An easy-to-implement hole mobility model, which accurately predicts low-field mobility in bulk MOSFETs and ultrathin-body (UTB) silicon-on-insulator FETs with different crystal orientations, is developed. The model accounts for the influence of the surface orientation and the inplane current-flow direction on effective masses, subband repopulation, and scattering rates. The effects induced by extremely small silicon thicknesses are also addressed. A good agreement with the experimental mobilities of bulk and UTB FETs with silicon thicknesses from 60 nm to values as small as about 2.7 and 2.3 nm is demonstrated for devices with (100) and (110) substrates, respectively.