Self-heating-related instabilities have been investigated in p-channel polycrystalline-silicon thin-fllm transistor, showing an anomalous transconductance (g m ) increase. The g m increase is a fingerprint of edge effects, resulting from a buildup of positive trapped charge in the gate oxide at the channel edges. This was confirmed by the annihilation of such positive charges obtained by sequential hot-carrier bias-stress experiments. From the analysis of the edge effects in devices with different channel lengths, we were able, using 2-D numerical simulations, to deter mine the size of the defected edge regions to be 400 nm.