A semiconductor device includes a first metal-oxide semiconductor (MOS) transistor on a first substrate, a first interlayer dielectric (ILD) layer on the first MOS transistor, a second substrate on the first ILD layer, and a second MOS transistor on a second substrate. Preferably, the semiconductor device includes a static random access memory (SRAM) and the SRAM includes a first pull-up device, a second pull-up device, a first pull-down device, a second pull-down device, a first pass-gate device, a second pass-gate device, a read port pull-down device, and a read port pass-gate device, in which the read port pull-down device includes the first MOS transistor and the read port pass-gate device includes the second MOS transistor.