The electrical instabilities in p-channel polysilicon TFTs induced by negative bias temperature stress (NBTS) and self-heating have been investigated. From NBTS experiments performed at different temperatures and gate bias, we derived an empirical relationship that provides the T and electric field dependence of the interface state generation. To explain the device instability related to self-heating we considered a spatially non uniform interface state distribution, as a non uniform transverse electric field is present during bias stress. The interface state distribution can be deduced using the empirical relationship, determined from NBTS experiments, and considering the spatial distribution of the oxide electric field, obtained from numerical simulations. Using the so determined interface state distribution it was possible to perfectly reproduce not only the transfer characteristics but also the asymmetry observed in the …