SEMICONDUCTOR DEVICE

PURPOSE:To obtain a switching element which can operate at a high speed by connecting an MOSFET and a bipolar transistor in parallel, and connecting the gate of the MOSFET through a diode with the base of the bipolar transistor. CONSTITUTION:When an input pulse voltage from a drive circuit 1 exceeds the sum of a breakdown voltage of a Zener diode 10, a forward voltage drop of a high speed switching diode 11 and a forward voltage drop between B and E of a bipolar transistor Tr 50, the input pulse voltage is applied through diodes 10, 11 to become a base current to the base 28 of the Tr 50. The base current almost flows between a P-type base layer 23 and an N type emitter layer 24 in the Tr 50, saturated between B and E, and the Tr 50 is turned ON in a bipolar operation. At this time, since the MOSFET 40 is already turned ON, a voltage between C/D terminal and E/S terminal is low, and the base current flows between the B and C of the bipolar Tr is considerably smaller than the normal case, or may not flow at all. Accordingly, the Tr 50 is quasi- saturated or switched in an active range to enable a high speed switching operation.