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Papers by Vikram Kumar

Research paper thumbnail of Phosphorylation-dependent interaction of osteopontin with its receptors regulates macrophage migration and activation

Neutrophil-independent macrophage responses are a prominent part of delayed-type immune and heali... more Neutrophil-independent macrophage responses are a prominent part of delayed-type immune and healing processes and depend on T cellsecreted cytokines. An important mediator in this setting is the phosphoprotein osteopontin, whose secretion by activated T cells confers resistance to infection by several intracellular pathogens through recruitment and activation of macrophages. Here, we analyze the structural basis of this activity following cleavage of the phosphoprotein by thrombin into two fragments. An interaction between the C-terminal domain of osteopontin and the receptor CD44 induces macrophage chemotaxis, and engagement of ␤ 3 -integrin receptors by a nonoverlapping N-terminal osteopontin domain induces cell spreading and subsequent activation. Serine phosphorylation of the osteopontin molecule on specific sites is required for functional interaction with integrin but not CD44 receptors. Thus, in addition to regulation of intracellular enzymes and substrates, phosphorylation also regulates the biological activity of secreted cytokines. These data, taken as a whole, indicate that the activities of distinct osteopontin domains are required to coordinate macrophage migration and activation and may bear on incompletely understood mechanisms of delayed-type hypersensitivity, wound healing, and granulomatous disease. J. Leukoc. Biol. 72: 752-761; 2002.

Research paper thumbnail of Low Resistance Ti/Al/Mo/Au Ohmic Contacts for AlGaN/GaN Heterostructure Field Effect Transistors

Physica Status Solidi (a), 2002

A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic con... more A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic contacts to AlGaN/GaN heterostructure field effect transistors (HFET). A contact resistance as low as 0.20 Wmm and a specific contact resistivity as low as 4.5 Â 10 --7 Wcm 2 were obtained using a pre-metallization surface treatment with SiCl 4 plasma at a self-bias voltage of --300 V in a reactive ion etching (RIE) system. X-ray photoelectron spectroscopy (XPS) measurements of the SiCl 4 plasma-treated surface revealed an increase in the N vacancies thereby increasing the donor concentration at the surface. Also a blue shift of the peak energy of the Ga 3d photoelectrons was observed showing that the Fermi level moved closer to the conduction band at the surface of the AlGaN.

Research paper thumbnail of A comparative study of surface passivation on AlGaN/GaN HEMTs

Solid-state Electronics, 2002

Using a Si 3 N 4 layer as passivation layer, effects of surface passivation on device performance... more Using a Si 3 N 4 layer as passivation layer, effects of surface passivation on device performances have been investigated. After passivation, devices exhibited better pinch-off characteristics and lower gate leakage current. For a device with a gate-length of 0.25 lm, the I dss increased from 791 to 812.2 mA/mm and the peak extrinsic transconductance increased from 207.2 to 220.9 mS/mm. The f T and f MAX values decreased from 53 and 102.5 to 45.9 and 90.5 GHz, respectively, due to the increase of parasitic capacitances. Microwave noise measurements showed that devices exhibited 0:2-0:25 dB increase in minimum noise figure (NF min ) after passivation. Ó

Research paper thumbnail of Thermally-stable low-resistance Ti/Al/Mo/Au multilayer ohmic contacts on n-GaN

Journal of Applied Physics, 2002

... Kinetics of defect formation and annealing. Structure of solids and liquids; crystallography.... more ... Kinetics of defect formation and annealing. Structure of solids and liquids; crystallography. ... However, among the various problems limiting the performance of these devices are the need to form ohmic contacts with very low resistance, smooth morphology, and good edge ...

Research paper thumbnail of Phosphorylation-dependent interaction of osteopontin with its receptors regulates macrophage migration and activation

Neutrophil-independent macrophage responses are a prominent part of delayed-type immune and heali... more Neutrophil-independent macrophage responses are a prominent part of delayed-type immune and healing processes and depend on T cellsecreted cytokines. An important mediator in this setting is the phosphoprotein osteopontin, whose secretion by activated T cells confers resistance to infection by several intracellular pathogens through recruitment and activation of macrophages. Here, we analyze the structural basis of this activity following cleavage of the phosphoprotein by thrombin into two fragments. An interaction between the C-terminal domain of osteopontin and the receptor CD44 induces macrophage chemotaxis, and engagement of ␤ 3 -integrin receptors by a nonoverlapping N-terminal osteopontin domain induces cell spreading and subsequent activation. Serine phosphorylation of the osteopontin molecule on specific sites is required for functional interaction with integrin but not CD44 receptors. Thus, in addition to regulation of intracellular enzymes and substrates, phosphorylation also regulates the biological activity of secreted cytokines. These data, taken as a whole, indicate that the activities of distinct osteopontin domains are required to coordinate macrophage migration and activation and may bear on incompletely understood mechanisms of delayed-type hypersensitivity, wound healing, and granulomatous disease. J. Leukoc. Biol. 72: 752-761; 2002.

Research paper thumbnail of Low Resistance Ti/Al/Mo/Au Ohmic Contacts for AlGaN/GaN Heterostructure Field Effect Transistors

Physica Status Solidi (a), 2002

A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic con... more A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic contacts to AlGaN/GaN heterostructure field effect transistors (HFET). A contact resistance as low as 0.20 Wmm and a specific contact resistivity as low as 4.5 Â 10 --7 Wcm 2 were obtained using a pre-metallization surface treatment with SiCl 4 plasma at a self-bias voltage of --300 V in a reactive ion etching (RIE) system. X-ray photoelectron spectroscopy (XPS) measurements of the SiCl 4 plasma-treated surface revealed an increase in the N vacancies thereby increasing the donor concentration at the surface. Also a blue shift of the peak energy of the Ga 3d photoelectrons was observed showing that the Fermi level moved closer to the conduction band at the surface of the AlGaN.

Research paper thumbnail of A comparative study of surface passivation on AlGaN/GaN HEMTs

Solid-state Electronics, 2002

Using a Si 3 N 4 layer as passivation layer, effects of surface passivation on device performance... more Using a Si 3 N 4 layer as passivation layer, effects of surface passivation on device performances have been investigated. After passivation, devices exhibited better pinch-off characteristics and lower gate leakage current. For a device with a gate-length of 0.25 lm, the I dss increased from 791 to 812.2 mA/mm and the peak extrinsic transconductance increased from 207.2 to 220.9 mS/mm. The f T and f MAX values decreased from 53 and 102.5 to 45.9 and 90.5 GHz, respectively, due to the increase of parasitic capacitances. Microwave noise measurements showed that devices exhibited 0:2-0:25 dB increase in minimum noise figure (NF min ) after passivation. Ó

Research paper thumbnail of Thermally-stable low-resistance Ti/Al/Mo/Au multilayer ohmic contacts on n-GaN

Journal of Applied Physics, 2002

... Kinetics of defect formation and annealing. Structure of solids and liquids; crystallography.... more ... Kinetics of defect formation and annealing. Structure of solids and liquids; crystallography. ... However, among the various problems limiting the performance of these devices are the need to form ohmic contacts with very low resistance, smooth morphology, and good edge ...

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