The enhancement of Q factor in RPCVD SiGe varactors by the structural modification of the base-collector junction

We designed two silicon germanium (SiGe) varactors enhanced in Q factor through a structural modification by using a cost-effective SiGe heterostructure bipolar transistor (HBT) process, a conventional reduced-pressure chemical vapor deposition (RPCVD). As a result, the suggested structures showed a superiority in Q factor (160/GHz/pF at 2.5 GHz) to the conventional one (70/GHz/pF), even with neither a change in process nor an additional mask. We attributed the enhancement of Q factor to the structural feature of the varactors and quantitatively analyzed it with a lumped element model.