Self-consistent Schroedinger-Poisson calculations have been employed to investigate theoretically the mobility extraction for hetero-MOSFETs or HMOSFETs with a thin gate oxide. In this particular case, the contribution of the cap layer to the gate capacitance is crucial. It is shown that there is a considerable error in the effective mobility when extracted with the common constant capacitance approximation and that application of the split-CV method is essential.