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June 2005 NASA Electronic Parts and Packaging Program

Prototype Silicon Germanium Power Diodes at Extreme Temperatures

Richard Patterson, NASA Glenn Research Center

Ahmad Hammoud, QSS Group, Inc. / NASA GRC Malik Elbuluk, University of Akron

Rufus Ward & Randall Kirschman, GPD Optoelectronics Corp. Scope Space exploration missions require electronics that are able to withstand operation in extreme temperature environments. Diodes are used extensively in electronic circuits for power management and logic control; however, limited data exist on their performance and reliability at cryogenic temperatures and under thermal cycling. Silicon germanium diodes, which represent a new generation of these devices, are considered as potential candidates for cryogenic operation due to the utilization of band gap engineering in their design. This report summarizes the preliminary results obtained on the evaluation of six prototype samples of silicon germanium (SiGe) power diodes at extreme temperatures. The findings of this work can provide insight on the suitability of these devices for inclusion in electronics designed for use in extreme temperature space exploration missions. Test Procedure Prototype engineering samples of silicon germanium power devices, which were developed by GPD Optoelectronics Corporation under a DARPA/ONR contract, were evaluated at extreme temperatures between -195 °C and +85 °C. Performance characterization was obtained in terms of their forward voltage-current characteristics, using a Sony/Tektronix 370A programmable curve tracer, at specific test temperatures. Cold-restart capability, i.e. power switched on while the devices were at a temperature of -195 °C, was also investigated. A temperature rate of change of 10 °C per minute was used, and a soak time of at least 20 minutes was allowed at every test temperature. The effects of thermal cycling under a wide temperature range on the operation of these diodes were also investigated. The devices were exposed to a total of 12 cycles between -195 °C and +85 °C at a temperature rate of 10 °C per minute. Following the thermal cycling, measurements were then performed at the test temperatures +20, -195, and +85 °C. Table I shows specifications of these prototype diodes [1].

Table I. Specifications of diodes [1].

Parameter (Unit) SGE-N54 SGD-P51 SG-21-41 SG-21-42 SG-22-41 SG-22-42 Forward Current (A) 10 10 5 5 5 5 Breakdown Voltage (V) -125 +100 >100 >100 >100 >100 Oper. Temp. (°C) -253 to +70 -253 to +70 -253 to +70 -253 to +70 -253 to +70 -253 to +70 Packaging TO-3 TO-3 TO-3 TO-3 TO-3 TO-3

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Test Results Temperature Effects The forward voltage-current characteristics of the SiGe power diodes were obtained at test temperatures of 20, -50, -100, -150, -195, and +85 °C. The voltage-current characteristics of the six diodes are shown in Figure 1. It can be seen that all six diodes exhibited, in general, similar temperature dependency as they all underwent a gradual increase in forward voltage as test temperature was decreased. The magnitude of this forward voltage, however, varied from one diode to another. For example, while the forward voltage drop of diode SGE-N54 changed from about 0.7V at room temperature to about 1.3V at -195 °C, the forward voltage drop of diode SG-22-41 increased from about 0.6V to about 0.85V when test temperature decreased from 20 °C to -195 °C. At high temperatures, i.e. +85 °C, all diodes exhibited improvement, again with varying degrees, in their forward voltage drop. Cold Re-Start Cold-restart capability of the six SiGe power diodes was investigated by allowing the devices to soak at -195 °C for 20 minutes without electrical bias. Power was then applied to the device under test, and measurements were taken on the forward voltage-current characteristics. All diodes did perform cold re-start at -195 °C, and the results obtained were similar to those obtained earlier at that temperature. Effects of Thermal Cycling The effects of thermal cycling under a wide temperature range on the operation of the diodes were investigated by subjecting them to a total of 12 cycles between -195 °C and +85 °C at a temperature rate of 10 °C/minute. Forward voltage-current characteristics of the power diodes were then taken at +20,-195, and +85 °C. This post-cycling data is shown in Figure 2 at these selected temperatures. All diodes, with the exception of SG 21-42 and SG 22-42 units, exhibited exact behavior in their switching characteristics before and after cycling. The SG 21-42 and SG 22-42, however, displayed little change in their voltage-current profiles after cycling. These changes were not significant, and the forward voltage drop of these two diodes remained the same at each given test temperature. This limited thermal cycling also appeared to have no effect on the physical integrity of these power diodes as no structural deterioration or packaging damage was observed. Conclusions Prototype engineering samples of silicon germanium power devices were developed and provided by GPD Optoelectronics Corporation for evaluation at extreme temperatures. The six diodes were evaluated in terms of their forward voltage-current characteristics at specific test temperatures between -195 °C and +85 °C. The effects of thermal cycling under a wide temperature range on the operation of these silicon germanium diodes and cold-restart capability were also investigated. All diodes were able to maintain operation between -195 °C and +85 °C, and were able to cold re-start at -195 °C. Thermal cycling

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on the diodes did not have much effect on either their switching characteristics or packaging. It is important to note that these silicon germanium power diodes were prototype engineering samples and optimization in their final design and packaging would lead to improvement in performance and efficiency. Further testing under long term cycling is, however, required to fully establish the reliability of these devices and to determine their suitability for extended use in extreme temperature environments. References [1]. GPD Optoelectronic Corporation, Preliminary Data Sheets for Silicon

Germanium Power Diodes. Acknowledgements This work was performed under the NASA Glenn Research Center GESS Contract # NAS3-00145. Funding was provided by the NASA Electronic Parts and Packaging (NEPP) Program. The management of GPD Optoelectronics Corporation is appreciated for providing the diode samples.

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Figure 1. Forward voltage-current characteristics of diodes as a function of temperature.

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Figure 2. Pre- & post-cycling characteristics of diodes at selected temperatures.

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Figure 2(Cont.). Pre- & post-cycling characteristics of diodes at selected temperatures