Advanced concepts of THz resonant-tunnelling-diode oscillators

Abstract

One of the topics the thesis is focused on is the concept of chip-sized double-resonant-tunneling diode (RTD) patch antenna oscillators capable of operating in the sub-THz and THz frequency regions. The concept offers the advantages of simplicity, compactness, high isolation of the oscillator from the external biasing circuitry, and substantial output powers comparable with the output powers of more complex chip-sized oscillators. Implementing this concept into a practical design with 1.6 nm barrier RTDs resulted in an output power of 10 μW at the fundamental frequency of 525 GHz and 70 μW at 330 GHz. These results represent an order-of-magnitude increase in the output power compared to previous reports on patch-antenna RTD oscillators. In addition, the oscillators were fabricated using only optical lithography. Further, from the analysis of the designed oscillators' limiting factors, the hindering mechanisms for achieving higher frequencies were identified in the inductance of the slant bridges connecting the RTDs and the patch antenna and in the parasitics of the used RTD. Therefore, conical vias replacing the slant bridges and 1.0 nm barrier RTDs were used in the design's next iteration, reducing the influences of those parasitics. These changes resulted in the oscillator operating with an output power of 9 μW at the fundamental frequency of 1.09 THz, 15 μW at 0.98 THz, and up to 27 μW at the lower frequencies of 620-660 GHz. These parameters are close to the state-of-the-art level for all other types of RTD oscillators at around 1 THz. We further demonstrate that there is much room for further improvement of the parameters of these oscillators. The constructed oscillators were used as sources for sub-THz FMCW radar and optical coherent tomography. These two examples represent potential real-life applications of RTD oscillators. For these applications, an RTD oscillator operating at 680 GHz, with a bandwidth of 38 GHz, and output power of 23 μW provided a spatial resolution of 4 mm. The frequency tuning of the RTD oscillator was done by the sweep of its bias voltage. The shape of the sweep needed to be corrected to increase the system's accuracy. Next, we show that the theoretical maximum output power of a simple RTD oscillator is fundamentally limited by the radiation conductance of its antenna and by a maximum RTD voltage swing for which the RTD still shows negative conductance. As such, this maximum output power marks an upper bound for the RTD oscillators. The RTD current peak density and the RTD capacitance then influence how close we can get to this bound. The value of the upper bound gives us a tool for comparing the theoretical performance of different antennas. Lastly, the thesis presents a model for analyzing dynamic large-signal characteristics of double-barrier RTDs. The model is based on analyzing dynamical trajectories in phase space, defined by the RTD bias and electron density in the RTD quantum well. We demonstrate that the large-signal admittance of RTDs can be accurately represented with a simple equivalent circuit composed of a capacitor, inductor, and two resistors (RLRC). These circuit elements correspond to large-signal relaxation time, geometrical RTD capacitance, and low- and high-frequency resistors. The same structure of the circuit was previously derived to model small-signal RTD admittance; however, in the large-signal case, the model's parameters depend on the AC-signal amplitude. Further, we show that the large-signal RTD relaxation time can be shorter or longer than the small-signal one. For the RTD oscillators, a shorter RTD relaxation time increases the gain of the RTD at high frequencies, which allows one to get higher output power at these frequencies. The availability of an accurate, general, but relatively simple, physics-based model for analyzing large-signal RTD dynamics removes one of the main obstacles to the further improvement of sub-THz and THz RTD oscillators. This model allows one to drop the quasi-static approximation of the RTD parameters in the large-signal analysis of the RTD oscillators. We also show that the model's parameters can be estimated, relying only on a directly measurable DC I-V curve and on a few other RTD parameters, which could be easily estimated with simple DC calculations.