The Bent Coplanar Waveguide at Sub-Terahertz Frequencies

Recent advancements in the field of high speed electronics put increasing pressure on the development of active devices that can operate at increasingly faster rates. However, at the frequency range of several GHz and above, loss and dispersion in the interconnecting transmission lines are dramatically enhanced. The elaborate characterization of such structures then becomes instrumental in the overall performance of ultrafast circuits. This thesis focuses on this aspect and presents a broad-band experimental characterization of coplanar transmission lines that extends to the THz range. Our samples were fabricated on semi-insulating GaAs substrates using a liftoff process. Both coplanar waveguides and coplanar strips with different dimensions were fabricated. Various straight and bent lines with different geometries were included. The transmission lines were excited by picosecond electrical transients generated by the illumination of photoconductive switches embedded in the lines. The time evolution of these transients was recorded by a subpicosecond electro-optic sampling system. Subsequent analysis of the experimental results was initially carried out in the time domain. With the development of a suitable pulse generation scheme and data processing algorithms, the analysis was also extended to the frequency domain. It included a study of the various transmission lines which incorporated the effects of lateral dimensions, ground plane width, bends, and wirebonds together with the evolution of the CPW even mode. When the transmission lines were excited by steplike electrical transients, our time-domain studies were based on the study of propagation velocity and the evolution of the signal's rise time. With the implementation of picosecond pulses the analysis was also extended to the qualitative comparison of loss and dispersion in the lines. Quantitative results on the effective dielectric permittivity, dispersion, and attenuation were extracted by frequency domain analysis that extended to frequencies up to 1 THz. With these results we provide a complete high-frequency characterization of the coplanar transmission lines.