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Produced by the NASA Center for Aerospace Information (CASI) https://ntrs.nasa.gov/search.jsp?R=19790010044 2020-03-22T01:16:29+00:00Z 1k^ .-t REC EIVED •> hHS,4 ST/ FACILjTy 'HESS OW n (NASA-CB-161136) TRANSISTOR STEP STRESS N79-18215 PROGRAM FOR JANTX2N2605 Final Report (DCA Reliability Lab., Sunnyvale, Calif.) 32 p HC A03/MF A01 CSCL 09A Onclas133/33 14170 TRANSISTOR STEP STRESS TESTING PROGRAM MSFC/NASA CONTRACT NUMBER MASB-31944 FINAL REPORT FOR J ANT X 2N2605 F F f- E- JANUARY 1979 Prepared For GEORGE C. MARSHALL SPACE FLIGHT CENTER NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Marshall Space flight Canter, Alabama 35812 OCA RELIABILITY LABORATORY SPECIAL PRODUCTS DIVISION 915 BENICIA AVE SUNNYVALE, CALIFORNIA 94086 •lT'-T lT. A:1► RELIABILITY LABORATORY JANTX2N2605 I FOREWORD This report is a summary of the work performed on NASA Contract NAS8-31944. The investigation was • conducted for the National Aeronautics and Space IAdministration, George C. Marshall Space Flight Center, Huntsville, Alabama. The Contracting Officer's Technical Representative was Mr. F. Villella. The short-term objective of this preliminary study of Itransistors, diodes, and FETS is to evaluate the relia- bility of these discrete devices, from diffe, ent manu- facturers, when subjected to power and temperature step stress tests. IThe long-term objective is to gain more knowledge of accelerated stress testing for use in future testing of Idiscrete devices, as well as to determine which type of stress should be applied to a particular device or I design. This report is divided as follows: description of tests, figures, tables, and appendix. I I I I I ii JANTX2N2605 TABLE OF CONTENTS Page 1.0 INTRODUCTION . . . . . . . . . . . . . . . . 1 2.0 TEST REQUIREMENTS . . . . . . . . . . . . . . . . 1 2.1 Electrical . . . . . . . . . . . . . . . . 1 2.2 Stress Circuit . . . . . . . . . . . . . . 1 2.3 Group I - Power Stress . . . . . . . . . . 1 2.4 Group II - Temperature Stress I . . . . . 2 2.5 Group III - Temperature Stress II . . . . 2 3.0 DISCUSSION OF TEST RESULTS . . . . . . . . . . 3 I3.1 Group I - Power Stress . . . . . . . . . . 3 3.1.1 Raytheon . . . . . . . . . . . . . . . . . 3 I 3.1.2 National Semiconductor . . . . . . . . . . 3 3.1.3 Statistical Summary - Group 1 . . . . . . 4 3.2 Group II - Temperature Stress I . . . . . 4 3.2.1 Raytheon . . . . . . . . . . . . . . . . . 4 3.2.2 National Semiconductor . . . . . . . . . . 4 I 3.2.3 Statistical Summary -Group II . . . . . 5 3.3 Group III - Temperautre Stress II . . . . 5 I3.3.1 Raytheon . . . . . . . . . . . . . . . . . 5 3.3.2 National Semiconductor . . . . . . . . . . 6 3.3.3 Statistical Summary - Group III . . . . . 7 I 4.0 FINAL DATA SUMMARY . . . . . . . . . . . . . . . . 7 5.0 CONCLUSIONS . . . . . . . . . . . . . . . . . . . 7 iii JANTX2N2605 LIST OF ILLUSTRATIONS 1 I I I I I Figure Title Page 1 Power/Temperature Stress Circuit. . . . . . . . . 9 2 Cumulative Percent Failures Versus Junction Temperature, Raytheon . . . . . . . . . . . . . . 10 3 Time Steps Versus Junction Temperature, Raytheon . . . . . . . . . . . . . . . . . . . . . 11 4 Cumulative Percent Failures Versus Junction Temperature, National Semiconductor . . . . . ... 12 5 Time Steps Versus Junction Temperature, National Semiconductor . . . . . . . . . . . . . . 13 A-1 SIN 7078, National Semiconductor, 160X. . . . . . 26 A-2 SIN 7117, Raytheon, 160X . . . . . . . . . . . . . 26 LIST OF TABLES Table Title Page 1 Test Flow Diagram . . . . . . . . . . . . . . . . 14 2 Parameters and Test Conditions. . . . . . . . . . 15 3 Power Stress Burn-In Conditions . . . . . . . . . 15 4 Group I - Power Stress Data Summary . . . . . . . 16 5 Group II - Temperature Stress I Data Summary. . . 18 6 Group III - Temperature Stress II Data Summary. . 19 7 Final Data Summary . . . . . . . . . . . . . . . . 20 8 Step Stress Catastrophic Failure Summary. . . . . 