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Electrical ratings of electronic components from the viewpoint of reliability
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World Abstracts on Microelectronics and Reliability
Reliability study of wire bonds to silver plated surfaces. Proc. Electron. Components Cot!f Arlington, Va., May 16 18, 1977. p. 175. The infuence of temperature and environment on bond integrity was investigated for both aluminum ultrasonic and gold thermocompression wire bonds made to silver plate. Storage of aluminum/silver bonds at 85~C 85°; R.H. (no bias) resulted in progressive bond strength degradation and a shift in the primary failure mode from wire breaks to bond lifts. Degradation was attributed to massive aluminum corrosion. Bond degradation was not observed for the gold/silver system during storage at 85~C/857; R.H. (no bias} for 1500 hours. In addition to humidity testing gotd'silver bonds were isothermally aged at 150, 250, 350, 450 and 540°C for a minimum of 10 minutes at each temperature to a maximum of 2000 hours at 150, 1500 hours at 250 and 350, 1000 hours at 450 and 500 hours at 540~C. Although rapid silver diffusion was observed in samples aged above 150°C, the system was determined to be mechanically reliable. An effective silver diffusion coefficient for the wire bond system was calculated from destructive wire pull data.
Plastic outgassing induced wire bond failure. R. E. THOMAS, V. WINCHELL, K. JAMES and T. SCHARR. Proc. Electron. Components Conf. Arlington, Va., May 16 18, 1977. p. 182. A multitude of molding compound formulations are continually being made available to the semiconductor industry. To maintain device reliability, it is necessary to evaluate materials not only for their advantageous properties, but also their detrimental characteristics. One area of particular concern is wire bound degradation resulting from outgassed products from the plastics during high temperature applications. A tmique test method has been instituted to separate plastic outgassing related bond degradation from other molding compound variables such as mechanical stress. In this study, An AI wire bond degradation is directly attributed to the outgassed products of certain molding compounds. A correlation between molded devices and the test method which isolates the bonds to gaseous environment has been established for accelerated 200°C testing.
Obtaining high reliability performance from commercial quality opto-isolators. I. DOSHAY and M. KALASHIAN.Proc. Electron. Components Conf. Arlington, Va., May 16-18,
ponents Cot~ Arlington, Va., May 16 18, 1977. p. 429. A polymer sealed ceramic, LSI microcircuit package containing up to six nitride-passivated beam-leaded logic arrays and a thick film multilayer interconnection network was designed and evaluated. Selection of the sealing potymcr was based on the study of curing, outgassing and water vapor transmission properties of candidate epoxies. The withstand capability of these devices to harsh environmental stresses was studied. Of particular interest was a humid ambient with and without electrical bias. Finally, the devices were subjected to a qualification-like sequence of tests to determine whether, in a polymer sealed package, they could pass all the M1L-STD 883. Method 5005. Class B tests.
The consideration of reliability in a fully automatic assembly system of small signal transistors. Y. SAKAMOTO,Y. KOYDO, M. EJIRI, T. TAKAGI,H. KAWASEand T. OsHIo. Proc. Electron. Components Conf. Arlington, Va., May 16 18, t977. p. 146. A computer controlled automatic assembly system for small signal transistors was successfully introduced to mass production line in early 1975. Since major portions of reliability failure of semiconductor devices are related to poor assembly operation, careful research and development was performed for the system to improve reliabi• lity, which included machine and system design, chip-coating, and lead frame configutation. As a result, considerable improvement was obtained, and percent defective in the field was reduced from 50 ~ 100p.p.m to less than 20 p.p.m which is 0.002°o. This shows an increase of approximately 500~, in reliability.
Reliability problems in reflow soldering of Ag- and Pd/Agterminated chip components. G. J. EWELL. Proc. Electron. Components Conf. Arlington, Va., May 16-18 1977. p. 206. Four potential reliability problems related to reflow soldering of thick film Ag- and Pd/Ag-terminated chip components are discussed. The nature of each problem and the solutions investigated for that problem are explained. It is concluded that reflow soldering of Ag- and Pd/Ag-terminated chip components can be used if a rigorous program of part inspection and screening is carefully developed and used and if detailed soldering procedures are closely followed.
