A method includes growing an epitaxy semiconductor layer over a semiconductor substrate. The epitaxy semiconductor layer is of a first conductivity type. A Lateral Insulated Gate Bipolar Transistor (LIGBT) is formed at a front surface of the epitaxy semiconductor layer. After the LIGBT is formed, a backside thinning is performed to remove the semiconductor substrate. An implantation is performed from a backside of the epitaxy semiconductor layer to form a heavily doped semiconductor layer. The heavily doped semiconductor layer is of a second conductivity type opposite the first conductivity type.