High-injection barrier effects in SiGe HBTs operating at cryogenic temperatures

We demonstrate that high-injection barrier effects associated with the collector-base silicon-germanium (SiGe) to silicon (Si) heterojunction are an important design constraint for SiGe heterojunction bipolar transistors (HBTs) operating at cryogenic temperatures. Due to its thermally activated nature, these barrier effects can have important dc and ac consequences at cryogenic temperatures even when undetectable under room temperature operation. We use measured results from advanced SiGe HBTs and Si BJTs over a wide temperature range, in combination with simulation, to shed light on the design issues associated with these high-injection barrier phenomena.