Hot-Carrier Degradation in Low-Temperature Polycrystalline Silicon n-Channel Lightly Doped Drain Thin-Film Transistors

The hot-carrier degradation mechanism in low-temperature polycrystalline silicon (poly-Si) n-channel lightly doped drain (LDD) thin-film transistors (TFTs) is investigated. The degradation is characterized by transconductance degradation (Delta G(m)) at low drain voltage (V(d)) and decreases in substrate current (I(sub)) and kink current at high V(d). It is assumed that the trapped negative charges (acceptor-type trap states) in the gate edge region, mostly outside and partly inside the gate, contribute to hot-carrier degradation after stress under current saturation bias. The degradation presumably first occurs outside the gate in the LDD region and, with increasing stress time, the electron-trapped region expands toward the channel under the gate owing to the saturation of electrons in the trap states. At high-V(d) bias, a decrease in the lateral electric field in the gate-edge region due to the presence of negative charges reduces I(sub) and the drain current I(d), thereby reducing the kink current. At low-V(d) bias, the current flow near the upper interface in the gate-edge region decreases after the stress owing to the generated negative charges. The negative charges lead to the decrease in I(d) in the deep-gate voltage V(g) region in V(g)-I(d) characteristics. (c) 2009 The Japan Society of Applied Physics