A comparison of modeling approaches for current transport in polysilicon-channel nanowire and macaroni GAA MOSFETs
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A comparison of modeling approaches for current transport in polysilicon‑channel nanowire and macaroni GAA MOSFETs Aurelio Mannara1 · Gerardo Malavena1 · Alessandro Sottocornola Spinelli1 · Christian Monzio Compagnoni1 Received: 30 June 2020 / Accepted: 26 September 2020 © The Author(s) 2020
Abstract In this paper, we compare quantitatively the results obtained from the numerical simulation of current transport in polysiliconchannel MOSFETs under different modeling assumptions typically adopted to reproduce the basic physics of the devices, including the effective medium approximation and the description of polysilicon as the haphazard ensemble of monocrystalline silicon grains separated by highly defective grain boundaries. In the latter case, both pure drift-diffusion transport and a mix of intra-grain drift-diffusion and inter-grain thermionic emission are considered. Interest is focused on cylindrical nanowire and macaroni gate-all-around structures, due to their relevance in the field of 3-Dimensional NAND Flash memories, focusing not only on the average behavior but also on the variability in the electrical characteristics of the devices. Keywords Polysilicon · Nanowire MOSFETs · Macaroni MOSFETs · Semiconductor device modeling
1 Introduction The recent transition of NAND Flash arrays from planar to 3-dimensional (3-D) [1, 2] has boosted interest towards deca-nanometer polysilicon-channel MOSFETs. The mainstream technological solution to vertically stack many layers of memory cells in 3-D NAND Flash arrays, in fact, is the so-called punch-and-plug process and allows to create strings of cylindrical gate-all-around (GAA) memory transistors in series connection along a silicon channel with polycrystalline structure [3–6]. The presence of grains separated by highly defective grain boundaries in the polysilicon channel of the strings introduces some relevant issues in the operation and in the reliability of the array. Among them, it is worth mentioning (i) a severe limitation to the string current, especially at low temperature [7]; (ii) transient instabilities in the string current, due to the dependence of the average occupancy of the traps at the grain boundaries on the bias history of the string [8–10]; (iii) variability in the cell threshold voltage ( VT ) and in its temperature dependence, due to the haphazardness in the configuration of the * Aurelio Mannara [email protected] 1
Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milan, Italy
polysilicon grains [11, 12]; (iv) the worsening of the amplitude statistics of random telegraph noise (RTN) affecting cell VT , due to the contribution to percolative channel conduction of the nonuniform inversion of the intra-grain and inter-grain regions [13–19]. The adoption of a thin annular channel instead of a full nanowire was demonstrated to be a successful solution to mitigate some of these issues [20, 21]. In that case, which is commonly referred to as macaroni channel, the inner par
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