The local rise of lattice temperatures in enhancement MOSFET's is analyzed by the frequency shift of the silicon optical phonon using Raman spectroscopy with submicron spatial resolution. Operating the devices beyond saturation a source drain asymmetry is observed corresponding to the heat dissipation profile peaked at the pinch off region. A reduction of channel length at standard conditions leads to a local temperature increase due to the enhanced electrical power. An anisotropic behaviour of the temperature distribution is also found in the substrate surrounding the MOSFET, which is related to the geometric shape of the heat source. The experimental results are in good agreement with a simnple model calculation based on device parameters.