1977. p. 536. Techniques are presented for effectively screening plastic DIP opto-isolators for latent dark current leakage and premature current transfer end-of-life, with resultant low laboratory field failure rates. Investigations are described which included long life tests conducted at TRW Components Laboratories and vendor test laboratories, and field experience with TRW/Data Systems and TRW/VIDAR/Digital Products equipment. Resultant from these investigations were discovery of environmental stress conditions for predictable rates of dark current leakage acceleration and annealing, and for criteria and test conditions to identify latent premature end-of-life current transfer characteristics. These findings were instrumental in the development of customized burn-in and screening techniques to effectively surface parts with latent infant mortality characteristics. Field failure rates were improved to less than 20 failures per billion hours. Laboratory test experience projects to failure rates of less than 13 failures in a billion device-operating hours at 25°C for these specially screened devices. This is approximately 100 times improvement over prior life experience of similar commercial and "high rel" devices, thus resulting in significant maintenance cost savings.
filament lamps, both inert-gas filled and evacuated, was studied. The development of facets can influence life in two ways: by causing a decrease in temperature, and by influencing the mechanical stability through formation of notches at d.c. operation below 2700 K. The way in which the effective emissivity and electrical resistance are influenced by mass loss and by facet formation is investigated. An analysis of burn-through is given. In about 70°i, of all cases the site of initially highest temperature coincides with the final highest-temperature spot. For evacuated lamps this value is somewhat higher and the defect responsible for the initial spot appears to be a minimum in wire diameter. For gas-filled lamps the correlation between final and initial spot is somewhat lower. In this case the defects responsible for burn-through appear to be minima either in coil pitch or in wire diameter, both are of about equal importance.
Reliability assessment of beam-lead sealed-junction integrated circuits in polymer sealed packages. K. B. LASCH, W. N. SCHAITTER and R. ILGENDRITZ. Proc. Electron. Corn-
21 (January 1977). Reliability of any electronic instrument depends upon derating policy, redundancy, proper selection and screening of components, adequate consideration of environmental conditions, proper handling and the
Incandescent lamp filaments: facet-cooling, failure mechanisms. W. LEMS, H. KINKARTZ and W. LECHNER. Philips Res. Rep. 32, 82 (1977). The life of incandescent single-coil
Electrical ratings of electronic components from the viewpoint of reliability. J. S. BORA. The QR Journal (India) p.
World Abstracts on Microelectronics and Reliability reliability of interconnections and solder-joints. The design engineer can derate the electrical stresses of electronic components to the desired level provided manufacturers have correctly specified the ratings. This paper describes the experiments conducted on capacitors, resistors, toggle switches and transistors to find the correctness of the specified ratings. It was found out that the manufacturers either underestimate or overestimate their components regarding electrical ratings. A method to determine the ratings has also been given. It is emphasised that all manufacturers should follow a uniform procedure in deciding the ratings and that users should verify the validity of the specified ratings. Encapsulation, sectioning and examination of multilayer ceramic chip capacitors. G. J. EWELL and W. K. JONES. Proc. Electron. Components Conf Arlington, Va., May 16-18, 1977. p. 446. As the reliability of multilayer, ceramic capacitors becomes more critical, the destructive physical analysis (DPA) of lot samples will become mandatory. The techniques of DPA must be such that they induce no changes in the part's structure. Handling~ encapsulation, grinding, polishing and examination techniques are detailed herein. Tab lead capacitor. G. R. LOVE, E. D. McLAURIN and W. E. HUCKS. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 458. Differential thermal expansion between a monolithic chip capacitor and the common hybrid substrate materials gives rise to significant stresses in the chip. For conventional attach techniques and conventional chips, these stresses may exceed the strength of one or more of the components, chip, termination, substrate metallization, or solder, in the assembly with disastrous effects. Further, common choices of materials aggravate the basic problem by "building-in" opportunities for delayed failure due to slow formation of brittle intermetallics in the assembly. We have examined this problem in detail, and we offer a solution that incorporates material, process, and design innovations which essentially eliminate these problems as sources of device failure. Thermally stable tantalum oxide thin film capacitors. SHIGEHIKO SATO, KATSUAKI YANAGISAWA, SUSUMU OKAMOTO and HAJIME SASAKI. