A thermally annealed Ni/Pt/Au metal structure was employed as the gate contacts of AlGaN/GaN high electron mobility transistors (HEMTs), and their DC and RF performances were investigated. This gate structure markedly improved the Schottky characteristics such as the Schottky barrier height and leakage current. Regarding the DC characteristics, the maximum drain current and off-state breakdown voltage were increased from 0.78 A/mm (V g=1 V) to 0.90 A/mm (V g=3 V) due to the improved applicability of the gate voltage and from 108 V to 178 V, respectively, by annealing the gate metals. In addition, a reduction of the transconductance was not observed. Furthermore, even after the deposition of SiN x passivation film, the off-state breakdown voltage remained at a relatively high value of 120 V. Regarding the RF characteristics, the cut-off frequency and maximum oscillation frequency were also improved from 10.3 GHz to 13.5 GHz and from 27.5 GHz to 35.1 GHz, respectively, by annealing the gate metals whose gate length was 1 µm.

Absfrocl -Microwave waveforms of SiGe HBTs have been directly measured. The maximum operating region has been experimentally investigated by sweeping the load lines and power of the input signal. The device is found to operate beyond the conventional BVceo, while GaAs HBTs cannot survive at that voltage. The conventional BVceo is found lo limit the average Vc of the maximum load lines, but has no influence on the peak voltsge. Another BVcw measured with a voltage generator is proposed to represent the avalanche breakdown instead of the conventional one. IEEE M T -S Digest0-7803-7239~5/02/$10.00 D 2002 IEEE

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Feigin-Frenkel duality is the isomorphism between the principal W-algebras of a simple Lie algebra g and its Langlands dual Lie algebra L g. A generalization of this duality to a larger family of W-algebras called hook-type was recently conjectured by Gaiotto and Rapčák and proved by the first two authors. It says that the affine cosets of two different hook-type W-algebras are isomorphic. A natural question is whether the duality between affine cosets can be enhanced to a duality between the full W-algebras. There is a convolution operation that maps a hook-type W-algebra W to a certain relative semi-infinite cohomology of W tensored with a suitable kernel VOA. The first two authors conjectured previously that this cohomology is isomorphic to the Feigin-Frenkel dual hook-type W-algebra. Our main result is a proof of this conjecture.

Current and voltage waveforms of GaAs HBTs at 1 GHz have been directly measured using a microwave waveform measurement system. The maximum operating region has been experimentally investigated by sweeping load lines. The limits of a small input power are found to come from the thermal runaway and avalanche breakdown of the device. With large input power, the HBT is found to operate beyond the DC limit of thermal runaway.