Optimization of sub-THz and THz resonant-tunneling-diode oscillators with symmetrical slot antennas

Abstract

The first topic of the thesis is focused on the analysis of how the oscillation frequency limitation of the symmetrical slot-antenna RTD oscillators depends on the parasitic elements of the circuit, especially on the contact resistance and the spreading resistance. In the high frequency regime, the RTD areas are quite small, and the top contact resistance has a large impact; however, the spreading resistance plays a small contribution on the total parasitic resistance. The spreading resistance is dominant for large RTD areas corresponding to the low-frequency regime of the RTD oscillators. For the simple analysis of maximizing the oscillation frequencies of RTD oscillators, by adjusting the effects of these parasitic elements, one can estimate the parameters of the RTD oscillators, such as the length and the width of the slot antenna, and the RTD-mesa area in order to achieve the maximum oscillation frequencies.Further, the thesis shows that the output power of symmetrical-slot-antenna RTD oscillators depends on the slot-antenna width. Although the dependence is not very strong for wider slots, it is essential for fine tuning the oscillator performance. In this work, we present an extensive set of experimental data on RTD oscillators with different slot widths. The data reasonably agree with the theoretical analysis and support its validity. Some oscillators exhibit the output-power level close to the state-of-the-art for fundamental-frequency symmetrical slot-antenna RTD oscillators in the frequency range of 150-400 GHz. Specifically, we report 283 μWat 184 GHz and 82 μW at 368 GHz. The RTD we used in this work has 1.6 nm thick barrier with the peak current density is only ∼ 5.8 mA/μm2. In addition, the RTD oscillators were fabricated using only optical lithography. Lastly, the thesis presents an island slot-antenna RTD oscillator, where the contact n++ layer is removed everywhere except for a small island under the RTD. At frequencies around 2 THz, this design leads to a significant reduction (by a factor of ≈ 2) of the total ohmic losses at the conducting surfaces of the slot antenna. With this design, we demonstrate the highest radiated power for RTD oscillators in the frequency range 1.6-1.74 THz with around 2.2 μW at the fundamental frequency of 1.74 THz. The RTD used in this work has 1.0 nm thick barrier, and its peak current density is 23.6 mA/ μm2. We further demonstrate that the oscillation frequency above 2 THz should be reached if the RTD contact resistance is reduced to 1-1.5 Ωμm2.