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 374. Thin film capacitors have been developed which can withstand temperatures as high as 300°C for more than one hour. This enables capacitors to be used at higher application temperatures and to be assembled by TC bonding at high substrate temperature or by solder reflow techniques. The dielectric was tantalum oxide formed by anodization of sputtered fl tantalum films. It was heat-stabilized at high temperature and then reanodized to the desired voltage for the removal of dielectric defects caused by the heattreatment. Except for the thermal stability, no significant differences in electrical characteristics were noted between the developed and conventional tantalum oxide thin film capacitors. But the former was more stable than the latter when life-tested under various conditions of voltage, temperature and humidity. The production yield of the thermally stable tantalum oxide thin film capacitors was more than 95~,0 when a tolerance of 5~o was allowed for capacitance values. Failure mechanisms in solid electrolytic capacitors. J. BRETTLE and N. F. JACKSON. Electrocomp. Sci. Technol. 3, 233 (1977). The failure mechanism of solid tantalum and alurninium capacitors have been investigated using a combination of electrical measurements and electron microscopy. The capacitor dielectric was examined before and after life testing and changes correlated with electrical measurements. M.R. 1 6 / 6
:~
643
The basic mechanism of failure of solid tantalum capacitors is found to be field crystallisation of the essentially amorphous dielectric oxide. The growth of higher conductivity crystalline oxide during operation of the capacitors causes an increase in leakage current and may result in catastrophic failure. The effect of field crystallisation can be minimised by using high purity tantalum to reduce the number of crystallisation nucleation sites. Since crystalline growth is primarily dependant on applied voltage, high voltage capacitors are much more susceptible to failure than low voltage units. There appears to be no long term failure mechanism in solid aluminium capacitors. However, a particular problem with these units is that they are difficult to make. This is because the anodic layer is chemically less stable in the case of aluminium than in the case of tantalum. The attack is initiated during the deposition of manganese oxide by pyrolysis from manganese nitrate solution and developed by the reform process. Solid aluminium capacitors often have a lower capacitance and higher initial leakage current than comparable solid tantalum units; however, the leakage current decreases on life tests and their reliability is high. Special lot acceptance tests for multilayer ceramic chip capacitors. W. N. WESSBERGand G. J. EWELL. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 452. Detailed, rigorous tests for acceptance of lots of multilayer ceramic chip capacitors are described. Electrical tests are listed, and illustrations of part defects detectable by external examination and by internal examination through destructive physical analysis (DPA) are given. Structural integrity and electrical stability are emphasized. Influence of defects in the silicon wafer on the properties of silicon oxide films formed by reactive sputtering. D. KROPMAN, M. VINNAL and P. PUTK. Vacuum 27 (3) 125 (1977). It is well known that the properties of SiO 2 films, formed by reactive sputtering in oxygen, sensitively depend upon the details of chemical pretreatment of the Si surface and on the conditions of oxide formation. The influence of bulk defects in the silicon wafer has, however, not been adequately studied. Bombardment of the silicon surface with electrons and ions during the initial stages of the oxide formation is accompanied by extensive interaction with the surface defects, leading to changes in the charge of these defects and properties of the deposited film in general. If vacancies are the dominant defects, then a positive charge is created in the oxide film. If both vacancies and stacking faults are present, then both positive and negative charge can appear in the SiO2 film. Which non-sinusoidal voltage may shorten the life of a capacitor? MOtSEY LERNER. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 468. Heat dissipation in a capacitor, caused by a non-sinusoidal voltage of an arbitrary wavcform, is discussed. It is shown how to estimate the maximum and the minimum of possible losses in a capacitor when its frequency response is unknown. The width of the band of possible losses is chosen to indicate the degree of danger of overheating of a particular cpacitor by a particular non-sinusoidal voltage. Examples of a saw-tooth voltage and of a trapezoidal voltage are investigated.
