A semiconductor device includes a substrate, a pair of transistor devices and an isolation region. The pair of transistor devices are disposed over the substrate. Each of the pair of the transistor devices includes a channel, a gate electrode over the channel, and a source/drain region alongside the gate electrode. The isolation region is disposed between the source/drain regions of the pair of the transistor devices. The isolation region has a first doping type opposite to a second doping type of the source/drain regions.