The parametric subharmonic oscillator operates by energy transfer from the pump frequency to the oscillator frequency through a nonlinear energy storage element-in the present case, the nonlinear capacitance of a semiconductor diode. This paper examines both the requirements on the diode for satisfactory performance in this circuit and the limitations on oscillator performance which arise from the nature of the semiconductor diode. The analysis shows that abrupt junction diodes must have a Q of at least four at the oscillation frequency if there is to be any useable energy transfer, and that graded junction diodes must have a Q of six. The time constant governing the rise of the envelope of the subharmonic waveform is a marked function of the stray capacitances; this function is examined in detail. The choice of bias voltage to obtain the fastest possible rise time involves consideration of the stray capacitance, the Q of the available diode, and limitations imposed by excessive pump power requirements. For negligible stray capacitance, it is shown that the subharmonic wave-form can rise by a factor e in 1.3 cycles of the subharmonic frequency for an abrupt junction diode, or in 1.9 cycles for a graded junction diode. The principles involved in the design of the semiconductor diode are examined and the choice of materials, impurity distributions, and fabrication techniques are discussed. A new diode encapsulation intended for direct mounting in microstrip transmission line is described. Measurement of appropriate diode parameters is vital to diode research.