In one general aspect, an apparatus can include a substrate having a semiconductor region, and a trench defined in the semiconductor region and having a sidewall. The apparatus can include a shield electrode disposed in the trench and insulated from the sidewall of the trench by a shield dielectric, the shield dielectric having a low-k dielectric portion and a high-k dielectric portion. The apparatus can include a gate electrode disposed in the trench and at least partially surrounded by a gate dielectric, and an inter-electrode dielectric disposed between the shield electrode and the gate electrode.