Dynamic Response of Amorphous In-Ga-Zn-O Thin-Film Transistors for 8K×4K Ultra-High Definition Active-Matrix Liquid Crystal Displays
Eric K.-H. Yu, Rui Zhang, Linsen Bie, Alex Kuo, and Jerzy Kanicki
Our objective is to evaluate and compare the dynamic response of a-Si:H and a-IGZO TFTs and their potential application to pixel circuits for UHD AM-LCDs. For this purpose, we have fabricated test circuits consisting of one a-Si:H or a-IGZO TFT connected in series with a storage capacitor. In our results, a-Si:H TFTs are insufficient for 8K×4K UHD AM-LCDs. Only the a-IGZO TFT is fully capable of supporting 8K×4K resolution at 480 Hz.
The impact of the gate signal falling edge tFE on feedthrough voltage ΔVP is thoroughly studied. We find that at the small-tFE limit, channel charge redistribution and overlap capacitance feedthrough both contribute to ΔVP. At the large-tFE limit – approximately three orders of magnitude above the transit time – accumulated channel charges are almost completely released through the drain electrode and do not contribute to ΔVP. In this case, ΔVP becomes entirely independent of VGH.
The size of Cst is shown to have a strong impact on the ΔVP for a-IGZO TFTs, but has negligible influence on its charging behavior. After normalizing for TFT geometry and structure, ΔVP of a-IGZO TFTs is shown to be very similar to a-Si:H TFTs. Increasing the size of Cst can reduce the ΔVP, and for a-IGZO TFTs this can be done without sacrificing charging behavior.