A MOS capacitor includes a substrate having a capacitor forming region thereon, an ion well having a first conductivity type in the substrate, a counter doping region having a second conductivity type in the ion well within the capacitor forming region, a capacitor dielectric layer on the ion well within the capacitor forming region, a gate electrode on the capacitor dielectric layer, a source doping region having the second conductivity type on a first side of the gate electrode within the capacitor forming region, and a drain doping region having the second conductivity type on a second side of the gate electrode within the capacitor forming region.