This paper describes a new low-pass filter topology based on tapered periodic structures. These filters exhibit interesting characteristics in terms of compactness, return loss, insertion loss, selectivity, and the suppression of spurious frequency bands. Hybrid prototypes with a 1-GHz cutoff frequency, based on a coplanar-waveguide technology, and using both low-cost and high-performance substrates, have been fabricated and measured. Spurious frequency bands can be suppressed to below 22 dB at frequencies up to 20 GHz. Passband ripples are negligible, and the return loss is better than 20 dB. A two-section filter has a length of 0.2 and exhibits a 120-dB/dec selectivity, while a six-section filter is 0.51 long and has a 560-dB/dec selectivity. A design procedure has been established. These filters are compatible with monolithic microwave integrated circuit technologies in which the capacitors can be realized as metal-insulator-metal structures.

The dynamical electrical properties of annealed low-temperature-grown GaAs layers moderately doped with beryllium are studied using photoconductive sampling. Picosecond electrical pulses are sampled using an interdigitated metal-semiconductor-metal structure. These time-resolved experiments supported by numerical simulation allow us to determine electron and hole mobilities of, respectively, 540 and 90 cm 2 /V/s. We also demonstrate that the free-electron trapping process is saturating under high-optical-density excitation. From these experiments, we get an electron trapping cross section of n =1ϫ 10 −13 cm 2 .

Terahertz radiation from CdxHg1−xTe samples excited by femtosecond Ti:sapphire laser pulses were measured by using an ultrafast photoconductive antenna manufactured from low-temperature grown GaAs. Terahertz fields radiated by the samples of all three investigated alloy compositions with x=0, 0.2, and 0.3 were of the same order of magnitude. No azimuthal angle dependence of the radiated signal was detected, which evidences that linear current surge effect is dominating over nonlinear optical rectification.

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