The Research Of Drain Current Models For Double-Gate Poly-Si Thin-Film Transistors Based On Surface Potential

Poly-Silicon thin-film transistors(poly-Si TFTs) have been widely used in the field of liquid crystal displays, three-dimensional integrated circuit and SRAM.However, with the decrease of channel length, short-channel effects become more and more serious.Traditional single-gate devices are hard to meet the future needs of the integrated circuit. Double-gate poly-Si TFTs are new kinds of devices developed in recent years, including constructions of junction and junctionless. Due to the better subthreshold characteristics,lower leakage current and smaller short-channel effects,double-gate poly-Si TFTs have become more development potential in the industry currently.Therefore, building reasonable electrical models for double-gate poly-Si TFTs, and making them suitable for circuit simulator, has important practical significance for the integrated circuit industry.The main goal of the paper is to develop drain current models for junction and junctionless double-gate poly-Si TFTs suitable for circuit simulator, which includes the following issues.Firstly, for the undoped or lightly doped junction double-gate poly-Si TFTs,one-dimensional Poisson’s equation contains free carrier,exponential tail state and constant deep state is proposed. Then, the relation between the center potential and surface potential is derived by using superposition method. Through fitting method and Newton midpoint method we can get center potential, then accurate surface potential is obtained.The surface potential calculation not only effectively improve the computational efficiency, but also does not need smoothing function to connect different operating regimes.Compared with numerical calculation, the maximum absolute error of surface potential is only millivolt range.Then, a drain current model is derived consistent with Pao-Sah model,which covers from subthreshold region to above-threshold region. Since the model and its first derivatives are continuous, it is easy to implement the model into circuit simulator.Meanwhile, compared the model with different experimental results verified that the proposed model is capable of accurately predicting the drain current characteristics of junction double-gate poly-Si TFTs.Secondly, for the heavily doped junctionless double-gate poly-Si TFTs, "finite-difference" method is used to establish relation for surface potential and center potential.By fitting method and Newton midpoint method we can get center potential,then accurate surface potential is obtained.The surface potential calculation has clear physical meaning and its algorithm is simple.While compared with the numerical results, the accuracy of the proposed calculation is also proved.Based on the above calculation, the compact drain current model for junctionless double-gate poly-Si TFTs is developed, describing accumulation,hybrid,partly-depleted and fully-depleted regimes.Due to the advantage of the short simulation time,the model is suitable for circuit simulator.Furthermore, Comparisons among the model,2D numerical data and experimental results of transfer and output characteristic curves with different devices ensure the validity of the model.In conclusion, drain current models based on surface potential for junction and junctionless double-gate poly-Si TFTs are proposed in this paper. The proposed models, which have the advantages of clear physical meaning and simple calculation,can describe all the operating regimes. The models are compared with the numerical calculation, 2D simulation results and experimental data, which are fully verified its applicability and accuracy and can serve as a solid foundation for the practical application.