Effect of oxidation on the breakdown characteristics of ahminium diffused junctions. B. JAYANTBALIGA.Solid-St. Electron. 20, 555 (1977). High voltage power rectifiers and thyristors are usually fabricated with deep diffused junctions using aluminum or gallium as the p-type dopant. Aluminum and Gallium have a relatively large diffusion coeffi-
World Abstracts on Microelectronics and Reliability
Reliability study of wire bonds to silver plated surfaces. Proc. Electron. Components Cot!f Arlington, Va., May 16 18, 1977. p. 175. The infuence of temperature and environment on bond integrity was investigated for both aluminum ultrasonic and gold thermocompression wire bonds made to silver plate. Storage of aluminum/silver bonds at 85~C 85°; R.H. (no bias) resulted in progressive bond strength degradation and a shift in the primary failure mode from wire breaks to bond lifts. Degradation was attributed to massive aluminum corrosion. Bond degradation was not observed for the gold/silver system during storage at 85~C/857; R.H. (no bias} for 1500 hours. In addition to humidity testing gotd'silver bonds were isothermally aged at 150, 250, 350, 450 and 540°C for a minimum of 10 minutes at each temperature to a maximum of 2000 hours at 150, 1500 hours at 250 and 350, 1000 hours at 450 and 500 hours at 540~C. Although rapid silver diffusion was observed in samples aged above 150°C, the system was determined to be mechanically reliable. An effective silver diffusion coefficient for the wire bond system was calculated from destructive wire pull data.
Plastic outgassing induced wire bond failure. R. E. THOMAS, V. WINCHELL, K. JAMES and T. SCHARR. Proc. Electron. Components Conf. Arlington, Va., May 16 18, 1977. p. 182. A multitude of molding compound formulations are continually being made available to the semiconductor industry. To maintain device reliability, it is necessary to evaluate materials not only for their advantageous properties, but also their detrimental characteristics. One area of particular concern is wire bound degradation resulting from outgassed products from the plastics during high temperature applications. A tmique test method has been instituted to separate plastic outgassing related bond degradation from other molding compound variables such as mechanical stress. In this study, An AI wire bond degradation is directly attributed to the outgassed products of certain molding compounds. A correlation between molded devices and the test method which isolates the bonds to gaseous environment has been established for accelerated 200°C testing.
Obtaining high reliability performance from commercial quality opto-isolators. I. DOSHAY and M. KALASHIAN.Proc. Electron. Components Conf. Arlington, Va., May 16-18,
ponents Cot~ Arlington, Va., May 16 18, 1977. p. 429. A polymer sealed ceramic, LSI microcircuit package containing up to six nitride-passivated beam-leaded logic arrays and a thick film multilayer interconnection network was designed and evaluated. Selection of the sealing potymcr was based on the study of curing, outgassing and water vapor transmission properties of candidate epoxies. The withstand capability of these devices to harsh environmental stresses was studied. Of particular interest was a humid ambient with and without electrical bias. Finally, the devices were subjected to a qualification-like sequence of tests to determine whether, in a polymer sealed package, they could pass all the M1L-STD 883. Method 5005. Class B tests.
The consideration of reliability in a fully automatic assembly system of small signal transistors. Y. SAKAMOTO,Y. KOYDO, M. EJIRI, T. TAKAGI,H. KAWASEand T. OsHIo. Proc. Electron. Components Conf. Arlington, Va., May 16 18, t977. p. 146. A computer controlled automatic assembly system for small signal transistors was successfully introduced to mass production line in early 1975. Since major portions of reliability failure of semiconductor devices are related to poor assembly operation, careful research and development was performed for the system to improve reliabi• lity, which included machine and system design, chip-coating, and lead frame configutation. As a result, considerable improvement was obtained, and percent defective in the field was reduced from 50 ~ 100p.p.m to less than 20 p.p.m which is 0.002°o. This shows an increase of approximately 500~, in reliability.
Reliability problems in reflow soldering of Ag- and Pd/Agterminated chip components. G. J. EWELL. Proc. Electron. Components Conf. Arlington, Va., May 16-18 1977. p. 206. Four potential reliability problems related to reflow soldering of thick film Ag- and Pd/Ag-terminated chip components are discussed. The nature of each problem and the solutions investigated for that problem are explained. It is concluded that reflow soldering of Ag- and Pd/Ag-terminated chip components can be used if a rigorous program of part inspection and screening is carefully developed and used and if detailed soldering procedures are closely followed.
