Characterization of Polycrystalline Silicon Thin-Film Transistors

Abstract

The nonlinear behavior of the transfer characteristics of polycrystalline silicon thin-film transistors (poly-Si TFTs) at the threshold voltage was analyzed. The threshold voltage VT was defined as the gate voltage giving half of the maximum transconductance (GmMAX/2). The nonlinear parameter ΔV was also introduced as the maximum transconductance divided by the differential of Gm at VT. VT and ΔV increased as the density of defect states increased. Moreover, ΔV increased as the energy level of defects increased near the band edge. ΔV gave the carrier density and the Fermi level at the silicon surface at VT. VT and ΔV also gave the density of occupied defects states at VT. Analysis using VT and ΔV was applied to the characterization of n-channel TFTs fabricated with laser crystallization and H2O vapor annealing. VT and ΔV were 0.90 and 0.81 V, respectively. They gave the Fermi level and the densities of electron carrier and occupied defect states at the threshold voltage as 0.91 eV, 2.2×1010 and 1.8×1011 cm-2, respectively.