Terahertz Microstrip Elevated Stack Antenna Technology on GaN-on-Low Resistivity Silicon Substrates for TMIC

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

The Investigation of Flowering Control in Late/Rare Flowering Lolium Perenne

Flowering in Lolium perenne (perennial ryegrass) results in reduced digestibility and its inhibition would enhance forage quality. Flowering regulation has been well studied in Arabidopsis thaliana (Simpson and Dean, 2002) and orthologs of Arabidopsis flowering genes underlying heading date Quantitative Trait Loci (QTL) have been identified in rice (Yano, M et al., 2000). However it is not clear yet how universally applicable such studies are to Lolium. The project goals are to characterise the gene expression profiles of late/rare flowering L. perenne plants to determine factors affecting flowering and to map the genes involved in the flowering process. Initial studies, reported here, have focussed on the ability of 6 plant lines from the Oak Park breeding programme, previously identified as rare or non-flowering under natural day length conditions, to flower in controlled environments

Control of threshold voltage in E-mode and D-mode GaN-on-Si metal-insulator-semiconductor heterostructure field effect transistors by in-situ fluorine doping of atomic layer deposition Al2O3 gate dielectrics

We report the modification and control of threshold voltage in enhancement and depletion mode AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors through the use of in-situ fluorine doping of atomic layer deposition Al2O3. Uniform distribution of F ions throughout the oxide thickness are achievable, with a doping level of up to 5.5 × 1019 cm−3 as quantified by secondary ion mass spectrometry. This fluorine doping level reduces capacitive hysteretic effects when exploited in GaN metal-oxide-semiconductor capacitors. The fluorine doping and forming gas anneal also induces an average positive threshold voltage shift of between 0.75 and 1.36 V in both enhancement mode and depletion mode GaN-based transistors compared with the undoped gate oxide via a reduction of positive fixed charge in the gate oxide from +4.67 × 1012 cm−2 to −6.60 × 1012 cm−2. The application of this process in GaN based power transistors advances the realisation of normally off, high power, high speed devices

Sulfuric acid and hydrogen peroxide surface passivation effects on AlGaN/GaN high electron mobility transistors

In this work, we have compared SiNx passivation, hydrogen peroxide, and sulfuric acid treatment on AlGaN/GaN HEMTs surface after full device fabrication on Si substrate. Both the chemical treatments resulted in the suppression of device pinch-off gate leakage current below 1 μA/mm, which is much lower than that for SiNx passivation. The greatest suppression over the range of devices is observed with the sulfuric acid treatment. The device on/off current ratio is improved (from 104–105 to 107) and a reduction in the device sub-threshold (S.S.) slope (from ∼215 to 90 mV/decade) is achieved. The sulfuric acid is believed to work by oxidizing the surface which has a strong passivating effect on the gate leakage current. The interface trap charge density (Dit ) is reduced (from 4.86 to 0.90 × 1012 cm−2 eV−1), calculated from the change in the device S.S. The gate surface leakage current mechanism is explained by combined Mott hopping conduction and Poole Frenkel models for both untreated and sulfuric acid treated devices. Combining the sulfuric acid treatment underneath the gate with the SiNx passivation after full device fabrication results in the reduction of Dit and improves the surface related current collapse

Novel Shielded Coplanar Waveguides on GaN-on-Low Resistivity Si Substrates for MMIC Applications

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (σ < 40 Ω.cm). Transmission losses (S 21 ) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si.This work was supported in part by the EPSRC III-V national center pump-priming scheme

Impact of stress in ICP-CVD SiN x passivation films on the leakage current in AlGaN/GaN HEMTs

The impact of the stress in room temperature inductively coupled plasma chemical vapour deposited (ICP-CVD) SiN x surface passivation layers on off-state drain ( I DS-off) and gate leakage currents ( I GS) in AlGaN/GaN high electron mobility transistors (HEMTs) is reported. I DS-off and I GS in 2 μm gate length devices were reduced by up to four orders of magnitude to ∼10 pA/mm using a compressively stressed bilayer SiN x passivation scheme. In addition, I on/ I off of ∼10 11 and subthreshold slope of 68 mV/dec were obtained using this strain engineered surface passivation approach

GaN on Low-Resistivity Silicon THz High-Q Passive Device Technology

In this paper, viable transmission media technology has been demonstrated for the first time on GaN on low-resistivity silicon) substrates (ρ < 40 $\Omega$·cm) at H-band frequencies (220-325 GHz). The shielded-elevated coplanar waveguide (CPW) lines employ a standard monolithic microwave integrated circuit compatible air bridge process to elevate the CPW traces above a 5-μm layer of benzocyclobutene on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was used. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of-34 dB. To our knowledge, these results are the best reported to date for GaN-based technology.This work was supported by the EPSRC under Grant EP/N014820/1

High Performance GaN High Electron Mobility Transistors on Low Resistivity Silicon for X -Band Applications

This letter reports the RF performance of a 0.3-μm gate length AlGaN/AlN/GaN HEMT realized on a 150-mm diameter low-resistivity (LR) (σ <; 10 Ω · cm) silicon substrate. Short circuit current gain (fT) and maximum frequency of oscillation (fMAX) of 55 and 121 GHz, respectively, were obtained. To our knowledge, these are the highest fT/fMAX values reported to date for GaN HEMTs on LR silicon substrates.This work was supported by the Pump-Priming Scheme–EPSRC National Centre for III–V Technologies

Vertical leakage mechanism in GaN on Si high electron mobility transistor buffer layers

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices.This work was funded by the Engineering and Physical Sciences Research Council under Grant Code Nos. EP/K014471/1 and EP/N01202X/1 and the European Research Council under the European Community's Seventh Framework Programme Grant Agreement No. 279361 (MACONS)

High-performance MMIC inductors for GaN-on-low-resistivity silicon for microwave applications

Novel MMIC spiral inductors on GaN-on-low-resistivity silicon (LR-Si) substrates ( σ<40 Ω⋅cm ) are demonstrated with enhanced self-resonance frequency ( fSRF ) and Q -factor. The developed technology improves inductor performance by suppressing substrate coupling effects using air-bridge technology above benzocyclobutene dielectric as an interface layer on the lossy substrate. A 0.83-nH spiral inductor with peak Q -factor enhancement of 57% ( Q=22 at 24 GHz) and maximum fSRF of 59 GHz was achieved because of the extra 5- μm elevation in air. An accurate broad-band model for the fabricated inductors has been developed and verified for further performance analysis up to 40 GHz. The proposed inductors utilize cost-effective, reliable, and MMIC-compatible technology for the realization of high-performance RF GaN-on-LR Si MMIC circuits for millimeter-wave applications