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This letter presents the first experimental study of the mobility in 50-nm gate length (LG) pMOSFETs highly strained by a contact etch stop layer. Thanks to an advanced characterization method, the mobility is in-depth studied versus the inversion charge density, the gate length and the temperature. The physical origin of the more than 50% mobility enhancement at LG=50 nm is proven to be the low effective mass of the top valence band rather than any scattering modification. This mobility gain is maintained even at high effective field. This explains the 30% ION enhancement at 50-nm gate length, which is among the best results at such a dimension.