A
solid state
relay composed of a series connected pair of LDMOSFETs has a minimized output
capacitance. Each LDMOSFET is configured to have a
silicon layer of a first conductive type, a drain region of the first conductive type diffused in the top surface of the
silicon layer, a well region of a second conductive type diffused in the
silicon layer in a laterally spaced relation from the drain region, and a source region of the first conductive type diffused within the well region to define a channel extending between the source region and a confronting edge of the well region along the top surface of the silicon layer. Each LDMOSFET is of an SOI (
Silicon-On-Insulator) structure composed of a silicon substrate placed on a supporting plate, a
buried oxide layer on the silicon substrate, and the silicon layer on the
buried oxide layer. The well region is diffused over the full depth of the silicon layer to have its bottom in contact with the
buried oxide layer, so that the well region forms with the silicon layer a P-N interface only at a small area adjacent the channel. Because of this reduced P-N interface and also because of the buried
oxide layer exhibiting a much lower inductive
capacitance than the silicon layer, it is possible to greatly reduce a drain-source
capacitance for minimizing the output capacitance of the
relay in the non-conductive condition.