SiGe HBT performance limits

How high a performance can be achieved in silicon-based bipolar transistors? An answer to such a question surely considers many assumptions. New discoveries continue to affect the critical aspects of device operation such as charge storage, carrier transport, and parasitics. Other discoveries affect the processing of the device, leading to even better ways to make the device structurally ideal. One example of a historic discontinuity in device fabrication and operation is the development of production-ready SiGe epitaxy. Before the advent of SiGe epitaxy, predictions toward device limits would likely have made certain assumptions regarding emitter charge storage or minority carrier diffusion and this would clearly be off the mark due to the significant advancement in SiGe band engineering. More recently, the incorporation of carbon has provided a boost, strongly affecting the diffusion of dopants and thus providing a greater control over the device structure. Similar innovations are expected to continue to provide a boost to the device operation, and so continually change the assumptions that may go into predicting limits of device operation.