Limitations of Output Power and Efficiency of Simple Resonant-Tunneling-Diode Oscillators

We show that the maximum output power of a simple resonant-tunneling-diode (RTD) oscillator is fundamentally limited by the radiation conductance of its antenna and by a maximum RTD voltage swing. For stand-alone RTD oscillators with common simple antenna types, this power level most likely cannot exceed 1 mW at sub-THz and THz frequencies. The RTD current density, conductance, and capacitance have no direct influence on the maximum power level but those RTD parameters determine how close one can get to the maximum. We show that the maxima of the output power and dc-to-RF conversion efficiency are achieved for different sets of the parameters of RTD oscillators and not simultaneously. We demonstrate slot-antenna RTD oscillators with up to 0.5-mW output power at ${\approx \!100}\,$ GHz and with 5 ${\%}$ conversion efficiency, which could be increased further up to 8 ${\%}$ , although at the expense of reduced output power. Composite RTD oscillators with separate resonators and radiators can possibly overcome the limitations of simple oscillators.