PURPOSE: To obtain a high speed switching seiconductor element by a method wherein a capacitor nearly the same or more with input transition capacity of the element is connected in parallel with the circuit of the emitter and the base of the semiconductor element.

CONSTITUTION: When the capacitor of the extent of 0.8W3 times of input transition capacity of the power transistor 1 is connected in parallel with the circuit of the emitter and the base of the transistor thereof, a spike and ringing can be controlled without reducing efficiency to enable to suppress generation of noise, and moreover destruction of the transistor 1 itself also can be checked. When a switching electric power source is constructed using the transistor formed according to this construction, an outside circuit for shaping of a waveform is also made unnecessary, the device can be made in a small type and in light weight, and the high speed switching device of high efficiency and low noise can be obtained.