Interface Trap Effect on the n-Channel GaN Schottky Barrier-Metal-Oxide Semiconductor Field-Effect Transistor for Ultraviolet Optoelectronic Integration.

Ultraviolet (UV) photodetectors are key devices required in the industrial, military, space, environmental, and biological fields. The Schottky barrier (SB)-MOSFET, with its high hole and electron barrier, and given its extremely low dark current, has broad development prospects in the optoelectronics field. We analyze the effects of trap states on the output characteristics of an inversion mode n-channel GaN SB-MOSFET using TCAD simulations. At the oxide/GaN interface below the gate, it was demonstrated that shallow donor-like traps were responsible for degrading the subthreshold swing (SS) and off-state current density (Ioff), while deep donor-like traps below the Fermi energy level were insignificant. In addition, shallow acceptor-like traps shifted the threshold voltage (Vt) positively and deteriorated the SS and on-state current density (Ion), while deep acceptor-like traps acted on a fixed charge. The output characteristics of the GaN SB-MOSFET were related to the resistive GaN path and the tunneling rate due to the traps at the metal (source, drain)/GaN interface. For the UV responses, the main mechanism for the negative Vt shift and the increases in the Ion and spectral responsivity was related to the photo-gating effect caused by light-generated holes trapped in the shallow trap states. These results will provide insights for UV detection technology and for a high-performance monolithic integration of the GaN SB-MOSFET.

Site-Specific Glycan Microheterogeneity Evaluation of Aflibercept Fusion Protein by Glycopeptide-Based LC-MSMS Mapping.

The evaluation of the protein glycosylation states of samples of aflibercept obtained from three different regions was conducted by site-specific N-linked glycan microheterogeneity profiling. Glycopeptide-based nano-LC MSMS mapping of tryptic-digested samples of each aflibercept lot provided site-specific information about glycan microheterogeneity on each of the five N-glycosylation sites (two sites in the VEGFR-1 region, two sites in the VEGFR-2 region, and one site in the human IgG Fc region). Next, the glycopeptide-mapping results obtained from the three different aflibercept lots were compared to evaluate the similarity between the samples. Three aflibercept lots showed a high degree of similarity in glycan composition, fucosylation level, sialylation level, and branching, when all five N-glycosylation sites were assessed together as a group. On the other hand, noticeable variations between lots in the glycan types and sialylation levels on the two sites of the VEGFR-2 domain were observed when each of the five N-glycosylation sites were assessed using the glycopeptide-based method. The presence of N-glycolylneuraminic acid (NeuGc) glycans, which may mediate adverse immune reactions in antibody therapeutics, were also detected on the sites of VEGFR1 and VEGFR2 domains, but not on the IgG Fc domain site. These results imply that analyses of the glycosylation profiles of fusion proteins containing multiple N-glycosylation sites, such as aflibercept, being done as a part of quality control for the therapeutics manufacturing process or for biosimilar development, can be done with a more applicable outcome by assessing each site separately.

A Schottky-Type Metal-Semiconductor-Metal Al0.24Ga0.76N UV Sensor Prepared by Using Selective Annealing.

Asymmetric metal-semiconductor-metal (MSM) aluminum gallium nitride (AlGaN) UV sensors with 24% Al were fabricated using a selective annealing technique that dramatically reduced the dark current density and improved the ohmic behavior and performance compared to a non-annealed sensor. Its dark current density at a bias of -2.0 V and UV-to-visible rejection ratio (UVRR) at a bias of -7.0 V were 8.5 × 10-10 A/cm2 and 672, respectively, which are significant improvements over a non-annealed sensor with a dark current density of 1.3 × 10-7 A/cm2 and UVRR of 84, respectively. The results of a transmission electron microscopy analysis demonstrate that the annealing process caused interdiffusion between the metal layers; the contact behavior between Ti/Al/Ni/Au and AlGaN changed from rectifying to ohmic behavior. The findings from an X-ray photoelectron spectroscopy analysis revealed that the O 1s binding energy peak intensity associated with Ga oxide, which causes current leakage from the AlGaN surface, decreased from around 846 to 598 counts/s after selective annealing.

GaN-Based Ultraviolet Passive Pixel Sensor on Silicon (111) Substrate.

The fabrication of a single pixel sensor, which is a fundamental element device for the fabrication of an array-type pixel sensor, requires an integration technique of a photodetector and transistor on a wafer. In conventional GaN-based ultraviolet (UV) imaging devices, a hybrid-type integration process is typically utilized, which involves a backside substrate etching and a wafer-to-wafer bonding process. In this work, we developed a GaN-based UV passive pixel sensor (PPS) by integrating a GaN metal-semiconductor-metal (MSM) UV photodetector and a Schottky-barrier (SB) metal-oxide-semiconductor field-effect transistor (MOSFET) on an epitaxially grown GaN layer on silicon substrate. An MSM-type UV sensor had a low dark current density of 3.3 × 10-7 A/cm² and a high UV/visible rejection ratio of 10³. The GaN SB-MOSFET showed a normally-off operation and exhibited a maximum drain current of 0.5 mA/mm and a maximum transconductance of 30 µS/mm with a threshold voltage of 4.5 V. The UV PPS showed good UV response and a high dark-to-photo contrast ratio of 10³ under irradiation of 365-nm UV. This integration technique will provide one possible way for a monolithic integration of the GaN-based optoelectronic devices.

