Investigation of scattering mechanisms for scaled mosfets

The scattering mechanisms are investigated for MOSFETs with gate length varying from 1000 nm to 32 nm in this paper. Although the carrier mobility should theoretically be independent on the gate length, using the universal mobility model it is found that as the gate length reduces the Coulombic scattering decreases while the phonon scattering and the surface roughness scattering increase with a decreasing effective mobility. It is revealed that the decreasing effective substrate doping concentration N_a related to the reduced threshold voltage V_t is the dominant reason. However, the dominant mechanism varies from the Coulombic scattering for the long gate length to the phonon scattering for the short gate length at medium effective field (about 1 MV/cm).

Introduction

Modern MOSFETs become smaller and smaller for a higher integration and current density. However, there are degradations in performance due to some non-ideal factors, such as hot carriers, the effect of velocity saturation and so on. Effective mobility is one of the most important parameters to measure the degradation because it reflects intrinsic properties of devices. It is well known that the carrier mobility depends on gate voltage, body bias, gate oxide thickness and channel doping, but is less dependent on gate length. At least three scattering mechanisms contribute to the mobility degradation, namely, Coulombic scattering , phonon scattering , and surface roughness scattering According to Mattiesen's rule, the simple model proposed by K.Y. Lim et. al. is employed^[2]. The total effective mobility is given by

$${1\over \mu_{\rm eff}}={1\over \mu_{co}}+_{l}{1\over \mu_{ph}}+{1\over \mu_{sr}}={1\over \mu_{1}}+{E_{\Phi}^{1/3}\over h}+{E_{eff}^{2}\over \mu_{3}}, \eqno{\hbox{(1)}}$$ where the is considered as a constant at room temperature and high field and is the effective vertical field at Si surface. By substituting , the fitting parameters, a, “b” and “c” represent the Coulombic, phonon and surface roughness scattering probability, respectively. In this paper, the scattering mechanisms are investigated for MOSFETs with gate length varying from 1000 nm to 32 nm.