a semiconductor device having an improved voltage control oscillator circuit is provided . the voltage control oscillator circuit includes , in combination , a variable - capacitance element and at least one bipolar transistor on a single semiconductor substrate . the variable - capacitance element includes reversely serially connected pn junctions , and junctions are formed by a single common collector layer and separated base layers on the common collector layer . the capacitance of the variable - capacitance element is generated between respective base layers of the pn junctions with the common collector layer , and varies in correspondence with the voltage applied to the common collector layer .