A microscopic study of transport in thin base silicon bipolar transistors

A comprehensive study of electron transport within silicon bipolar transistors with varying base widths is conducted using a rigorous microscopic model, the scattering matrix approach. Results are presented for the base transit time, current, and the velocity and concentration profiles as a function of basewidth. Unlike previous analyses which artificially impose boundary conditions for the distribution function at the edges of the base, this study rigorously treats transport through both the emitter and collector space charge regions. The results are then compared with other transport models and reveal that even for very thin bases a properly modified drift-diffusion expression gives surprisingly good agreement with the rigorous model for the transit time and current. Finally, an analysis of the distribution function at the collector edge of the base shows that the electron velocity can exceed the thermal velocity (2k_BT/πm*) in a zero-field region, contrary to the assumptions of several authors