Low-frequency noise has been investigated in self-aligned (SA) and gate overlapped lightly doped drain (GOLDD) polysilicon thin-film transistors (TFTs). We found that, at low V-ds both device structures are characterised by 1/f noise, originating from carrier number fluctuations. However, when the SA devices were operated in the kink regime, an excess noise was observed, in contrast to GOLDD TFTs. This has been explained by attributing the excess noise to supplemental oxide charge fluctuations induced by hot-carrier injection into the gate oxide. By using two-dimensional numerical simulations, we show that the hot-electron emission current in SA polysilicon TFTs is much higher than in the GOLDD structure.