Self-Heating Effects in p-Channel Polysilicon TFTs Fabricated on Different Substrates

The self-heating-related instability was studied in p-channel polysilicon TFTs fabricated on glass, stainless steel (SS) and polyimide (PI) substrates. We found that while the devices fabricated oil glass and PI presented substantial device degradation when operated in the self-heating region, the devices fabricated on SS were very stable when bias stressed under similar conditions. From extensive analysis of the phenomenon through numerical simulations, we found that the device degradation could be reproduced perfectly by interface state generation and uniform positive charge injection into the gate oxide along the channel. Additional defects, located in narrow (100 nm) regions at the gate edges, were also introduced to fully reproduce the apparent field effect mobility enhancement. In order to explain the role of the substrate on the stability, we analyzed three different structures by using 3-dimensional numerical simulations, coupling the thermodynamic and the transport models. The results from the simulations clearly demonstrate that devices fabricated on SS operate at much lower temperatures, thus explaining the observed better stability.