A semiconductor device is provided, which includes a substrate, a gate and a gate contact. The substrate has a well region, which has a source, a drain and a channel region extending between the source and the drain. The gate is on the well region and extends across the channel region. The gate contact is directly on the gate and vertically overlaps with the channel region. The gate contact has a strip shape of which a ratio of a length to a width is at least 2. The gate contact includes a gate conductive plug and a gate contact dielectric. The gate conductive plug directly contacts the gate. The gate contact dielectric surrounds side surfaces of the gate conductive plug and has a frame shape. A dielectric constant of the gate contact dielectric is substantially greater than 4.9.