A device includes a semiconductor region in a semiconductor chip, a gate dielectric layer over the semiconductor region, and a gate electrode over the gate dielectric. A drain region is disposed at a top surface of the semiconductor region and adjacent to the gate electrode. A gate spacer is on a sidewall of the gate electrode. A dielectric layer is disposed over the gate electrode and the gate spacer. A conductive field plate is over the dielectric layer, wherein the conductive field plate has a portion on a drain side of the gate electrode. A deep metal via is disposed in the semiconductor region. A source electrode is underlying the semiconductor region, wherein the source electrode is electrically shorted to the conductive field plate through the deep metal via.