Kink effect in short-channel polycrystalline silicon thin-film transistors

Excess current, induced by impact ionization (kink effect) has been investigated in short-channel polysilicon thin-film transistors (TFTs). We have shown, both experimentally and by using two-dimensional (2-D) numerical simulations, that the output characteristics are substantially degraded by the kink effect as the channel length is reduced. In particular, we have shown that the excess current, triggered by the impact ionization and enhanced by the parasitic bipolar transistor action, scales nearly as L^-2, thus making very difficult the downscaling of polysilicon TFTs. Such L dependence has been clarified through a detailed analysis of the current components obtained from 2-D numerical simulations. The analysis demonstrates that there are fundamental issues with the output characteristics, and it appears that the introduction of appropriate drain field relief structures will be necessary for the fabrication of short-channel polysilicon TFTs with high output impedance.