Performance and Reliability Tradeoffs of Power Amplifier Cells Using High-Performance and Medium Breakdown SiGe HBTs

In this work, the reliability and performance characteristics of silicon–germanium heterojunction bipolar transistor (SiGe HBT) cascode amplifier cells are investigated. In particular, this study investigates the tradeoffs of using transistors scaled for maximum performance or for increased breakdown voltage in the common-base stage of the cascode. The cascode structures are investigated for their dc operating limits, as well as their small- and large-signal performance, and their electrical reliability. Simulations and measurements are performed to determine how to minimize the performance tradeoffs and maximize the reliability improvement of each device type. It is shown that the difference in peak unity cutoff frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{T}$ </tex-math></inline-formula>) is much smaller between the high-performance (HP) and medium breakdown (MB) cascodes than for the individual devices, and that biasing the collector–base (CB) device past its base current reversal point further increases <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{T}$ </tex-math></inline-formula> of the cell by 10% or more. Reliability data show that more reliable cascode cell depends on the biasing condition and load line of the cell. Overall, results show that bias and load line can be changed to improve both the performance and reliability of cascode cells.