The present application discloses a semiconductor device comprising a source region and a drain region in an ultra-thin semiconductor layer; a channel region between the source region and the drain region in the ultra-thin semiconductor layer; a front gate stack above the channel region, the front gate comprising a front gate and a front gate dielectric between the front gate and the channel region; and a back gate stack below the channel region, the back gate stack comprising a back gate and a back gate dielectric between the back gate and the channel region, wherein the front gate is made of a high-Vt material, and the back gate is made of a low-Vt material. According to another embodiment, the front gate and the back gate are made of the same material, and the back gate is applied with a forward bias voltage during operation. The semiconductor device alleviates threshold voltage fluctuation due to varied thickness of the channel region by means of the back gate.