Assessment of transport parameters for the design of high speed Si/SiGe HBTs with compositionally graded base

In heterojunction bipolar transistors current gain cut-off frequencies f(t) are limited by time constants of electron transport across the base and th e collector space charge zone. This point of view favours the use of small widths of the base. However, such a choice would increase the base resistan ce which is detrimental to the maximum frequency of oscillation f(max). A w ay out of this trade-off is to accelerate minority carriers by using a grad ed or step base structure. Besides their potential to keep transit times lo w such sophisticated HBT structures may show, in addition, an active behavi our above the cut-off frequencies due to the transit time effect which is c ommonly ignored in bipolar transistors. A key parameter for that is the min ority diffusion constant of electrons in the SiGe base. Our experimental st udy on this parameter in the present work indicates a value that is above t hat of minority carrier transport in purl silicon. (C) 1998 Published by El sevier Science Ltd. All rights reserved.