In heterojunction bipolar transistors current gain cut-off frequencies f(t) are limited by time constants of electron transport across the base and the 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 resistance which is detrimental to the maximum frequency of oscillation f(max). A way out of this trade-off is to accelerate minority carriers by using a graded or step base structure. Besides their potential to keep transit times low such sophisticated HBT structures may show, in addition, an active behaviour above the cut-off frequencies due to the transit time effect which is commonly ignored in bipolar transistors. A key parameter for that is the minority diffusion constant of electrons in the SiGe base. Our experimental study on this parameter in the present work indicates a value that is above that of minority carrier transport in purl silicon.