Mobility enhancement in dual-channel P-MOSFETs

Hole mobility is characterized in P-MOSFETs with a layered substrate consisting of tensile strained Si cap on a compressively strained Si/sub 0.4/Ge/sub 0.6/ buried layer grown pseudomorphically to a relaxed Si/sub 0.7/Ge/sub 0.3/ virtual substrate. Besides the expected mobility enhancement in the strained Si cap and in the buried Si/sub 0.4/Ge/sub 0.6/ layer, a second peak in mobility versus total inversion carrier areal density curve was observed under strong inversion conditions in thin Si-cap layer samples. Qualitatively, this reversed mobility trend can be correlated to the transition of inversion conduction from the buried layer to the surface layer, but quantitative analysis reveals that the surface layer mobility in thin Si-cap samples needs to be substantially larger than that in thick-cap samples, if it can be assumed that mobility is a function of transverse field. Further analysis found that, if it is assumed that mobility is a function of inversion carrier density, measured mobility curves can be matched consistently with a single set of mobility-carrier-density relationship.