Acceleration parameters and reliability of SiGe HBTs during long-term high-Vce operation

Modem high-performance ASICs often must operate with transistor biases high enough that they are in the regime of avalanching. We have conducted long-term (up to 8800 hrs) accelerated lifetesting under avalanche stress conditions on IBM\´s 120 GHz f_t SiGe heteroj unction bipolar transistors ("SiGe7HP") to determine the relevant acceleration parameters, and estimate degradation for typical applications. Acceleration was achieved with V_ce bias up to 5.2 V (c.f. BV_ceo = 2.0 V) and with baseplate temperatures ranging from -40°C to 56°C, corresponding to emitter-base junction temperatures of 105°C and 200°C respectively. The dominant mode of wear-out was degradation (< 45% under high acceleration) of DC current gain β(at a collector current density of 1 mA/μm²), caused by increased base current, while the collector current remained largely unchanged. The device degradation was parameterized by fitting this data to an existing empirical model, modified to include an Arrhenius temperature dependent term. A negative activation energy was found, consistent with the avalanche carrier degradation mechanism. Detailed analysis was performed using the parameterization found in this work to estimate the β degradation during a typical application. The results indicate that β would degrade by about 4 ± 1% after 20 years. Since this degradation is insignificant for most integrated circuit applications, it is concluded that this technology is highly reliable during typical long-term high-V_ce bias operation.