We investigated the dependence of tunnel magnetoresistance (TMR) and its electric-field effect in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) on different underlayer (UL) materials. We observed enhancements in the TMR ratio and its temperature dependence as well as electric-field effect in MTJs with W UL, as compared to those in MTJs with a conventional Ta UL. This is attributed to better thermal stability of perpendicular magnetic anisotropy (PMA) in W/CoFeB/MgO, which sustains up to 380 degrees C, compared to that of Ta/CoFeB/MgO which starts to degrade at 270 degrees C. This demonstrates that the PMA in a CoFeB/MgO structure and its electric-field dependence can be enhanced by the careful selection of underlayer, opening the way for the realization of electric-field effect-driven spintronic devices. (C) 2017 Elsevier B. V. All rights reserved.