Ageing and thermal recovery of advanced SiGe heterojunction bipolar transistors under long-term mixed-mode and reverse stress conditions

The main reliability issue in SiGe heterojunction bipolar transistors (HBT) is the cumulative base current degradation which they may experience during circuit operation. This continuous transistor ageing is the result of the interplay between oxide interface trap creation and annihilation. Based upon long-term mixed-mode (up to 1000 h) and reverse (up to 100 h) stress tests this study discusses in detail the change of ageing rate over time and how this impacts the dependence on stress-voltages and stress-currents. Additionally, investigation of degradation as function of ambient temperature under both stress types reveals stress specific thermal behavior. These results are put together into an ageing function useful for integration into compact models and effectively replacing "life-time" definitions. At high enough junction temperatures degradation can be reversed, leading to an efficient thermal recovery of the HBTs within one hour independent of previous transistor degradation. Finally, with a simple stress test imitating the switching between mixed-mode ageing and thermal recovering states in HBT duty cycles, it could be demonstrated that DC mixed-mode stress used for standard reliability characterization represents an upper limit for degradation of SiGe HBTs in RF circuits. (C) 2015 Elsevier Ltd. All rights reserved.