21 9 Step Stress Parametric Failure Summary. . . . . . 22 iv JANTX2N2605 INTRODUCTION DCA Reliability Laboratory, under contract NAS8-31944 for NASA/Marshall Space F149ht Center, has compiled data for the purpose of evaluating the effect of power/temper- ature step stress when applied to a variety of semiconductor devices. This report covers the PNP transistor JANTX2N2605 manufactured by Raytheon and National Semiconductor. A total of 48 samples from each manufacturer was submitted to the process outlined in Table 1. In addition, two control sample units were maintained for verification of the electrical parametric testing. TEST REQUIREMLNTS r ^^+-- I i All test samples were subjected to the electrical tests outlined in Table 2 at each measurement point after completing the prior power/temperature step stress point. These tests were performed using the Fairchild Model 600 high-speed computer-controlled tester. Additional bench testing was also required on the devices. Stress Circuit The test circuit shown in Figure 1 was used to power all of the test devices during the power/temperature stress conditions. The current was set by I and the VCE was varied in order to comply with the specified power rating for this device. At least one of the devices was subjected to maximum rated power (MRP). All remaining devices were subjected to no less than 90% of MRP. See Figure 1 for load resistance values and voltages. Group I - Power Stress Thirty-two units, 16 from each manufacturer, were submitted 1 JANTX2N2605 to the power stress process. The transistors were stressed in 500-hour steps at 50, 100, 125, 150, and 175 percent of maximum rated power for a total of 2500 hours or until * 50% or more of the devices in a sample lot failed. Elec- trical measurements were performed on all specified elec- trical parameters after each power step. See Table 1. 2.4 Group II - Temperature Stress I Thirty-two units, 16 from each manufacturer, were submitted to the Temperature Stress I Process. Group II was'subjected to 1600 hours of stress at maximum rated power in increments of 160 hours. The temperature was increased in steps of +25°C, commencing at +75°C and terminating at +300°C or until 50% or more of the devices failed.* Electrical measurements were performed on all specified electrical parameters after each temperature step. See Table 1. 2.5 Group III - Temperature Stress II Thirty-two units, 16 from each manufacturer, were submitted to the Temperature STress II Process. Group III was subjected to 112 hours of stress at maximum rated power in increments of 16 hours. The temperature was increased in steps of +25°C, commencing at +150°C and terminating at +300°C or until 50% or more of the devices in a sample lot failed.* Electrical measurements were performed on all specified electrical parameters after each temperature step. See Table 1. I *Conditions for failure: A) Open or short I B) Leakage exceeds the MIL limit by 100 times C) Other parameters exceed MIL limits by 50% or more. I 2 1 3.0 DISCUSSION OF TEST RESUL'T'S JANTX2N2605 I3.1 Group I - Power Stress 3.1.1 Raytheon. The Raytheon sample lot completed the entire 2500-hour Group I testing with two catastrophic failures. The first failure occurred 10 hours into the 175% MRP step. Serial number,7099 failed the minimum h FE limit. The last failure occurred 150 hours into the 175% MRP step. Serial niunber 7103 failed the maximum ICBO limit. Typical characteristics of this sample lot's performance I were: 1) The mean value for I CBO changed 157.6pA from an initial I mean of 373.7pA to a final mean of 531.3pA. 2) The mean value for VCE(SAT) changed 11.32mV from an I initial mean of 98.38mV to a final mean of 109.7mV. 3) The mean value for hFE changed 7.2 from an initial mean of 208.8 to a final mean of 201.6. The control units for this sample lot remained constant Ithroughout the entire Group I testing. I 3.1.2 National Semiconductor. The National Semiconductor sample lot completed the entire 2500-hour Group . I testing with two catastrophic failures. The first failure occurred 10 hours into the 150% MRP step. Serial number 7038 failed the maximum ICBO limit. The last failure occurred 500 hours into the 175% MRP step. Typical characteristics of this sample lot's performance were: I1) The mean value for I CBO changed 1.88nA from an initial mean of 295.6pA to a final mean of 2.18nA. I2) The mean value for VCE(SAT) changed 620.14mV from an initial mean of 94.06mV to a final mean of 714.20mV. I3) The mean value for hFE changed 21.9 from an initial mean of 184.0 to a final mean of 162.1. 