A semiconductor device includes a substrate, a first transistor on the substrate, and a second transistor on the substrate. The first transistor has a first threshold voltage, and a channel region and source/drain regions of the first transistor are N-type. The second transistor has a second threshold voltage, a channel region of the second transistor is N-type and source/drain regions of the second transistor are P-type, and an absolute value of the first threshold voltage is substantially equal to an absolute value of the second threshold voltage.