In this letter, we present a new technology to increase the breakdown voltage of AlGaN/GaN high-electronmobility transistors (HEMTs) grown on Si substrates. This new technology is based on the removal of the original Si substrate and subsequent transfer of the AlGaN/GaN HEMT structure to an insulating carrier wafer (e.g., glass or polycrystalline AlN). By applying this new technology to standard AlGaN/GaN HEMTs grown on Si substrate, an AlGaN/GaN HEMT with breakdown voltage above 1500 V and specific on resistance of 5.3 mΩ • cm 2 has been achieved.
Drag reduction, shear and extensional rheometry, and
cryogenic transmission electron microscopy (cryo-TEM) were used to study aqueous solutions of one cationic surfactant,
Arquad 16-50 (5 mM), with three
isomeric counterions, 2-, 3-, or 4-Cl-benzoate at 12.5 mM. Each
isomer showed different types of rheological
and drag reduction behavior and different micellar structures. The
4-Cl system showed good drag reduction,
high apparent extensional viscosity, and a thread-like micellar
network, while the 2-Cl system showed
no drag reduction, low apparent extensional viscosity, and only
spherical micelles. The 3-Cl system was
drag reducing and had high extensional viscosities at 30 °C.
However, at 20 °C, the 3-Cl solution precipitated
at high shear or extensional rates, leading to loss of drag reduction
and low apparent extensional viscosity.
The 3-Cl at 20 °C showed threadlike structures in some pictures
and vesicles in others, presumably because
of variations in the level of shear the samples were subjected to
during sample preparation. The differences
in behavior are explained by the position of the chlorine group on the
benzoate ring. The hydrophobic
chlorine in the 2-Cl system must reside in an unfavorable position in
the aqueous phase and hence only
spherical micelles are formed, which leads to no drag reduction and
very low apparent extensional viscosity.
The chlorine groups in the 3-Cl and 4-Cl counterions reside in the
nonpolar hydrocarbon core of the micelles
and, hence, stable elongated micelles can form. A schematic
micellar phase diagram is proposed to summarize
the transformations of surfactant molecular aggregates at different
temperatures and external forces.
The physical degradation of AlGaN/GaN high electron mobility transistors during OFF-state stress experiments has been systematically studied. Oxide particles and stringers were found to form along the gate edge of stressed devices. When the gate electrode is removed, pits are seen to have formed underneath each particle. The observed room-temperature oxidation process is strongly dependent on the duration of the electrical stressing and the electric field. Moreover, the oxidation can be significantly reduced in vacuum (3 × 10−5 Torr), with a corresponding 30% reduction of current collapse. Finally, a degradation process with electric-field-driven oxidation of the AlGaN surface has been proposed.
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