3 ^♦^il^ JANTX2N2605 The control units for this sample lot remained constant throughout the entire Group I testing. 3.1.3 Statistical Summary - Grour I ITable 4 outlines the results of Group I - Power Stress Process for all of the specified electrical parameters Iand all measurement points for both Raytheon and National Semiconductor. I3.2 Group II - Temperature Stress I I 3.2.1 Raytheon. The Raytheon sample lot completed a total of 1120 hours of Group II testing before the lot was stopped because 50% of the devices failed. The first failure occurred 160 hours into the + , 100°C-temperature step. Serial I number 7111 failed the maximum h FE limit. The next two failures occurred 160 hours into the +200°C-temperature step. Serial number 7109 failed the minimum h FE limit, Iand serial number 7117 failed the maximum ICBC limit. The last five failures occurred 160 hours into the +225°C- I temparature step. Serial numbers 7107 and 7108 failed the minimum hFE limit. Serial numbaers 7112, 7114, and I 7120 failed the maximum ICBC limit. Typical characteristics of this sample lot's performance were: I 1) The mean value for ICBO changed 200.2nA from an initial mean of 310.6pA to a final mean of 200.5nA. 2) The mean value for VCE(SAT) changed 4.09mV from an initial mean of 93.31mV to a final mean of 97.40mV. I3) The mean value for hFE changed 85.4 from an initial mean of 217.8 to a final mean of 132.4. IThe control units for this sample lot remained constant throughout the entire Group II testing. I3.2.2 National Semiconductor. The National Semiconductor lot 4 11i{-:i1 JANTX2N2605 completed 1120 hours of croup II testing before the lot was stopped because more than 50% of the devices failed. The first six failures occurred 160 hours into the +200°C- temperature step. Serial number 7071 failed the minimum hFE limit. Serial numbers 7046, 7049, 7052, 7074, and 7075 failed the maximum ICBO limit. The last four failures occurred 160 hours into the +225° C-temperature step. Serial t number 7047 and 7072 failed the maximum I CBO limit. Serial number 7073 failed the maximum V limit. SerialCE (SAT) number 7077 failed the minimum h FE limit. Typical^charac- teristics of this sample lot's performance were: 1) The mean value for ICBO changed 114.2nA from an initial mean of 304.4pA to a final mean of 114.5nA. 2) The mean value for VCE(SAT) changed 2.01V from an initial mean of 90.81mV to a final mean of 2.10V. 3) The mean value for hFE changed 69.1 from an initial mean of 177.0 to a final mean of 107.90. The control units for this sample lot remained constant throughout the entire Group II testing. 3.2.3 Statistical Summary - Group II Table 5 outlines the results of Group II - Temperature Stress I testing for all of the specified electrical parameters and all of the measurement points pertaining to both Raytheon and National Semiconductor. 3.3 Group III - Temperature Stress II 3.3.1 Raytheon. The Raytheon sample lot completed 96 hours of Group III testing before the lot was stopped because more than 50% of the devices failed. The 13 catastrophic failures occurred 16 hours into the +275°C-temperature step. Serial numbers 7123, 7129, 7136, 7137, 7138; and 7139 failed the maximum I CBO limit. Serial numbers 7125, 5 JANTX2N2605 7126, 7127, 7130, 7131, 7134, and 7135 failed the maximum i VCE(SAT) limit. Typical characteristics of this sample I lot's performance were: 1 1) The mean value for ICBO changed 865.5nA from an initial - 0 mean of 352.9pA to a final mean of 865.9nA. I 2) The mean value for VCE(SAT) changed 3.77V from an initial mean of 97.82mV to a final mean of 3.87V. 3) The mean value for hFE changed 50.4 from an initial I mean of 208.9 to a final mean of 158.5. The control units for this sample lot remained constant throughout the entire Group III testing. 