The threshold voltages of low temperature polycrystalline silicon thin film transistors

Carriers transport and channel turn-on in low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) are different from that in MOSFETs. Physical meanings of several key threshold voltages for the channel inversion and turn-on are clarified. Channel inversion starts from the partial inversion of the Si grains at V_g = V_{inv_i}, and ends with the full inversion of all Si grains at V_g = V_{inv_f}. However, the channel turn-on voltage VON is still larger, which includes the channel inversion voltage V_inv, as well as required gate voltage to lower the grain boundary (GB) potential barrier height (qφ_b) to the level of 3kT. Drift current dominates both the ON-state and the subthreshold regions of TFTs. Carriers thermionic emission over GB barrier is the controlling transport mechanism, however, before the channel turn-on the carrier effective mobility of LTPS TFTs follows the Meyer-Neldel rule.