Comparative Glycopeptide Analysis for Protein Glycosylation by Liquid Chromatography and Tandem Mass Spectrometry: Variation in Glycosylation Patterns of Site-Directed Mutagenized Glycoprotein.

Glycosylation is one of the most important posttranslational modifications for proteins, including therapeutic antibodies, and greatly influences protein physiochemical properties. In this study, glycopeptide mapping of a reference and biosimilar recombinant antibodies (rAbs) was performed using liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) and an automated Glycoproteome Analyzer (GPA) algorithm. The tandem mass analyses for the reference and biosimilar samples indicate that this approach proves to be highly efficient in reproducing consistent analytical results and discovering the implications of different rAb production methods on glycosylation patterns. Furthermore, the comparative analysis of a mutagenized rAb glycoprotein proved that a single amino acid mutation in the Fc portion of the antibody molecule caused increased variations in glycosylation patterns. These variations were also detected by the mass spectrometry method efficiently. This mapping method, focusing on precise glycopeptide identification and comparison for the identified glycoforms, can be useful in differentiating aberrant glycosylation in biosimilar rAb products.

Effect of Growth Temperature on the Structural and Electrical Properties of ZrO2 Films Fabricated by Atomic Layer Deposition Using a CpZr[N(CH3)2]3/C7H8 Cocktail Precursor.

The effect of growth temperature on the atomic layer deposition of zirconium oxide (ZrO2) dielectric thin films that were fabricated using a CpZr[N(CH3)2]3/C7H8 cocktail precursor with ozone was investigated. The chemical, structural, and electrical properties of ZrO2 films grown at temperatures from 250 to 350 °C were characterized. Stoichiometric ZrO2 films formed at 250-350 °C with an atomic ratio of O to Zr of 1.8-1.9 and a low content of carbon impurities. The film formed at 300 °C was predominantly the tetragonal crystalline phase, whereas that formed at 350 °C was a mixture of tetragonal and monoclinic phases. Electrical properties, such as capacitance, leakage current, and voltage linearity of TiN/ZrO2/TiN capacitors fabricated using the thin ZrO2 films grown at different temperatures were compared capacitor applications. The ZrO2 film grown at 300 °C exhibited low impurity content, predominantly tetragonal crystalline structure, a high dielectric permittivity of 38.3, a low leakage current of below 10-7 A/cm² at 2 V, and low-voltage linearity.

Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1-xN Buffer Layer.

The UV-to-visible rejection ratio is one of the important figure of merits of GaN-based UV photodetectors. For cost-effectiveness and large-scale fabrication of GaN devices, we tried to grow a GaN epitaxial layer on silicon substrate with complicated buffer layers for a stress-release. It is known that the structure of the buffer layers affects the performance of devices fabricated on the GaN epitaxial layers. In this study, we show that the design of a buffer layer structure can make effect on the UV-to-visible rejection ratio of GaN UV photodetectors. The GaN photodetector fabricated on GaN-on-silicon substrate with a step-graded AlxGa-xN buffer layer has a highly-selective photoresponse at 365-nm wavelength. The UV-to-visible rejection ratio of the GaN UV photodetector with the step-graded AlxGa1-xN buffer layer was an order-of-magnitude higher than that of a photodetector with a conventional GaN/AlN multi buffer layer. The maximum photoresponsivity was as high as 5 × 10-² A/W. This result implies that the design of buffer layer is important for photoresponse characteristics of GaN UV photodetectors as well as the crystal quality of the GaN epitaxial layers.

Retroviral gene therapy: safety issues and possible solutions.

The recent incidents of leukemia development in X-SCID patients after a successful treatment of the disease with retroviral gene therapy raised concerns regarding the safety of the use of retroviral vectors in clinical gene therapy. In this review, we have tried to re-evaluate the safety issues related to the use of retroviral vectors in human clinical trials and to suggest possible appropriate solutions to the issues. As revealed by the X-SCID incident, oncogenesis caused by retroviral insertional activation of host genes is one of the most prominent risks. An ultimate solution to this problem will be in re-engineering retroviral vectors so that the retroviral insertion takes place only at the desired specific sites of the host cell chromosome. This is, however, a technically demanding tasks, and it will take years to develop retroviral vectors with targeted insertion capability. In the mean time, the use of chromatin insulators can reduce chances for retrovirus-mediated oncogenesis by inhibiting non-specific activation of nearby cellular proto-oncogenes. Co-transduction of a suicidal gene under the control of an inducible promoter could also be one of the important safety features, since destruction of transduced cells can be triggered if abnormal growth is observed. Additionally, conditional expression of the transgene only in appropriate target cells via the combination of targeted transduction, cell type-specific expression, and targeted local administration will increase the overall safety of the retroviral systems. Finally, splitting of the viral genome, use of self-inactivating (SIN) retroviral vectors, or complete removal of the coding sequences for gag, pol, and env genes is desirable to virtually eliminate the possibility of generation of replication competent retroviruses (RCR).