3.3.2 National Semiconductor. The National Semiconductor sample lot completed a total of 96 hours of Group III testing Ibefore the lot was stopped because more than 50% of the devices failed. The first failure occurred 16 hours I into the +150°C-temperature step. Serial number 7084 failed the minimum hrE limit. The next nine failures I occurred 16 hours into the +275°C-temperature step. Serial number '7053, 7058, and 7082 failed the maximum ICBO limit. Serial numbers 7059, 7083, and 7085 failed the maximum I VCE(SAT) limit. Serial numbaers 7062, 7079, and 7081 failed the minimum hFE limit. Typical characteristics of this sample lot's performance were: 1) The mean value for I CBO changed 285.3nA from an initial mean of 336.7pA to a final mean of 285.6nA. 2) The mean value for VCE(SAT) changed 2.15V from an initial mean of 90.60mV to a final mean of 2.24V. I 3) The mean value for hFE changed 4.2 from an initial mean of 188.4 to a final mean 184.2. I The control units for this sample lot remained constant throughout the entire Group III testing. 6 JANTX2N2605 3.3.3 Statistical Summary - Group III Table 6 outlines the results for Group III - Temperature Stress II testing for each of the specified electrical I parameters and all measurement points for both Taytheon and National Semiconductor. -5 1 4.0 FINAL DATA SUMMARY Table 7 statistically summarizes the change in the mean value from the zero-hour data to the final data. The graphs of Figures 2 and 4 plot the cumulative percent failures versus the temperature stress level for Group II - Temperature Stress I, and Group III - Temperature Stress II. The graphs of Figures'3 and 5 plot the time step for Group II (160 hours) and Group III (16 hours) versus the temperatures T 1 and T2 calculated from Figures 2 and 4. Tables 8 and 9 summarize the failures encountered for I all three stress groups. The failures are separated into catagories: catastrophic failures in Table 8 and parametric failures in Table 9. The data from Table 8 was used as a source for the graphs its Figures 2 and 4. Figures 2 and 4 were used as a source for the graphs in 3 and 5 respectively. Junction temperature is plotted on an inverse hyperbolic scale. 1 5.0 CONCLUSIONS Groups II and III testing proved to be the most detrimental of the three groups. Samples from both manufacturers exhibited several microamperes of hysteresis in the BV CEO curve trace, but not in the other configuration. The k I pattern suggests th% presence of contamination. The flow of metal from the gold leads and the intermetallics is one source of such contaminants. 7 The hysteresis currents in the base can inject base cur- rent which is not part of the known base drive. From this the h FE reads high. At the same time the contaminant may reduce emitter effi- ciency by reducing the lifetime of holes in the base. This tends to lower the hFF. 1 So the migration of contaminants during burn-in can have two opposite effects on the hFR reading, and the net re- sult depends upon which effect predominates in the indi- vidual sample at the moment of failure. When carried to I extremes gold migration in the base region will develop surface and junction leakage problems, and collector- emitter shorts. I Possibly these Failures could have been delayed to higher stress levels if aluminum wires were used in place of gold, by removing the fast-migrating gold from the surface of + the die. I A plot showing cumulative failure distribution for Groups II i III was drawn for the Raytheon and National Semi- conductor sample lots (Figures 2 and 3 and 4 and 5 