A Physical Model for Grain-Boundary-Induced Threshold Voltage Variation in Polysilicon Thin-Film Transistors

Abstract

Grain boundaries (GBs) in the channel region of polysilicon thin-film transistors (poly-Si TFTs) lead to large variations in the performance of TFTs (delay and power). In this paper, we present a physical model to characterize the GB-induced transistor threshold voltage variations considering not only the number but also the position and orientation of each GB in 3-D space. The estimated threshold voltage variations show a good agreement with experimental data and simulations performed by a numerical 3-D drift-diffusion device simulator. Using the proposed model, the impact of GBs on TFTs for various grain sizes, device sizes, and source–drain voltages is discussed in detail. Specifically, when the grain size is comparable to the size of the device, we observed that threshold voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(V_{\rm th})$</tex></formula> variations significantly increase, and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm th}$</tex></formula> -distributions are non-Gaussian. Finally, using our model, we predict and demonstrate the GB-induced variations under different crystallization methods, such as sequential lateral solidification.