1977. p. 536. Techniques are presented for effectively screening plastic DIP opto-isolators for latent dark current leakage and premature current transfer end-of-life, with resultant low laboratory field failure rates. Investigations are described which included long life tests conducted at TRW Components Laboratories and vendor test laboratories, and field experience with TRW/Data Systems and TRW/VIDAR/Digital Products equipment. Resultant from these investigations were discovery of environmental stress conditions for predictable rates of dark current leakage acceleration and annealing, and for criteria and test conditions to identify latent premature end-of-life current transfer characteristics. These findings were instrumental in the development of customized burn-in and screening techniques to effectively surface parts with latent infant mortality characteristics. Field failure rates were improved to less than 20 failures per billion hours. Laboratory test experience projects to failure rates of less than 13 failures in a billion device-operating hours at 25°C for these specially screened devices. This is approximately 100 times improvement over prior life experience of similar commercial and "high rel" devices, thus resulting in significant maintenance cost savings.
filament lamps, both inert-gas filled and evacuated, was studied. The development of facets can influence life in two ways: by causing a decrease in temperature, and by influencing the mechanical stability through formation of notches at d.c. operation below 2700 K. The way in which the effective emissivity and electrical resistance are influenced by mass loss and by facet formation is investigated. An analysis of burn-through is given. In about 70°i, of all cases the site of initially highest temperature coincides with the final highest-temperature spot. For evacuated lamps this value is somewhat higher and the defect responsible for the initial spot appears to be a minimum in wire diameter. For gas-filled lamps the correlation between final and initial spot is somewhat lower. In this case the defects responsible for burn-through appear to be minima either in coil pitch or in wire diameter, both are of about equal importance.
Reliability assessment of beam-lead sealed-junction integrated circuits in polymer sealed packages. K. B. LASCH, W. N. SCHAITTER and R. ILGENDRITZ. Proc. Electron. Corn-
21 (January 1977). Reliability of any electronic instrument depends upon derating policy, redundancy, proper selection and screening of components, adequate consideration of environmental conditions, proper handling and the
Incandescent lamp filaments: facet-cooling, failure mechanisms. W. LEMS, H. KINKARTZ and W. LECHNER. Philips Res. Rep. 32, 82 (1977). The life of incandescent single-coil
Electrical ratings of electronic components from the viewpoint of reliability. J. S. BORA. The QR Journal (India) p.
World Abstracts on Microelectronics and Reliability reliability of interconnections and solder-joints. The design engineer can derate the electrical stresses of electronic components to the desired level provided manufacturers have correctly specified the ratings. This paper describes the experiments conducted on capacitors, resistors, toggle switches and transistors to find the correctness of the specified ratings. It was found out that the manufacturers either underestimate or overestimate their components regarding electrical ratings. A method to determine the ratings has also been given. It is emphasised that all manufacturers should follow a uniform procedure in deciding the ratings and that users should verify the validity of the specified ratings. Encapsulation, sectioning and examination of multilayer ceramic chip capacitors. G. J. EWELL and W. K. JONES. Proc. Electron. Components Conf Arlington, Va., May 16-18, 1977. p. 446. As the reliability of multilayer, ceramic capacitors becomes more critical, the destructive physical analysis (DPA) of lot samples will become mandatory. The techniques of DPA must be such that they induce no changes in the part's structure. Handling~ encapsulation, grinding, polishing and examination techniques are detailed herein. Tab lead capacitor. G. R. LOVE, E. D. McLAURIN and W. E. HUCKS. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 458. Differential thermal expansion between a monolithic chip capacitor and the common hybrid substrate materials gives rise to significant stresses in the chip. For conventional attach techniques and conventional chips, these stresses may exceed the strength of one or more of the components, chip, termination, substrate metallization, or solder, in the assembly with disastrous effects. Further, common choices of materials aggravate the basic problem by "building-in" opportunities for delayed failure due to slow formation of brittle intermetallics in the assembly. We have examined this problem in detail, and we offer a solution that incorporates material, process, and design innovations which essentially eliminate these problems as sources of device failure. Thermally stable tantalum oxide thin film capacitors. SHIGEHIKO SATO, KATSUAKI YANAGISAWA, SUSUMU OKAMOTO and HAJIME SASAKI. