面向AMOLED显示的含退化抑制结构的新型薄膜晶体管设计与研究

AMOLED display is the mainstream of the next generation Flat Panel Display (FPD). Thin-film transistors (TFT) are key devices in driving AMOLED displays. TFTs operated in the pixel driving circuit, being subjected to continuous DC and AC electrical stresses, would gradually degrade. With the trend that FPDs being developed to higher definition and higher grey scales, TFTs in the driving circuits must operate at higher frequency, so that device dynamic degradation becomes dominant. It lowers the performance of the pixel driving circuit as well as that of FPD products. Thus, one must design more complicated driving circuit, trying to compensate the degradation of TFT characteristics, however, with the trade-offs of lower product yield and lower pixel density. In this project, a new approach is proposed to meet the challenge. At one side of the channel of a conventional TFT, a novel anti-degradation structure can be built, which can directly suppress the dynamic degradation from the device level. According to the non-equilibrium PN junction degradation model, during transition of the voltage pulses, the novel structure can inject carriers into the channel, preventing the channel from entering the non-equilibrium state, thus greatly suppressing the dynamic hot-carrier degradation of the TFTs. To realize the carrier injection into the channel, the novel structure is based on a complimentarily doped substrate terminal and a metal-semiconductor Schottky diode, respectively for the low temperature poly-Si TFTs and the oxide semiconductor TFTs. With optimal design of the anti-degradation structure, operation lifetime of the TFTs in the pixel driving circuit is expected to be largely extended to 10 times longer. The proposed technology holds great potential in application of the FPD industry.

AMOLED是新一代平板显示的主流，薄膜晶体管(TFT)是驱动AMOLED的核心器件。像素驱动电路中，在持续的交直流电应力作用下，TFT特性会发生退化。随着平板显示日益向高清和高画质方向发展，TFT工作于高频驱动之下，动态退化将占据主导，导致像素驱动电路及显示性能的下降，现有的解决方案基于像素驱动中补偿电路的设计，驱动电路变得更加复杂，降低了显示产品的良率和像素密度。本课题提出了一种新方案，在传统TFT沟道的一侧增加退化抑制结构，可以从器件层面直接抑制其动态退化。根据非平衡PN结退化模型，该结构可以在脉冲电压转换时，向沟道注入载流子，抑制沟道中非平衡态的产生，从而大幅抑制器件的动态热载流子退化。针对低温多晶硅和氧化物半导体TFT，分别设计并优化含互补掺杂衬底端和金半肖特基结的抑制结构，实现向沟道的载流子注入，预期可大幅提高TFT动态工作寿命十倍以上，该项技术对平板显示产业有重要的应用价值。

随着平板显示向高PPI和高刷新率方向发展，工作于高频驱动下的TFT，动态退化将占据主导。这成为像素驱动电路及显示性能下降的主要因素，现有解决方案都基于电路设计的补偿，缺乏器件级别可靠性增强的手段。本项目提出新方案，在TFT沟道的一侧设置互补掺杂的载流子注入极，在器件动态时瞬时注入少数载流子，从器件层面直接抑制动态热载流子退化机制，而不牺牲器件性能。该退化抑制结构应用于N型多晶硅TFT，在典型动态工作条件下器件寿命可提高100倍，远优于LDD结构。应用于P型多晶硅TFT发现严重影响器件可靠性的第二阶段退化基本被抑制。在此基础上，对载流子注入极进行了优化设计，确定了退化抑制效果最佳的条件，并对TFT像素驱动电路和GOA电路的退化抑制进行了研究。对于IGZO TFT，课题研究表明在动态工作条件下，直流退化机制仍占主导，其退化抑制结构仍需进一步研究。本项目提出的退化抑制结构在平板显示行业具有广阔的应用前景。