respect- ively). Figures 2 and 3 display the data for Raytheon used to calculate an activation energy of 2.37eV. Figures I 4 and 5 display the data for National Semiconductor used to calculate an activation energy of 1.84eV. A broken circle around a marked point, on the graph, in- dicates a freak failure not calculated as part of the regression line. A solid circle around a marked point indicates an isolated main failure point. The regression line was calculated using the least squares method. I 8 L HBecause of visual defects caused by the extreme heat of the stress tests, serial numbers 7130, 7133 and 7135 were not calculated as part of the regression line. The activation energy was calculated from the formula: 8.63 X 10 -5 eV/°K E tn 11 1 1 — -- 1 —N – ( I eV t 2 T1+273 T 2 +2 73 Where: t, 0 step of Group 11 • Temp Stress I - 160 hrs. t2 = step of Group 111 - Temp Stress 11 a )6 hrs. T , a temperature in 0 C of 16% failure for Group 11. T2 - temperature in 0 C of 16% failure for Group 111. BA 0 N I zN x E-4 1 z h I I I _ I I I IZ M ll1 1 m( z^ I I Vr m Of 7_ M Z I N JANTX2N2605 a^ 7 0 w M _W H d dN N l 0 ♦1 N .N C 1- a z aI- O w 3 > rk^1 0! + O N ? L 11 O NN •- 0 c m a. 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O W LLJ ^ CL a LLJ N J ^ ^ U ro w a^ U N w N a a^ LO rd a a; g oUl)8 r- o ^ 8 M M M M N N N N H H o^ (Do) 28nlV83dW31 N011ONn * W F- O I ^t Z •t: QN EA N 8 ti 4n^ U 0 tf^ W JANTX2N2605 Q x0 p U A zO U H W a zO H HQ z U U 00 0 er O Ln co Ln cm M M e-^ it rl 11 N II W E4 H O C U 0 U •rl I O ro 0 —4 41 ro z o ^ cn LM cr ro Q D L ND Q' W E-E MO E-. 0 4J U i; h N N O ti N pv N O N ti N 0 ti LO qt tf tf M M N N N N N ri ^, E H (Jo) 38f1 83MR NOliONn 13 JANTX2N2605 TA!1LE I I TEST FL, )W DIAGRAM INITIAL ELECTRICAL TESTS Per Table II (2)* 16)* (16)* 1 (16)* Temperature Step Temperature Step 'Non-Operating Power Stress Stress 1 Stress 11 Control Group TA = 25°C 100 Percent MRP 100 Percent MRP Note 3 Note 4 0.50 MRP 500 Hours TA 75°C TA 150°C Note 1 t = 160 Hours t = 16 Hours 50.0Hours TA 100°C TA 175°C Note 1 t = 160 Hours t = . 16 Hours 1.25 MRP T =1250C T = 200°C 500 Hours A A t = 160 Hours t = 16 Hours Note 2 I 1.5 MRP 500 Hours 25°C Steps 25°C Steps Note 1 1.75 MRP TA = 300'. TA = 300°C 500 Hours t = 160 Hours t = 16 HoursI Note 2 *Quantity per manufacturer (Raytheon & National Semiconductor) I NOTES: 1) Electrical measurements per Table 2 were made at 50, 150, 250 and 500 hours. i2) Electrical measurements per Table 2 were made at 10, 25, 50, 150, 250 and 500 hours. 3) Electrical measurements per Table 2 were made at the end of each 160 hours. 4) Electrical measurements per Table 2 were made at the end of each 16 hours. I 14 • .. J y ^- it111:1 TAIL PARAMETERS ANC PARAMETER CONDITIONS I CBO @V CB = -50V V CE(SAT) @IC -IOmA & I V = -.5mA hFE @V CE _ -5 V & I C = -IOUA NOTES: IN ADDITION, ANY OPEN OR SHORT SHALL TAIL POWER STRESS C I = ,SCE 12.5V 25.OV 31.2V 37.5V 43.7V NI 440 0 O ^` t0 ^N A^ N R z X E4 I^ E _O G I O i O 0 0 0 u1 ^ Q ? M a+N MMM^ — z N E ?' I ►>n a' o ^ O 1 zo 11 IIU1 MOCO -T O L--T I a• ^ NU. > Q 00 co O U1L C•V d' (N NQ ¢Q to Ini. J X Q > > > > > > > > rn rE+cu rEE O oO o n of11 — O 99 a1 O^ M O> mI Z m^ 0000wE ^gy '^ E11 Q W ^ E-+ 2! 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JANTX2N26Ua 0 W Wa 1 i I Q r I^ o W ^ > m ^ O C1 O .^. 0 O O W Z 1 I a m at O ^ ^ m ^ O O N O a b {I^` • • • i i• tl1 •pp d^ a I Q I t I Q lowm $ o W N ^ ~o 0 G O O — ifl O^ 0 W 41 F = 1 i 1 1 1 1 f 4O C ^ m o 0 0 0 0 0 o aQ 0-6 u u♦ u• Ui 0• u• uf u• u• u•IA WW FH • Ul d0w u c O v Z Wt!t 2. J W Z x o > f- ac 2 a O' d W 11. x x W ~O 1 r t 1 1 1 1 1 1 1 1 1 U 1 I 1 1 t 1 1 1 I I 1 i Q m 2 a y H 0 O O 0 0 c 0 0 0 0 0 0 0 O 0 o O O 0 o O 0 O O —0 w O1 1 1 r co co 1 I 1 1 I 1 1 Q I 1 f 1 N I I I i 1 1 Q LL o o a o ^n 0 0 0 0 0 0 0 0 0 o c 0 0 0 0 0 0^ ^^ ^ sp' ^ fp ! si i pi # K^ ^ ^ ! f ^ W ^[ O Q 8 R ^bt = p O O x: c p ^ Q t: C„ M1 ^^(tt S O ^ t^'+ O Iff ^^I1 8 ppp^ yp1fHH r O^• ^ ^ N Otee• Mr Or O^ ^• r N ^ ^ •^ r N '^ Nr Q W Q QU. W F- N d w HN G WJ Q 9 rN et v i w W N S a 4.4 C M a R L uj W W d d cr O J J — U.a d w w ^ r - x ^ r — J .J J 0 0 = _ x - - - x X¢ 1X O W v r to w u_ u oO O N W i L"OON;k 1 pw - 22 FAILURE M hFE V I CBO @ I C a B ED -volts - InitialI BV CEO BV CEO -nA- @V CB BV EBO - IOUA; =IBED Red. @ Test Seq. Initial S/N -volts- -volts- -50 V -volts- V = -5V CE 10 nA No.: Rej. for 7050 107 H 156 0.5nA 9.5 120 0.83 01 (2500 hFE 7077 100 100 0.2nA 10.2 0.27 0.78 15(225'C) CAT 7078 92 H 124 O.1nA 10.0 454 0.82 15(2250C) h I INTERNAL VISUAL INSPECTION: All the National Semiconductor samples exhibit AuAl 2 ("purple plague") intermatallic formation where the gold wires contact the emitter and base aluminum metallization. I See Figure A-l. S/N 7078 has edge cracks which are directed toward the center of the die. These cracks are a reliability hazard, but are not the cause of the present failure. I *h trace. present. FE **h Cannot meet stated test conditions. (Leaky) FE trace very leaky. D drift H - h •,,fsteresis Inv = inversion R - . resistive S = soft Uns = unstable 24 ^r LL 10 111^^^J1 4-Y.. at•^ MSFC STEP-STRESS TEST FAILURI' ANALYSIS TRANSISTORS JANTX2N2605 Date J/N 2CN242-05 B P/N 2N2605 (PNP) MFR National FAILURE VERIFICATION: Max = lOnA Limits = 100-300 I hFE V - CnA - @ I C volts -- 0 Initial 1 BV CEO BV CEO @V = CB BVEBO 0uV -1 _ - 5V @ IB EO = Re j . @ Test Seq. Initial S/N - volts - - volts - -50 V - volts - CE lOnA No.: Rej. for 17111 96H 126 O.1nA 8.8 219* 0.79 05 00000 CAT 7112 25 -► 62 H 30 -► 90 D 50pA 8.7 149* 0.80 15 (22rC) ICBO 7117 0.74 12 R -- 7.7 150 0.80 13 (200oC) hFF Beta me sured at 0.5 volts Vr.F because f low V 1 INTERNAL VISUAL INSPECTION: 1 All the Raytheon samples exhibit AuAl 2 ("purple plague") intermetallics where the gold wires contact the emitter I I I ( and base aluminum metallization. See Figure A-2. *h FE **h trace present. Cannot meet stated test conditions. (Leaky) FE trace very leaky. D—= drift H = hysteresis Inv = inversion R =•resistive S = soft Uns = unstable25 u. a • 11 i^^^ MSFC STEP--STRESS TEST F FAILUR?' ANALYSIS ^^'- TRANSISTORS JANTX2N2605 Date 78 J/N 2CN242-05 B P /N 2N2605 (PNP) MFR Raytheon S/C FAILURE VERIFICATION: max/=10nA Limits = 100-300 DO^1^. JANTX2N2605 FIGURE A-1 S/N 7078, National Semiconductor, 160X Overall die view showing "purple plague" formation on base and emitter metallization. FIGURE A-2 S/N 71.17, Raytheon, 160X Overall die view showing "purple plague" formation on base and emitter metallization. 26 I^ JANTX2N2605 IAll six samples in this analysis exhibit several micro- amperes of hysteresis in the BVCEO curve trace, but not in the other configurations. The pattern suggests the presence of contamination in (or on) the base diffusion. I The flow of metal from the gold leads and the interma- tallics is one source of such contaminations. IThe hysteresis currents in the base can inject base , current which is not a part of the known base drive. Thus the calculated hFE reads high. At the same time the contaminants may reduce emitter Iefficiency by reducing the lifetime of holes in the base. This tends to lower hFE' I Thus the migration of contaminants during the burn-in can I have two opposite effects on the h FE reading, depending upon which effect predominates in the individual sample at the moment of failure. When carried to extremes, gold migration in the base region will develop surface and junction leakage problems, and collector-emitter I shorts. SIN 7112 is an example of such a surface problem, SIN 7078 is an example of rising h FE , SIN 7077 exhibits ` falling hFE , and SIN 7117 demonstrates an approaching I collector-emitter short. ` Possibly these failures could have been postponed to higher stress levels if aluminum wires were used instead I of gold to prevent the fast-migrating gold from contami- nating the surface of the die. I 27

Transistor step stress program for JANTX2N2605

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