Based on an analytical approach, developed by Jain and Roulston , the different contributions to the bandgap narrowing at T=0K are calculated for highly p-type doped Si and strained Si1-xGex layers for Ge-concentrations between 0 and 30%. The different assumptions will be highlighted with special emphasis on the procedure we used to deal with the non-parabolic aspect of the valence band. This result will be used to calculate the apparent bandgap narrowing in these layers. These theoretical results will then be compared to the experimental results obtained on mesa-type Heterojunction Bipolar Transistors with strained Si1-xGex-base and poly-emitter. The Ge-concentration in these layers was between 0 and 16% while the B-doping was varied between 5.10(17)/cm3 and 5.10(18)/cm3.