Abstract: A semiconductor integrated circuit device having an SOI structure is capable of preventing occurrence of leak current flowing from a diffusion layer even when a semiconductor element having a pn-junction is included in the semiconductor substrate. The semiconductor integrated circuit device having the SOI structure is formed with a semiconductor layer, or SOI layer, on a p-type semiconductor substrate through a buried insulating film and further with semiconductor circuit elements serving as functional elements at the SOI layer thus formed. As a protection transistor to protect the semiconductor circuit elements, a MOSFET may be formed in which n-type diffusion layers are formed in the semiconductor substrate. The n-type diffusion layers of the MOSFET are to be surrounded by p-type diffusion layers more highly doped than the semiconductor substrate.

Abstract: In a semiconductor integrated circuit device having an input protection circuit element such as a diode formed in the semiconductor substrate, the leak current is suppressed. An nMOS transistor and a pMOS transistor that constitute a CMOS inverter circuit are formed using a SOI structure. An n-type diffusion layer and p-type diffusion layer are formed within the semiconductor substrate to thereby construct a protective diode that forms an input protection circuit for the CMOS inverter circuit. By surrounding the outer periphery of the n-type diffusion layer with the p-type diffusion layer, the depletion layer that is formed at an interface between the semiconductor substrate and a buried insulation film therein is cut off by the p-type diffusion layer, thereby suppressing the leak current between the n-type diffusion layer and the p-type diffusion layer.

Abstract: A semiconductor device of SOI structure formed by the mesa isolation method, which can sufficiently reduce the wiring capacitance even if the width of the isolation trench is large. An SOI layer which constitutes an element region is formed by forming a buried oxide film in a silicon substrate, forming an isolation trench in the buried oxide film and burying an isolating material in the isolation trench. By the formation of the SOI layer with the isolating material, a dummy SOI layer is formed in a field part other than the element region. Then, by the formation of a MOSFET gate wiring on the dummy SOI layer, the wiring capacitance is reduced. Furthermore, the dummy SOI layer is completely depleted when the MOSFET threshold value is applied to the gate wiring.