Evaluation of SiGe:C HBT intrinsic reliability using conventional and step stress methodologies

The reliability of SiGe:C HBT devices fabricated using the Freescale’s 0.35-μm RF-BICMOS process was evaluated using both conventional and step stress methodologies. This device technology was assessed to determine its capability for various power amplifier applications (e.g., WLAN, Bluetooth, and cellular phone), which are more demanding than conventional circuit designs. The step stress method was developed to allow a rapid evaluation of product reliability, as well as, a quick method to monitor product reliability. For all tests the collector current IC and collector voltage VC were kept constant throughout the test, and the current gain β (IC/IB) was continuously monitored. The nominal bias condition was VC = 3.5-V and JC = 50-kA/cm2 (or 0.5-mA/μm2). The “failure criterion” for all reliability evaluations was −10% degradation in β from the initial value at the start of each stress test or interval. The median time to failure (MTTF) at a junction temperature (TJCN) of 150 °C for the conventional stress test was 1.86E6-h, and the thermal activation energy was 1.33-eV. In contrast for the temperature step stress tests the combined results gave an MTTF at TJCN = 150 °C of 5.2E6-h and a thermal activation energy of 1.44-eV. Considering the differences in the two test methods, these results are quite close to one another. The intrinsic reliability of this device at the nominal bias condition and TJCN = 150 °C is more than adequate for a 5-year system life.