In this study, we report the optimisation of ohmic contact formation on AlGaN/GaN on low resistivity silicon. To achieve this, a strategy of uneven AlGaN / GaN was introduced through patterned etching of the substrate under the contact. Various pattern designs (holes, horizontal lines, vertical lines, grid) and varied etch depth (above and below the 2-dimensional electron gas) were investigated. Further, a study of planar and non-planar ohmic metallisation was investigated. Compared to a traditional fabrication strategy, we observed a reduced contact resistance from 0.35 Ω.mm to 0.27 Ω.mm by employing a grid etching approach with a "below channel" etch depth and non-planar ohmic metallisation. In general, measurements of "below channel" test structures exhibited improved contact resistance compared to "above channel" in both planar and non-planar ohmic metallisation.

Abstract-In this work a novel ultra-low loss transmission media for RF GaN-on-low-resistivity silicon (LR-Si) substrates (σ < 40 Ω.cm) has been successfully demonstrated. The developed shielded-microstrip lines achieve comparable performance to those on semi-insulating (SI) GaAs substrates with transmission loss of 0.9 dB/mm for frequencies up to 67 GHz. Line performance was further enhanced by additional elevation of the shielded-microstrip lines using air-bridge technology above a 5 µm layer of benzocyclobutene (BCB) on shielded metalized ground planes. Transmission loss of 0.6 dB/mm for frequencies up to 67 GHz was obtained as a result of the extra elevation. Structure parameters were designed and optimized based on EM simulation for best performance. The work shows that the RF energy coupled into the substrate was eliminated, indicating the suitability of III-V-on-LR Si technology for millimeter-wave applications.Index Terms-GaN-based HEMTs, low-resistivity silicon substrates, Microstrip lines, benzocyclobutene (BCB), Millimeter-wave.

Abstract-In this paper we demonstrate various configurations of THz microstrip antenna on GaN-on low resistivity silicon substrates (p < 40 O.cm). To reduce the losses caused by the substrate and to enhance the antenna performance, the driven patch is shielded by a ground plane and silicon nitride, with BeB as the inset layer between them. Second patch (elevated patch) is suspended in air using gold posts, which makes the design stack configuration. Here, study of various design performances has been represented by changing the shape of the antenna between rectangular and circular, optimising the BeB and stack height and evaluating performance of stack using air and BeB as dielectric. Better fabricated performance was obtained when the patch was elevated in air and by using rectangular-circular stack configuration with BeB and elevation height of 5 !-lm. 3D EM model showed directivity, gain, and radiation efficiency as high as 8.3 dB, 3.4 dB, and 32 % respectively, a significant improvement over single or stack configuration antenna. Better simulated gain (6.7 dB) was obtained with the BeB height of 30 !-lm using a single antenna and highest gain and directivity (7.5 dB and 8.8 dB respectively) for stack antenna of height 15!-lm .To the authors' knowledge this is the first time such a study has been carried out at Terahertz frequency and this developed technology is suitable for high performance III-V material on low resistivity/ high dielectric substrates.

Abstract-In this paper we demonstrate a THz microstrip stack antenna on GaN-on-low resistivity silicon substrates (ρ < 40 Ω.cm). To reduce losses caused by the substrate and to enhance performance of the integrated antenna at THz frequencies, the driven patch is shielded by silicon nitride and gold in addition to a layer of benzocyclobutene (BCB). A second circular patch is elevated in air using gold posts, making this design a stack configuration. The demonstrated antenna shows a measured resonance frequency in agreement with the modeling at 0.27 THz and a measured S11 as low as -18 dB was obtained. A directivity, gain and radiation efficiency of 8.3 dB, 3.4 dB, and 32% respectively was exhibited from the 3D EM model. To the authors' knowledge, this is the first demonstrated THz integrated microstrip stack antenna for TMIC (THz Monolithic Integrated Circuits) technology; the developed technology is suitable for high performance III-V material on low resistivity/high dielectric substrates.

Single-photon avalanche diode (SPAD) detectors are of significant interest for numerous applications, including light detection and ranging (LIDAR), and quantum technologies such as quantum-key distribution and quantum information processing. Here we present a record low noise-equivalent-power (NEP) for Ge-on-Si SPADs using a pseudo-planar design, showing high detection efficiency in the short-wave infrared; a spectral region which is key for quantum technologies and hugely beneficial for LIDAR. These devices can leverage the benefits of Si avalanche layers, with lower afterpulsing compared to InGaAs/InP, and reduced cost due to Si foundry compatibility. By scaling the SPAD pixels down to 26µm diameter, a step change in performance has been demonstrated, with significantly reduced dark count rates (DCRs), and low jitter (134ps). Ge-on-Si SPADs were fabricated using photolithography techniques and characterised using time-correlated single-photon counting. The DCR reaches as low as kilocount/s at 100K for excess bias up to ~5%. This reduction in DCR enables higher temperature operation; e.g. the DCR of a 26µm diameter pixel at 150 K is approximately equivalent to a 100 µm diameter pixel at 77 K (100s of kilocounts/s). These low values of DCR, coupled with the relatively temperature independent single photon detection efficiencies (SPDE) of ~29% (at 1310nm wavelength) leads to a record low NEP of 7.7×10 −17 WHz −1/2 . This is approximately 2 orders of magnitude lower than previous similarly sized mesa-geometry Ge-on-Si SPADs. This technology can potentially offer a lowcost, Si foundry compatible SPAD operating at short-wave infrared wavelengths, with potential applications in quantum technologies and autonomous vehicle LIDAR.