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

Abstract

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 ≈ 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.

Impact of Slot Width on Performance of Symmetrical-Slot-Antenna Resonant-Tunneling-Diode Oscillators

“…2; (ii) we use a narrow-slot approximation, i.e., we assume that the slot-antenna radiation conductance and antenna loss resistance are independent of the slot width and determined only by the slot length; we also neglect the airbridge inductance compared to that of the slot antenna. At the lower side of the sub-THz frequency range, those are often accurate approximations [11]; in the middle of the sub-THz frequency range, these approximations have a semiquantitative character; at THz frequencies, the model is useful for a qualitative discussion, since parasitics become important, and the slot usually becomes quite short, i.e., it cannot be seen as a narrow one anymore.…”

“…When G p is neglected, the factor |Y circ /Y a | is equal to 1, and it disappears in (4). Then, the output power of an RTD oscillator (4) can be represented [11] as:…”

“…THz frequency range is expected to be important for many applications [1], but the lack of sufficiently-simple solidstate room-temperature radiation sources hinders its practical use. Among available technologies, resonant-tunneling-diode (RTD) oscillators stand out as one of the most promising for practical use since sub-THz and THz RTD oscillators are very simple [2], [3], compact [4], they work at room temperature, cover the frequencies up to almost 2 THz [5], [6], can provide relatively high output power radiated into free space (several hundred µW by stand-alone oscillators and up to ≈ 10 mW at 0.45 THz in coherent-array configuration) [7]- [10], have high DC-to-RF conversion efficiency of up to 1-5 % [7], [9], [11], etc. A number of potential applications were also demonstrated with RTDs: wireless communication [12], [13], imaging [14], spectrometer [15], radars [16], [17], etc.…”

“…In this respect, other types of oscillators (e.g., patchantenna RTD oscillators) are more compact since they emit radiation directly into free space upwards, from the ground plane of the chip. In terms of their output power, both the slotand patch-antenna oscillators perform, in principle, similarly [11].…”

“…The slot length is chosen according to the target oscillator frequency: the higher the frequency, the shorter the slot. On the other hand, the slot width is typically chosen to be equal to 4 µm in the sub-THz and THz RTD oscillators [5], [11], [23]- [26]. Usually, the slot width is not used as a degree of freedom in the optimization of the oscillators.…”