Towie, Ewan Alexander
Extended models of Coulomb scattering for the Monte Carlo simulation of nanoscale silicon MOSFETs.
PhD thesis, University of Glasgow.
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The International Technology Roadmap for Semiconductors (ITRS) specifies that MOSFET logic devices are to be scaled to sub-10nm dimensions by the year 2020, with 32nm bulk devices ready for production and double-gate FinFET devices demonstrated down to 5nm channel lengths. Future device generations are expected to have lower channel doping in order to reduce variability in devices due to the discrete nature of the channel dopants. Accompanying the reduced channel doping is a corresponding increase in the screening length, which is even now comparable with the channel length. Under such conditions, Coulomb scattering mechanisms become increasingly complex as the scattering potential interacts with a larger proportion of the device.
Ionized impurity scattering within the channel is known to be an important Coulombic scattering mechanism within MOSFETs. Those channel impurities located close to the heavily doped source and drain or both, will induce a polarisation charge within the source and drain. These polarisation charge effects are shown in this work to increase the net screening of the channel impurities, due to the inclusion of remote screening effects, and significantly decrease the scattering rate associated with ionized impurity scattering. Remote screening can potentially reduce the control by ionized channel impurities over channel transport properties, leading to an increased sub-threshold current.
A potential model has been obtained that is based on an exact solution of Poisson’s equation for an ionized impurity located close to one or both of these highly doped contact regions. The model shows that remote screening effects are evident within a few channel screening lengths of the highly doped contact regions. The resultant scattering model developed from this potential, which is based on the Born approximation, is implemented within a Monte Carlo simulator and is applied to MOSFET device simulation.
The newly developed ionized impurity scattering model, which allows for remote screening, is applied in the simulation of two representative MOSFET devices: the first device being a bulk MOSFET device developed for the 32nm technology generation; the second device is an Ultra-Thin-Body Double Gate (UTB DG) MOSFET developed for the forthcoming 22nm technology generation. Thorough investigative simulations show that for both the bulk MOSFET and the UTB DG MOSFET, that remote screening of channel impurities in these devices is not a controlling effect. These results prove that the current model for ionized impurity scattering employed in Monte Carlo simulations is sufficient to model devices scaled to at least the 22nm technology node, predicted to be in production in the year 2012.
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