a semiconductor device characterized by a field limiting ring formed by a number of field limiting cells that define wells which are laterally diffused to form a continuous equipotential ring between interior and exterior regions of a semiconductor device . a number of active cells are formed in the interior region , and are therefore delineated from the exterior region of the device . each of these active cells is a transistor , and preferably a field - effect transistor , whose structure is essentially identical to the field limiting cells , except that their wells are not merged but instead are isolated from each other . the field limiting ring increases the breakdown voltage and the ruggedness of device , and therefore enables the device to sustain high voltages when the device is in the off - state . the process does not require masking , implanting and diffusion steps for the sole purpose of forming the field limiting ring , but is instead fully integrated with the semiconductor process for forming the active cells . the field limiting cells also contribute to forward current conduction when the device is in the on - state , thereby lowering the on - resistance of the device .