Investigations On The Reliability Of Elevated-Metal Metal-Oxide Thin-Film Transistors Under AC Bias Stress

Amorphous indium-gallium-zinc oxide(a-IGZO)thin-film transistor(TFT)is an important TFT technology in active-matrix displays.Under practical operation conditions,a-IGZO TFTs suffer from both the DC bias stress and AC bias stress switching between ON and OFF state.In this work,stability of conventional and elevated-metal metal-oxide(EMMO)a-IGZO TFTs under AC bias stress is investigated.The degradation behaviors are analyzed and corresponding degradation mechanisms are proposed.Under pulse gate voltage(Vg)stress,the transfer characteristics of conventional a-IGZO TFTs shift positively and the degradation degree is dependent on the pulse repetition number.The device degradation is dominated by the dynamic mechanism,where electrons emitted from deep trap states would be exposed to the transient lateral coupled electric field and gain enough energy to overcome the interface barrier and inject into the gate insulator during pulse falling time(tf)transients.Under pulse drain voltage(Vd)stress,a positive shift of transfer characteristics is observed and the degradation is controlled by the DC mechanism.When Vg exists and Vd is high,electrons would be trapped at the interface between the gate insulator and channel or injected into the gate insulator.With Silvaco simulation,it's found that the lateral electric field in the depletion region near the source and drain is dominated by the transient lateral coupled electric field under pulse Vg stress,while it is still dominated by the dc electric field under pulse Vd stress,which explains the different dominant degradation mechanisms of conventional a-IGZO TFTs under such two kinds of pulse stress.Different from conventional a-IGZO TFTs,EMMO TFTs exhibit minimal device degradation under pulse stresses.Under pulse Vg stress,the contribution from dynamic degradation is negligible,which is the dominant one for conventional a-IGZO TFTs,which is attributed to the reduced deep acceptor-like trap states.Moreover,the negative shift of threshold voltage(Vth)after the pulse stress is smaller than that caused by the positive bias stress(PBS)with the same positive gate bias as in the pulse stress and the same effective PBS time,which is attributed to the negative shift of Vth induced during the PBS stage in the pulse stress being partially recovered during the negative bias stress(NBS)stage in the pulse stress.Under pulse Vd stress,EMMO TFTs also show negligible device degradation.The oxidizing heat-treatment process during device fabrication improves the quality of the channel a-IGZO and gate oxide,which in turn ensures the superior stability against pulse stresses.