Semiconductor device and electrical circuit device using thereof

+−++−A UMOSFET is capable of reducing a threshold voltage and producing a large saturation current. A typical UMOSFET according to the present invention includes: an N type SiC substrate constituting a drain layer; an N type SiC layer that is in contact with the drain layer and constitutes a drift layer; a P type body layer formed on the drift layer and being a semiconductor layer; an N type SiC layer constituting a source layer; a trench extending from the source layer to a predetermined location placed in the drift layer; a P type electric field relaxation region provided around and outside a bottom portion of the trench; and a channel region extending from the N type source layer to the P type electric field relaxation region and having an impurity concentration higher than that of the N type drift layer and lower than that of the P type body layer.