Study of Electron Velocity Overshoot in NMOS Inversion Layers

Wei-Kai Shih,Mahbub Rashed,Srinivas Jallepalli,Christine M Maziar,Al F Tasch Jr

doi:10.1155/1998/65364

Abstract

Non-local electron transport in nMOSFET inversion layers has been studied by Monte Carlo (MC) simulations. Inversion layer quantization has been explicitly included in the calculation of density of states and scattering rate for low-energy electrons while bulk band structure is used to describe the transport of more energetic electrons. For uniform, high-lateral field conditions, the effects of quantization are less pronounced due to the depopulation of electrons in the lower-lying subbands. On the other hand, Monte Carlo results for carrier transport in spatially varying lateral fields (such as those in the inversion layer of MOSFETs) clearly indicate that depopulation of the low-lying subbands is less evident in the non-local transport regime. Quasi-2D simulations have shown that, at high transverse effective field, the inclusion of a quantization domain does have an impact on the calculated spatial velocity transient.

Highlights

With the continued scaling of the feature size of MOS devices, carrier transport in the MOS inversion layers has entered the regime where non-local effects are no longer negligible
Quasi-2D Monte Carlo simulations that take into account the effects due to size quantization have been used to study carrier velocity overshoot in nMOS inversion layers
With a given ramp-field profile, the spatial transient velocity is observed to depend on the transverse effective field more strongly than on the substrate doping

Summary

Introduction

With the continued scaling of the feature size of MOS devices, carrier transport in the MOS inversion layers has entered the regime where non-local effects are no longer negligible. Velocity overshoot was found to account for approximately 20% of the disagreement in drain current between measurement and drift-diffusion simulation in a 0.12 lm SOI MOSFET [1]. Despite a few pioneering studies on probing the electron spatial transient velocity in compound semiconductor devices [2], a mature experimental technique that allows the direct measurement of carrier velocity in silicon MOSFETs has not been available. In order to accurately express these non-local effects in physically based models, the Monte Carlo (MC) technique has become increasingly indispensable for device scientists and engineers. With phenomenological surface-roughness scattering models, some conven-

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Conclusion