The disclosure provides a semiconductor device and a fabrication method thereof. The semiconductor device includes a substrate, a first and a second polysilicon layers on the substrate, a third polysilicon layer between the first and the second polysilicon layers, a first isolation layer adjacent with the first to the third polysilicon layers, a gate dielectric layer and a gate conductive layer in the third polysilicon layer, a second isolation layer on the gate conductive layer and the third polysilicon layer, a third isolation layer on the first the second isolation layers, a bit line via contact through the first and the third isolation layers, and a conductive layer on the bit line via contact and the third isolation layer. The third polysilicon layer has a concave portion between the first and the second polysilicon layers.