Construction of retroviral vectors with enhanced efficiency of transgene expression.

Retroviral vectors have been widely used in gene therapy due to their simple genomic structure and high transduction efficiency. We report a construction of Moloney murine sarcoma virus (MoMSV) and Moloney murine leukemia virus (MoMLV) hybrid-based retroviral vectors with significantly improved efficiency of transgene expression after stable incorporation into the host genome. In these vectors, the residual gag gene coding sequence located in the extended region of packaging signal was removed. These vectors, therefore, contain no coding sequence for the gag, pol, or env gene that can be used for homologous recombination with sequences introduced in the packaging system for a recombinant competent retrovirus (RCR) generation. A strong splice acceptor site obtained from the exon/intron junction of either the chimpanzee EF1-alpha gene or the human CMV major immediate early gene was placed downstream of the MoMSV packaging signal (Psi), significantly improving the efficiency of transgene expression. The 5' LTR U3 sequence was replaced with an extended human CMV major immediate early gene enhancer/promoter for a strong expression of full-length messages from the viral backbone, helping to maintain high levels of viral titer. These newly developed retroviral vectors should facilitate RCR-free gene transfer with significantly improved efficacy in clinical gene therapy trials.

A restrictive element 1 (RE-1) in the VIP gene modulates transcription in neuronal and non-neuronal cells in collaboration with an upstream tissue specifier element.

The vasoactive intestinal peptide (VIP) gene has been studied extensively as a prototype neuronal gene containing multiple cis-active elements that confer responsiveness to cell lineage, neurotrophic, and activity-dependent intrinsic and extrinsic cues. However, reporter genes containing the presumptive complete regulatory region 5' to the start of transcription do not confer tissue-specific gene expression in vivo. We therefore sought cis-regulatory elements downstream of the transcriptional start that might confer additional tissue-specific and tissue-restrictive properties to the VIP transcriptional unit. We report here a repressor element, similar to the canonical restrictive element-1 (RE-1), located within the first non-coding exon of the human VIP gene. The ability of this element to regulate VIP reporter gene expression in neuroblastoma and fibroblastic cells was examined. Endogenous VIP expression is high in SH-EP neuroblastoma cells, low but inducible in SH-SY5Y cells, and absent in HeLa cells. Endogenous RE-1 silencer factor (REST) expression was highest in SH-EP and HeLa cells, and significantly lower in SH-SY5Y cells. Transient transfection of a VIP reporter gene containing a mutated RE-1 sequence revealed an RE-1-dependent regulation of VIP gene expression in all three cell types, with regulation greatest in cells (SH-EP, HeLa) with highest levels of REST expression. Serial truncation of the VIP reporter gene further revealed a specific interaction between the RE-1 and a tissue-specifier element located 5 kb upstream in the VIP gene. Thus, REST can regulate VIP gene expression in both neuroblastic and non-neuronal cells, but requires coupling to the upstream tissue specifier element.

A calcium-initiated signaling pathway propagated through calcineurin and cAMP response element-binding protein activates proenkephalin gene transcription after depolarization.

Essential components of a signal-transduction pathway regulating activity-dependent neuropeptide gene transcription have been identified. Proenkephalin (PEnk) gene activation after depolarization of chromaffin cells with 40 mM KCl was blocked by the voltage-sensitive calcium-channel blocker methoxyverapamil (D600) (30 microM) and by calcineurin inhibition with 100 nM cyclosporin A or ascomycin but not by inhibiting new protein synthesis with 0.5 microg/ml cycloheximide. KCl-induced elevation of PEnk mRNA was distinct from activation of the PEnk gene by either cAMP or protein kinase C. Twenty-five micromolar forskolin- and 100 nM phorbol 12-myristate 13-acetate-induced elevations of PEnk mRNA were cycloheximide-sensitive and were not blocked by cyclosporin A or ascomycin. KCl stimulated Ser-133 phosphorylation of cAMP response element-binding protein (CREB) in chromaffin cells, and CREB phosphorylation was blocked by both ascomycin and D600. A reporter gene containing 193 bases of the PEnk gene 5' flank driving luciferase gene expression (pENK12-Luc) transfected into chromaffin cells was transcriptionally activated by KCl depolarization. Activation was blocked by both ascomycin and D600 and required an intact CREB binding site (ENKCRE2). An oligonucleotide containing the PEnk cAMP response element-2 was gel-shifted by both unstimulated and potassium-stimulated chromaffin cell nuclear extracts into a prominent complex supershifted by CREB antibodies. Finally, stimulation of transcription of the pENK12-Luc reporter by KCl in chromaffin cells was blocked by coexpression of the CREB antagonist A-CREB but not by the AP-1 antagonist A-Fos. Stimulus-transcription coupling after depolarization in chromaffin cells occurs via calcineurin-dependent activation of CREB, a pathway distinct from cAMP- or protein kinase C-initiated signaling and independent of immediate early gene regulation.