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Self-heating, a degradation mechanism of n-channel poly-Si thin-film transistors (TFTs) due to bias stress, has been investigated. The aim of this work is to study this effect in depth to be able to propose a device structure designed to reduce it. The variation of the threshold voltage (Vt) shift with the stress-pulsewidth is related to the temperature rise due to the self-heating effect that depends on the stress-pulsewidth. Electron trapping in the oxide caused by the bias stress is considered to be enhanced by the TFT temperature rise owing to the self-heating. We show that copper-film-based TFTs, which have a substrate made of an extremely thin glass layer and a copper film exhibit much reduced self-heating and thus a decrease of Vt shift caused by the bias stress. These observations are interpreted using numerical simulations to estimate the temperature rise in the poly-Si channel region due to Joule heating.