Subthreshold Current Model For Polycrystalline Silicon Thin Film Transistors

This paper proposes an analytical subthreshold drain current model for polycrystallinesilicon (poly-Si) thin film transistors (TFTs), which well reproduces the experimentalsubthreshold current of both metal-induced laterally crystallized (MILC) and excimer laserannealed (ELA) devices.Firstly, it is presented that the entire subthreshold region of poly-Si TFTs is dominatedby drift current, where the carrier effective channel mobility (μeff) is first found to followthe Meyer-Neldel rule (MNR). Extracted characteristic MN energies and estimatedactivation energies beginning from which MNR fails are all close to the optical phononenergy of Si for both MILC and ELA poly-Si TFTs, providing strong evidence to the MNR.Carrier thermionic emission over grain boundary (GB) barriers activated by multi-phononabsorption process is the origin of the MNR.Then, an analytical subthreshold drain current model is proposed. Channel inversionprocess in the subthreshold region is clarified. Channel inversion charge sheet density isthen modeled based on the gate-to-channel capacitance-voltage measurement. Followingthe observation of the MNR for μeffin the subthreshold region, a μeffmodel is constructed.By considering the carrier drift, an analytical subthreshold drain current model isdeveloped. Subsequently, expression for subthreshold swing (SS) is derived to analyze thesubthreshold property, and gate voltage induced GB barrier lowering effect is determinedto be the key mechanism for SS of poly-Si TFTs. Finally, an interpolation function isintroduced to get a unified current model which combines the proposed subthreshold drain curre nt model with a previousl y presented on-state region drain curre nt model.