A semi-empirical model for electron mobility at the SiC/SiO 2 interface
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A semi-empirical model for electron mobility at the SiC/SiO2 interface Nelson S. Saks Code 6813, Naval Research Laboratory Washington, D.C. 20375 U.S.A. [email protected]
ABSTRACT The mobility of electrons in inversion layers at SiC/SiO2 interfaces µinv has been characterized in 4H- and 6H-SiC using Hall effect measurements. In order to understand the cause of the low mobilities typically observed in SiC MOS devices, a semi-empirical mobility model has been developed based on a previous model for silicon inversion layers. Using this model, two scattering mechanisms, surface phonon and Coulomb scattering from high densities of electrons trapped at the SiC/SiO2 interface, are found to account reasonably well for the behavior of the mobility. The model employs a changing density of trapped electrons as a function of gate voltage to accurately model Coulomb scattering. Surprisingly, evidence of surface roughness scattering is not observed in any SiC MOS device. INTRODUCTION Current state-of-the-art SiC power MOSFETs have poor ON conductance due to high densities of traps Dit at the SiC/SiO2 interface [1]. These traps reduce transistor conductance by two roughly equal mechanisms: (1) removal of free electrons from the MOS inversion layer by trapping, and (2) reduction in the mobility of the remaining free electrons due to increased Coulomb scattering from all the trapped electrons [2]. The standard technique for characterizing mobility in MOS inversion layers is to measure MOSFET drain current as a function of gate voltage Vg, from which the effective inversion mobility µeff is calculated [3]. This approach is simple to implement but does not take electron trapping into account, which significantly underestimates the actual mobility in most SiC MOSFETs with high trap densities [2]. In this paper, Hall effect measurements are used to obtain both the Hall mobility µinv and the density of free electrons in the inversion layer ninv at each gate voltage. Because the Hall mobility is unaffected by charge trapping, it is a better measure of the actual electron inversion mobility than µeff. A further major advantage of Hall data is that ninv(Vg) can be used to calculate many useful parameters in the MOS structure at each gate voltage, including surface potential φs(Vg), effective inversion layer electric field Eeff(Vg), and interface trap density vs. trap energy Dit(Et) [4-7]. A few Hall effect studies of electron inversion mobility in SiC MOS devices have been reported previously [5,6]. Arnold and Alok [6] studied scattering by interface charges with a simple model which correlates uinv with the density of interface scattering charges Nscat using uinv/u0 = 1/(1+α*Nscat). Here we use a simple semi-empirical model based on a model developed by Schwarz and Russek for silicon MOS inversion layers which accounts for surface phonon
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scattering and screened Coulomb scattering [8]. Nscat is treated as a variable which changes with Vg, unlike refs. [6,8] where Nscat is constant. EXPERIMENTAL DETAILS Hall measurements w
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