The photothermal deflection technique is applied in transverse configuration to measure the thermal diffusivity of semiconductor wafers. The large size of these samples inhibits the possibility to make the probe beam skim the sample at a small height which is required for a direct thermal diffusivity measurement. To overcome this problem, three new experimental schemes are proposed, each one based on a different geometry of the heat diffusion (one-, two-, or three-dimensional scheme). In particular for the 3D experimental scheme, a new mirage setup is described which uses two crystalline prisms 6 mm apart from each other to let the probe beam skim 50±3 &mgr;m high over the sample surface, with a spot size of 22 &mgr;m. The main advantages of this setup, here discussed, are the obtained low probe beam height which is, moreover, independent of the sample dimensions, and the cheap technology to produce the necessary high-quality prisms. The performances of the new schemes have been tested by comparing, for well-known semiconductor wafers (InSb, InAs, InP, GaAs, GaP, Ge, and Si), the experimentally measured thermal diffusivity with the values reported in the literature. ©1997 American Institute of Physics.