A numerical solution of the Boltzmann equation for highpowered short pulse microwave breakdown in nitrogen Phys.Microwave energy coupling to nitrogen for breakdown conditions has been investigated. The experiments were performed in 25-Torr nitrogen using a 35-GHz l-,us microwave pulse focused to 33 kW /cm 2 . The plasma created during breakdown was observed to travel towards the source with a velocity of 4.6 X 10 6 cm/s. Spectroscopic measurements using a nitrogen-helium mixture indicate the average electron density over a 2-cm chord to be 4.4 X 10 12 cm~3, and the electron temperature to be 3.8 eV. Microwave cutoff was observed photographically, and suggests the peak electron density to be the collisional critical density. Computer simulations were also performed, and their results are in accord with the experimental results.

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Computer simulations of microwave coupling to a nitrogen-breakdown plasma have been performed at 25 Torr. Nonhydrodynamic ionization fronts are observed to propagate toward the radiation source under a variety of circumstances. Free-nitrogen-breakdown simulations in a spherical system show the propagation velocity of the breakdown wave can be as high as 5×106 cm/sec. An elementary theory is used for estimating the speed of the breakdown wave in one dimension. The results are in reasonable agreement with breakdown experiments.

A failure mechanism is observed for SiN2-passivated metal semiconductor field-effect transistor (MESFET) devices exposed to fast risetime DC video pulses. The intensity of the pulses is about 33% of the value required to cause single-pulse failure. The failure mechanism, which degrades performance by surface flashover and erosion of the passivation layer, eventually leads to sputtering of the gatesource metallization. The results are observed using a combination of optical, electron, and X-ray micrographs, plus MESFET terminal parameters.IEEE 1992 Microwave and Millimeter-Wave Monolithic Circuits Symposium 1