Speed comparison of digital BiCMOS and CMOS circuits

Today BiCMOS technologies are becoming more and more attractive and offer an interesting alternative to existing technologies. In the paper it will be shown that in digital circuits the bipolar transistor does not necessarily improve BiCMOS speed performance over CMOS in all circumstances. We shall show what conditions are necessary to achieve superior speed and how they can be met. The results depend on channel length and can be easily applied to downscaled BiCMOS technologies as well. The estimation is based on theoretical considerations and the results are compared to those achieved in already realised BiCMOS technologies.