Influence of base dopant out diffusion into the emitter in Si/SiGe heterojunction bipolar transistor using Monte Carlo simulations

In Si/SiGe HBTs the base is doped higher than the emitter and base out-diffusion causes problems because of the broadening of the base and the formation of a ''parasitic'' barrier. On the basis of CNET/CNS technology, Monte Carlo simulations of Si/SiGe emitter-base heterojunctions have been performed to study the influence of the Si/SiGe interface position with respect to the pn junction on the total transit time and the current gain. Only a few nanometer shift causes severe degradation on current gain, but a minimum total delay with an improvement of less than 20% is obtained for a 10 nm shift. However, detrimental effects on transit time appear with less than 20 nm shift. The control of the base dopant diffusion is, generally, not sharp enough to ensure the improvement of the transit time.