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

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

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

The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4

Macrophages are pivotal constituents of the innate immune system, vital for recognition and elimination of microbial pathogens(1). Macrophages use Toll-like receptors (TLRs) to detect pathogen-associated molecular patterns - including bacterial cell wall components, such as lipopolysaccharide or lipoteichoic acid, and viral nucleic acids, such as double-stranded (ds) RNA and in turn activate effector functions, including anti-apoptotic signalling pathways(2). Certain pathogens, however, such as Salmonella spp., Shigellae spp. and Yersiniae spp., use specialized virulence factors to overcome these protective responses and induce macrophage apoptosis(3). We found that the anthrax bacterium, Bacillus anthracis, selectively induces apoptosis of activated macrophages(4) through its lethal toxin, which prevents activation of the anti-apoptotic p38 mitogen-activated protein kinase(4). We now demonstrate that macrophage apoptosis by three different bacterial pathogens depends on activation of TLR4. Dissection of anti- and pro-apoptotic signalling events triggered by TLR4 identified the dsRNA responsive protein kinase PKR as a critical mediator of pathogen-induced macrophage apoptosis. The pro-apoptotic actions of PKR are mediated both through inhibition of protein synthesis and activation of interferon response factor 3.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62666/1/nature02405.pd

Impact of buffer charge on the reliability of carbon doped AlGaN/GaN-on-Si HEMTs

Charge trapping and transport in the carbon doped GaN buffer of an AlGaN/GaN-on-Si HEMT have been investigated. Back-gating and dynamic RON experiments show how the onset of leakage in the strain relief layer at a lower field than that through the upper part of the structure can result in serious long-term trapping leading to current collapse under standard device operating conditions. Controlling current-collapse requires control of not only the layer structures and its doping, but also the precise balance of leakage in each layer

Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors

We investigate the impact of a fluorine plasma treatment used to obtain enhancement-mode operation on the structure and chemistry at the nanometer and atomic scales of an InAlN/GaN field effect transistor. The fluorine plasma treatment is successful in that enhancement mode operation is achieved with a +2.8 V threshold voltage. However, the InAlN barrier layers are observed to have been damaged by the fluorine treatment with their thickness being reduced by up to 50%. The treatment also led to oxygen incorporation within the InAlN barrier layers. Furthermore, even in the as-grown structure, Ga was unintentionally incorporated during the growth of the InAlN barrier. The impact of both the reduced barrier thickness and the incorporated Ga within the barrier on the transistor properties has been evaluated theoretically and compared to the experimentally determined two-dimensional electron gas density and threshold voltage of the transistor. For devices without fluorine treatment, the two-dimensional electron gas density is better predicted if the quaternary nature of the barrier is taken into account. For the fluorine treated device, not only the changes to the barrier layer thickness and composition, but also the fluorine doping needs to be considered to predict device performance. These studies reveal the factors influencing the performance of these specific transistor structures and highlight the strengths of the applied nanoscale characterisation techniques in revealing information relevant to device performance.</jats:p