Abstract: A novel thin drift region device is proposed based on the electric field modulation in the substrate with thin epitaxial layer, which is called SB LDMOS (LDMOS with substrate bias). The SB LDMOS is characterized by infusing N<sup<+</sup< layer into the back of P type substrate to improve the breakdown voltage of the device. The mechanism of improving breakdown voltage could be considered that the high electric field under the drain is reduced by positive substrate bias, which causes the re-distribution of the bulk electric field in the drift region and the drain voltage is sustained by two PN junctions under the drain and the source. By solving 2-D Poisson's equation of potential in the drift region, the analytical solutions of breakdown voltage and surface electric field are obtained, from which the optimal relation between structure parameters could also be fixed. Numerical results indicate that the breakdown voltage of proposed device is increased by 63% in comparison to the LDMOS with P buried layer.