An analytical expression for base transit time in an exponentially doped base bipolar transistor

An analytical expression of base transit time for exponentially doped base, which is applicable to both homojunction and heterojunction bipolar transistors, is developed. The present treatment includes dopant dependent mobility variation, bandgap narrowing and finite velocity saturation effects in the calculation of base transit time and thus is a generalization of the results reported recently by Rosenfield and Alterovitz[6]. These effects degrade base transit time significantly and must be incorporated in the calculation. The finite velocity saturation effect will progressively play a pivotal role as the base width is scaled down and for double heterostructure bipolar transistor. The transition of base transit time from the ballistic and the drift-diffusion limit will occur at a longer base width if the base is exponentially doped with a sufficiently high doping index.