In an embodiment of this invention, if a state, in which duty ratio of a PWM signal is kept equal to or greater than a predetermined value, has been maintained for a first time period, the duty ratio of the PWM signal of a digital power amplifier is adjusted downward. The heating plate is heated by a heating liquid heated by a thermostatically controlled electrical heating element within the second chamber. The collector and emitter structure are preferably superlattices each comprising a plurality of periods of InAs and AlSb sublayers. The second layer includes the first metal but less oxide of the first metal than the first layer.