A device includes a semiconductor substrate, a well region in the semiconductor substrate, and a Metal-Oxide-Semiconductor (MOS) device. The MOS device includes a gate dielectric overlapping the well region, a gate electrode over the gate dielectric, and a source/drain region in the well region. The source/drain region and the well region are of opposite conductivity types. An edge of the first source drain region facing away from the gate electrode is in contact with the well region to form a junction isolation.