We proposed an improved method for evaluating the effective channel mobility (μeff), involving an appropriate definition of the threshold voltage (Vth) based on the ideal gate bias voltage – drain current (VG-ID) characteristics. Using this method, the dependence of μeff on the effective field (Eeff) could be evaluated even for SiC trench MOSFETs with large interface state density (Dit) values. The dominant influence on μeff in the low Eeff region was found to be Coulomb scattering caused by interface states at the SiC/SiO2 interfaces.

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Temperature characteristics of the channel mobility were investigated for 4H-SiC trenched MOSFETs in the range from 30 to 200 °C. The conventional model of channel mobility limited by carrier scattering is based on Si-MOSFETs and shows a greatly different channel mobility from the experimental value, especially at high temperatures. On the other hand, our improved mobility model taking into account optical phonon scattering yielded results in excellent agreement with experimental results. Moreover, the major factors limiting the channel mobility were found to be Coulomb scattering in a low effective field (<0.7 MV/cm) and optical phonon scattering in a high effective field.

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The effect of Al doping concentration (NA) at channel regions ranging from 1.0×1017 to 4.0×1017 cm-3 on the effective channel mobility of electron (μeff) and the threshold voltage (Vth) instability under the positive bias-temperature-stress conditions has been investigated througu the use of trench-gate 4H-SiC MOSFETs with m-face (1-100) channel regions. It was found that μeff degraded with an increase in NA. On the other hand, the increase of NA enlarged the Vth instability. These results indicate that NA has a large impact not only on the Vth value but also on the channel resistance and reliability in 4H-SiC trench MOSFETs.