integrated power semiconductor devices having improved high frequency switching performance , improved edge termination characteristics and reduced on - state resistance include gd - umosfet unit cells with upper trench - based gate electrodes and lower trench - based source electrodes . the use of the trench - based source electrode instead of a larger gate electrode reduces the gate - to - drain capacitance of the umosfet and improves switching speed by reducing the amount of gate charging and discharging current that is needed during high frequency operation . methods according to the present invention also include the steps of forming a trench in a semiconductor substrate and then forming an insulated source electrode in and adjacent a bottom of the trench by forming a first electrically insulating layer on a bottom and sidewall of the trench , then forming a first electrically conductive layer on the first electrically insulating layer and then forming a second electrically insulating layer on the first electrically conductive layer using a thermal oxidation technique , for example . an insulated gate electrode is then formed in and adjacent a top of trench . steps are also provided to form a source region of first conductivity type in the semiconductor substrate and form a second source electrode electrically connected to the source region and the insulated source electrode .