Implications of thermal instability on HBT power amplifier reliability

Thermal stability is one of several important performance requirements for RF power amplifier modules used for cellular applications. In this work, the conditions for thermal instability in an HBT array for shared and individual ballast configurations are evaluated using experimental results from RF biased life test (RFHTOL) and ADS simulations. The instability is modest under nominal operating conditions, but becomes more severe at elevated temperatures which are frequently used in accelerated reliability tests such as RFHTOL. Thermally unstable power cells exhibit current crowding where one or two cells hog most of the current, resulting in localized hot spots from increased power dissipation density. This paper discusses the link between reliability failures during RFHTOL reliability testing and the thermal stability of HBT arrays used in a power amplifier. Shared ballast arrays are shown to be less thermally stable than individually ballasted arrays and thus have more failures during RFHTOL testing. Electro-thermal results indicate the layout aspects of power amplifier module also play an important role in determining junction temperatures for various stages.