A semiconductor device 1 includes a plurality of amplifier circuits 2 connected in parallel between an input terminal RFin and an output terminal RFout. Each of the amplifier circuits 2 includes an HBT 3, an oscillation stabilizing circuit 4 connected between the input terminal RFin and a base B of the HBT 3, and a ballast resistor 5 connected between a bias terminal Bin and the base B of the HBT 3. The oscillation stabilizing circuit 4 includes a resistor 6 and a capacitor 7 connected in parallel. Thus, thermorunaway of the HBT 3 can be prevented using the ballast resistor 5, and the stability against oscillation can be improved using the oscillation stabilizing circuit 4 even at low frequencies.