Figure-of-Merit for Laterally Diffused MOSFETs with Rectangular and Semi-Circular Field Oxides

We derive an analytical relation between on-resistance and breakdown voltage for LDMOS devices with different Field-OXide (FOX) configurations. We consider two FOX shapes: rectangular and semi-circular. We solve the Poisson equation in Cartesian and polar coordinates to deduce the electric field and potential profiles for the rectangular case and the semi-circular case. We use a Taylor expansion approximation to simplify the on-resistance equation for the semi-circular case. We calculate the on-resistance under doping dependent electron mobility assumptions. We find and optimum value for drift doping concentration which minimizes the on-resistance at each breakdown voltage. We provide a range for the drift doping concentration in which LDMOS transistors with semi-circular FOX outperform LDMOS transistors with rectangular FOX. Finally, we validate our analytical findings with TCAD simulations.