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 374. Thin film capacitors have been developed which can withstand temperatures as high as 300°C for more than one hour. This enables capacitors to be used at higher application temperatures and to be assembled by TC bonding at high substrate temperature or by solder reflow techniques. The dielectric was tantalum oxide formed by anodization of sputtered fl tantalum films. It was heat-stabilized at high temperature and then reanodized to the desired voltage for the removal of dielectric defects caused by the heattreatment. Except for the thermal stability, no significant differences in electrical characteristics were noted between the developed and conventional tantalum oxide thin film capacitors. But the former was more stable than the latter when life-tested under various conditions of voltage, temperature and humidity. The production yield of the thermally stable tantalum oxide thin film capacitors was more than 95~,0 when a tolerance of 5~o was allowed for capacitance values. Failure mechanisms in solid electrolytic capacitors. J. BRETTLE and N. F. JACKSON. Electrocomp. Sci. Technol. 3, 233 (1977). The failure mechanism of solid tantalum and alurninium capacitors have been investigated using a combination of electrical measurements and electron microscopy. The capacitor dielectric was examined before and after life testing and changes correlated with electrical measurements. M.R. 1 6 / 6
:~
643
The basic mechanism of failure of solid tantalum capacitors is found to be field crystallisation of the essentially amorphous dielectric oxide. The growth of higher conductivity crystalline oxide during operation of the capacitors causes an increase in leakage current and may result in catastrophic failure. The effect of field crystallisation can be minimised by using high purity tantalum to reduce the number of crystallisation nucleation sites. Since crystalline growth is primarily dependant on applied voltage, high voltage capacitors are much more susceptible to failure than low voltage units. There appears to be no long term failure mechanism in solid aluminium capacitors. However, a particular problem with these units is that they are difficult to make. This is because the anodic layer is chemically less stable in the case of aluminium than in the case of tantalum. The attack is initiated during the deposition of manganese oxide by pyrolysis from manganese nitrate solution and developed by the reform process. Solid aluminium capacitors often have a lower capacitance and higher initial leakage current than comparable solid tantalum units; however, the leakage current decreases on life tests and their reliability is high. Special lot acceptance tests for multilayer ceramic chip capacitors. W. N. WESSBERGand G. J. EWELL. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 452. Detailed, rigorous tests for acceptance of lots of multilayer ceramic chip capacitors are described. Electrical tests are listed, and illustrations of part defects detectable by external examination and by internal examination through destructive physical analysis (DPA) are given. Structural integrity and electrical stability are emphasized. Influence of defects in the silicon wafer on the properties of silicon oxide films formed by reactive sputtering. D. KROPMAN, M. VINNAL and P. PUTK. Vacuum 27 (3) 125 (1977). It is well known that the properties of SiO 2 films, formed by reactive sputtering in oxygen, sensitively depend upon the details of chemical pretreatment of the Si surface and on the conditions of oxide formation. The influence of bulk defects in the silicon wafer has, however, not been adequately studied. Bombardment of the silicon surface with electrons and ions during the initial stages of the oxide formation is accompanied by extensive interaction with the surface defects, leading to changes in the charge of these defects and properties of the deposited film in general. If vacancies are the dominant defects, then a positive charge is created in the oxide film. If both vacancies and stacking faults are present, then both positive and negative charge can appear in the SiO2 film. Which non-sinusoidal voltage may shorten the life of a capacitor? MOtSEY LERNER. Proc. Electron. Components Conf. Arlington, Va., May 16-18, 1977. p. 468. Heat dissipation in a capacitor, caused by a non-sinusoidal voltage of an arbitrary wavcform, is discussed. It is shown how to estimate the maximum and the minimum of possible losses in a capacitor when its frequency response is unknown. The width of the band of possible losses is chosen to indicate the degree of danger of overheating of a particular cpacitor by a particular non-sinusoidal voltage. Examples of a saw-tooth voltage and of a trapezoidal voltage are investigated.
Effect of oxidation on the breakdown characteristics of ahminium diffused junctions. B. JAYANTBALIGA.Solid-St. Electron. 20, 555 (1977). High voltage power rectifiers and thyristors are usually fabricated with deep diffused junctions using aluminum or gallium as the p-type dopant. Aluminum and Gallium have a relatively large diffusion coeffi-