A semiconductor device includes a heterojunction semiconductor region (9), which forms a heterojunction with a drain region (2). The heterojunction semiconductor region (9) is connected to a source electrode (7), and has a band gap different from a band gap of a semiconductor substrate constituting the drain region (2). It is possible to set the size of an energy barrier against conduction electrons, which is formed between the drain region (2) and the heterojunction semiconductor region (9), into a desired size by changing the conductivity type or the impurity density of the heterojunction semiconductor region (9). This is a characteristic not found in a Schottky junction, in which the size of the energy barrier is inherently determined by a work function of a metal material. It is easy to achieve optimal design of a passive element in response to a withstand voltage system of a MOSFET as a switching element. It is also possible to suppress diffusion potential in a reverse conduction mode and to improve a degree of integration per unit area. As a result, it is possible to reduce the size of elements and to simplify manufacturing processes thereof. A semiconductor device having a Schottky junction region (10) is moreover disclosed.