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- PostDesign, fabrication and measurements of planar Goubau lines from 0.75 THz to 1.1 THz(2017) Cabello Sanchez, Juan; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceTerahertz (THz) waves have proven to possess a unique interaction with biomolecules, and therefore it’s interesting to study its influence using biosensors. Recent advances in heterodyne technology allow measuring on-wafer scattering parameters with Vector Network Analysers (VNA) up to 1.1 THz, which could be used for THz molecular spectroscopy. Compared to THz-Time Domain Spectroscopy (THz-TDS), this technique is expected to have greater sensitivity and dynamic range which would open new possibilities for near-field biomolecular spectroscopy. One possible set-up for near-field biosensing is using an on-wafer biosensor, which integrates microfluidic channels for the samples and electrical devices for the sensing. The Planar Goubau Line (PGL), a single conductor waveguide, has suitable properties for near-field on- wafer sensing since its geometry allows a correct sample deposition and a substantial part of the field travels on top of the substrate, where the samples will be located, thus increasing sensitivity. In this thesis, Planar Goubau Lines (PGL) have been designed, fabricated and measured for frequencies from 0.75THz to 1.1THz to be integrated as the electrical structure in a future on-wafer THz biosensor. Additionally, a design method for the layout of the Coplanar Waveguide (CPW) to PGL transition, needed for Ground-Signal-Ground (GSG) probe excitation, was developed to minimise reflections. Electromagnetic simulations were used for the design and analysis of the structures, and different software and port excitations were compared to achieve an accurate simulation environment. The importance of the choice of the substrate’s properties is investigated to increase the field on top of the substrate, reduce losses and limit the excitation of substrate modes. The fabrication process is described together with the VNA on-wafer S-parameter measurement set-up. To obtain conclusions for future optimisation of the structures, the fabricated structures were characterised, and different PGL widths and several CPW-PGL transitions were compared between 0.75 THz and 1.1 THz. In this frequency range, losses showed to be on average 5dB/mm and 13.3dB/mm for CPW and PGL, respectively. Despite the high losses per unit length happening at THz frequencies, the small size of the microfluidic channels allows its application in biosensing. The thesis shows that the transmission could be improved by eliminating the carrier wafer, limiting the use of the PGL to the sensing areas and shortening the CPW- PGL transitions. These guidelines would increase the signal-to-noise ratio for the sensor, increasing its potential to analyse THz-biomolecule interaction.
- PostFull 3D Modeling of Waveguide-Embedded Frequency Multiplier Arrays(2016) Hrubó, Gergely; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe frequency region between 300 GHz - 10 THz has long been in interest of many scientific and industrial branches. However, the traditional ways to provide continuous wave (CW) signals are inapplicable at these frequencies due to fundamental limitations. There have been many approaches to bridge the lack of sources, commonly referred as the THz gap, both from the electronics and photonics side. To overcome the input power limitation of solid state electronics, quasi-optical grid arrays have been presented in order to spatially combine contributions from a large number of active devices. Originally they operate in free space environment, but to gain a compact component for i.e. satellite applications, the device can be enclosed in a waveguide. The interaction between the large number of nonlinear devices and the confined field in a waveguide cavity poses great modeling challenges. The thesis is investigating different modeling techniques for waveguide embedded varactor frequency multiplier arrays. A Heterostructure Barrier Varactor (HBV) frequency tripler array operating at 250 GHz output frequency was experimentally tested and modeled with the traditional unit cell method and a novel approach using full 3D electromagnetic simulation. The full 3D model follows the abrupt changes of the measured results both as a function of frequency and input power due to the interaction between all the nonlinear elements. Furthermore, it enables the quantitative characterisation of individual diodes and provides knowledge of the total field distribution. In the evaluated example, higher order mode excitation was found due to the interaction between the filter and antenna array. An analysis to account for diode failures using the full 3D model is presented. Due to the results obtained from the full 3D model, a new output filter was designed for the device. By matching the filter’s periodicity to the antenna array, simulations show a decreased higher order mode excitation and therefore an increase of 1 dB in peak output power. RF measurements on the fabricated filter are reported, showing an increased output power at the lower edge of the measured band with a maximum increase of 0.8 dB at 243 GHz. Altogether, the full 3D model provides a useful complementary to the present tools for waveguide-embedded grid array design. Taking into account the resource needs of the two models, the unit cell model is recommended for a quick design synthesis and the full 3D model can be employed for further improvements.
- PostThe Impact of GaN-buffer C-doping Profile on Short-Channel Effects and Ohmic Contact Formation for Millimeter Wave III-Nitride HEMTs(2015) Simpanen, Ewa; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap; Chalmers University of Technology / Department of Microtechnology and NanoscienceThe InAlN/AlN/GaN HEMT has a high breakdown field which allows for excellent power performance. In order to increase frequency operation, a reduction in device size is required, inherently increasing the short-channel effects and limiting device efficiency and linearity. It is possible to mitigate DIBL and subthreshold swing by increasing the carrier confinement in the 2DEG channel of the HEMT through optimizing the buffer design with the use of GaN-buffer C-doping. The impact of different C-doping profiles on short-channel effects is investigated by the fabrication and characterization of two-finger U-shaped HEMTs with 2 x 25 μm gate widths and 50, 100 and 180 nm gate footprints. DC and S-parameter analysis show an increased immunity to short-channel effects for devices with constant levels of intentional C-doping to the buffer, but that the high frequency and power performance is impaired by a reduction of the on-state conductance. Best RF performance was achieved for a ramped C-doping profile with values of 𝑓𝑇 and 𝑓𝑚𝑎𝑥 of 66 and 180 GHz respectively. Although, these devices showed a deteriorated performance in terms of short-channel effects, including pinch-off, output conductance and current leakage. Consequently, there is a trade-off between the reduction of short-channel effects and an increase of 𝑓𝑇 and 𝑓𝑚𝑎𝑥. Ohmic contact formation and the reduction of contact resistance and specific contact resistivity is important to consider in order to increase the efficient operation of HEMTs. Ohmic contacts induce negligible voltage drop and heat generation compared to the bulk resistance of the semiconductor, but are difficult to realize in wide-bandgap materials. It is possible to assist the formation of ohmic contacts and lower the resistivity by contact recess etching down to the substrate channel and by altering the metallization. Ta-based contacts are considered for their low optimum anneal temperature, improving the sheet resistance of the heterostructure material. Ta/Al/Ta contacts to AlGaN/AlN/GaN heterostructures have been found to be reproducible and to have good performance with concern to contact resistance, specific contact resistivity, surface morphology and line edge acuity. The exclusion layer formed in between the contacts and the semiconductor during anneal, causes N-vacancies in the buffer and promotes tunnelling of carriers through the barrier, which in turn lowers the contact resistance. The lower resistivity of ZrN compared to TaN, yields an interest in Zr-based contacts. Zr/Al/Zr/Ta/Au contacts to AlGaN/AlN/GaN show low optimum anneal temperature and contact resistance but further research is required.