Modeling And Simulation Of ZnO Thin-film Transistors

With the development of information and technology, active driven flat-panel display technology is developing widely and deeply in the field of display. As the most important part of the active matrix liquid display device,thin film transistor has also been widely concerned and become the focus of research and the leading technology of flat-panel display filed.With the advantages of high mobility,suitable for low temperature production, high optical transparency,good uniformity, large band gap, and device stability,Zn O thin film transistor(TFT) has a wide range of potential applications in flexible and transparent electronics.Zn O TFT can not only solve the problem due to Si-based thin film transistors,such as non-transparent,poor light sensitivity and complicated fabricate process,but also can avoid the poor performances of organic thin film transistor like low mobility and high power etc.Hence, more and more research institutions begin their research journey of the Zn O thin film transistor.As the integrated circuit is developing at a high-speed, it is absolutely impossible to design and optimize the process condition just based on the way of process tape-out.Therefore, it is imperative to use digital simulation and emulation to realize the process flow of integrated circuit.While,so far,the research work of Zn O TFT is mainly on the process design and optimization;however, the modeling and simulation of Zn O TFT is very limited.Our work mainly focus on the modeling and simulation of double-gate Zn O TFT.The model,which is proposed and improved effectively fitting the device properties by other research institution,takes into account the gaussian distribution deep level states at the midgap and exponential distribution band tail states.Firstly,we compare the three working modes and determine the top-gate bottom-gate short working mode to be used in the ongoing study.Next,we study some aspects that influence the device properties,including deep level states and band tail states,source electrode and drain electrode placement and thickness,top-gate material.The results show that,deep level states and band tail states mainly influence the on-state characteristics and open voltage respectively,source and drain electrodes top contact is better than bottom contact.On the basis of the above conclusion, we propose the double-gate multi-layer insulator Zn O TFT(DGMI-TFT), whose gate insulator is Si O2-Hf O2-Si O2. By comparing the performances of DG-TFT and DGMI-TFT, we found that DGMI-TFT has more superior performances. The performance improvement mechanism is studied through the following aspects,such as electron concentration and energy band distribution in the channel,the variation of grain boundary barrier height,and also the electric field and potential in the channel. In addition,the influential factors of DGMI-TFT are also discussed.Higher permittivity material means better device performance.The smaller the thickness of high-k material,the larger percentage of high-k material,the better properties the device performs.Devices with different high-k materials show completely different gate control ability.Finally,we also research the relationship between the variation of grain boundary barrier height and threshold voltage,gaining the conclusion that the VGS when the barrier height is thelargest is identical with the threshold voltage that extracted by the simulator.