 

180 

6 0 

180 

230 

220 

6 0 

70 

70 

70 

10 

300 

Time 

To Failure 

2. Discussion of the Test Result$ 

Inspection of the tabulated results shows several interesting points: 

The voltage at failure varies between 1600 and 2100 volts, for a range of 

voltage class ratings from 200 to 1000 volts; there is no correlation between 

the voltage class on one hand and the voltage and currefit at failure on the 

other hand. 

The plot shown on the opposite page was made in an attempt to correlate 

voltage across the SCR with the avalanche current, for a specific voltage class. 

The number plotted represents the voltage class while the coordinates are the 

voltage and current at the time of failure. There is no readily apparent 

correlation or trend in the location of the failure points in this plot. 
3. Conclusions of the Tests on the 25266 SCRs 

The test results do not show any clear correlation between the voltage 

class of the SCR and the voltage at which it will fail under reverse transient. 

Furthermore, there is no correlation between the voltage and current at the 

time of failure. 

Therefore, the voltage class of the device cannot be used as a measure of 

the withstand capability under reverse transients beyond the values specified 

by the manufacturer. 

Thia loop RF filter in power cord 

volt probe 

\ Theae loops minidzed by built-in filter in ~6013 probe 
APPENDIX 

Notes on Two-Channel Transient Recordings 

The normal difficulties encountered in making single sweep transient 

recordings are increased when two rather than one signal are to be recorded. 

The perennial problem is that of ground loops, deliberate or unsuspected, which 

must be weeded out. 

Some of the failure ~scillograms recorded for this report included a current 

trace as well as a voltagC trace. The voltage measurements were obtained through 

a Tektronix P6013 high voltage probe while the current measurements were derived 

from a TGdVI Research Associates coaxial shunt. 

The oscilloscope used for the two-trace measurements was a Tektronix 555 

oscilloscope. 

The sketches on the opposite page shsw some of the most obvious "unsuspecEed" 

couplings in the power and measuring circuit, as well as the steps taken to 

eliminate them. 

The following circuit clements are connected directly to a cormon grounding 

point, which is connected to the building ground via bench ground: the low side 

of the DC high voltage sourke, the low side of the energy storage element (line 

or capacitor) and the shell of the coaxial shunt. One might also connect to 

this point the ground side of the high voltage probe. 

In addition, the oscilloscope power supply, through the line cord and the 

power transformer, is capacitively coupled to the ground, by a rather substantial 

stray capacitance. At high frequencies, it was also found that the perfotated 

oscilloscope cabinet can allow direct radiation into the oscilloscope circuitry. 

Therefore, a number of precautions were taken, some of which are rather 

obvious, some others had to be tried out and still did not entirely clean up the 

measurements, as we will see later. 
Volt probe connected 

Shunt not connected 

101 Volt probe not connected 

to scope 

Shunt connected 

- - 

Volt probe connected and 

grounded 

Shunt connected 

- 

Volt probe connected but not 

grounded 

Shunt connected 

- - 
First, an RF filter was inserted in the line cord of the oscilloscope, and 

the oscilloscope was plahed in a copper enclosure tied to the bench ground. 

This reduced to less thah 2% of the normal signal level the noise still displayed 

by the oscilIoscope, eved with its input shorted. 

The Tektronix P6013 probe has a built-in filter consisting of several turns 

of the probe lead wound oh a powder core offering a high impedance to high 

frequency circulating curtent in the sheath of the probe lead. However, this is 

not quite sufficient if ariother ground lead, such as the current lead from the 

shunt, would also be connected to the oscilloscope. 

This is illustrated in the four circuit arrangements shown on the opposite 

page* 

With the volt probe odly connected, circulating current in a ground loop 

is limited as the only possible loops involve stray capacitances from the scope 

to its grounded box and the built-in filter of the P6013 probe. Therefore, the 

first oscillogram, recorded with a 9.1 us rise time pulse applied to a test 

diode, can be expected to flCirly represent the voltage across the test piece. 

The current channel, which is not connected to the circuit, does however, display 

a smafl high frequency signal. 

The second oscillogram, recorded with the same applied pulse but with the 

volts probe disconnected, cah in a similar manner be expected to fairly represent 

the current in the test pieck. A small signal is displayed on the disconnected 

voltage channel. 

Connecting both channels produces the display recorded on the third 

oscillogram. While the main rise of the voltage signal is not seriously affected, 

it does-display a low amplitude, high frequency oscillation which was not present 

in the first oecillogram, and the signal before the main rise is definitely 

changed. The current trace is ilot changed as fat as major waveshape is concerned, 
but a rather large, high frequency oscillation persists for the duration of the 

current pulse display. 

A fourth ~scillogram corresponds to an arrangement with the current ptobi 

connected (both sides of the coax shunt are shown in schematic form by a single 

line) and the giound (sheath) side of'the voltage probe floating. This 

configuration, waich produces the two- hannel display used in d number of 'a b 

recordings in thid report, exhibits thersmallest distortion colapared to 8l&l& 

h 

voltage or current traces. P 

However, small high frequency signaS's are displayed during the initial 

part 

than 

of the pulse, which are more likely to represent spurious signal rather 

significant, actual device phenomena. 1 Therefore, caution is required in 

attempting to draw conclusion from some of the minor ripples which can be 

observed in a number of oscillograms within the main part of this report. 
References 

1. TIS 62GL191 

2. TIS 63GL133 

3. TIS 63GL132 

4. TIS 63GL144 

5. DF 63LC1342 

6. DF 64LC1605 

7. TIS 65ESP3 

Tests on IN679 Diode - effects of - waveshape 

- temperature 

- steady-state current 

- timing of surge 

- PIV 

Limited tests on GE 4JA4, 4JA10, IN1696 and other 

manufacturers. 

Tests on 1N679, 4JA4 - effects of rate of voltage rise. 

Tests on C20 SCR's - effects of - PIV 

- surge energy 

Tests on IN690 manufactured by G.E. and others. Limited 

tests on other devices. 

Tests on 1N539, 1N547, 1N560, 1N1117, IN1120 diodes 

also 2N525, 2H333, 2N2714 transistors. 

Tests at high speed on various devices - to be issued 

in first half of 1965 by the Laboratory Operation 

of the Power Transmission Division. 

8. AIEE Standard 1/59 Semiconductor Rectifier Components. 

Acknowledgements 

The test samples were selected, and representative failed units were 

dissected through the cooperation of Messrs. Bisson, Knaus, Jalbert, Lyon, 

and Werner of the Rectifier Components Department. 

Messrs. F.W. Gutmiller and J. Read of the Rectifier Components Department 

made helpful suggestions to improve the manuscript of this report. 

Mr. E. Pollard, of the Philadelphia Laboratory Operation, provided the 

test facilities for the high speed tests on the A92 rectifier. 

Dr. Storm of ATL suggested explanations on the junction behaviour during 

the fnterpretation of the measurements. 

These contributions, together with the financial support of the sponsoring 

departments, are gratefully acknowledged.