In this work, we investigated self-heating related instability in polysilicon thin film transistors (poly-Si TFTs) fabricated on polyimide (PI) substrates. Indeed, when joule heating becomes relevant, the temperature of the active layer can substantially rise, since the devices are fabricated on thermally insulating substrates. As a result, electrical instability is triggered and attributed to the generation of interface states, due to the Si-H bond breaking, and charge trapping into the gate insulator. In addition, by using 3-dimensional numerical simulations, coupling the thermodynamic and transport models, we analyzed the temperature distribution of the device under operating conditions and found that self-heating is more severe for devices fabricated on plastic substrates. (C) 2009 Elsevier B.V. All rights reserved.