SiGe HBTs offer significant performance advantage over Si BJTs with improved base resistance and base transit time. However, each faster technology iteration usually comes at the cost of reduced breakdown parameters. As a result, devices are often restricted to smaller operation biases, and rely on fully utilizing these imposed limits to attain maximum circuit performance. Both high voltage and high current operation have been shown to take a toll on a device’s aging behavior. One unintended consequence of technology scaling is that it forces designers to consider alternate circuit architectures and operate these devices under non-traditional modes such as reverse active, off-state, and saturation. Although not as optimal for device performance as forward active operation, these modes enable improved radiation hardening, power savings and even simpler circuit design.

A bipolar transistor’s operation can be categorized into four main modes as shown in the figure below based on the applied voltage to the base-emitter (V_BE) and base-collector (V_CB) junctions. Due to the asymmetrical junction engineering for vertical HBTs, the forward-active mode is usually the most optimized for device performance. Because it is the most utilized mode, it is also the most characterized for device reliability. In this paper, in addition to the mixed mode stress used to characterize SiGe bipolars, we will discuss some of the recent learning in SiGe HBT reliability in the other less-understood modes of operation. We begin with a brief overview of SiGe HBT reliability physics including hot carrier generation (Auger vs. Impact-Ionization), the temperature dependence of competing mechanisms and the physical damage responsible for electrical degradation. Using measurements and simulations, we discuss in detail the device degradation under off-state and saturation modes. We demonstrate how the hot carrier generation physics responsible for degradation in the forward-active mode is still applicable in the other regions of operation.