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At frequencies below 2 GHz, conventional microwave absorbers are limited in application by their thickness or narrow absorption bandwidth. In this paper, we propose and fabricate an ultra-thin broadband active frequency selective surface (AFSS) absorber with a stretching transformation (ST) pattern for use in the ultrahigh-frequency (UHF) band. This absorber is loaded with resistors and varactors to produce its tunability. To expand the tunable bandwidth, we applied the ST with various coefficients x and y to the unit cell pattern. With ST coefficients of x = y = 1, the tunability and strong absorption are concisely demonstrated, based on a discussion of impedance matching. On analyzing the patterns with various ST coefficients, we found that a small x/y effectively expands the tunable bandwidth. After this analysis, we fabricated an AFSS absorber with ST coefficients of x = 0.7 and y = 1. Its measured reflectivity covered a broad band of 0.7–1.9 GHz below −10 dB at bias voltages of 10–48 V. The total thickness of this absorber, 7.8 mm, was only ∼λ/54 of the lower limit frequency, ∼λ/29 of the center frequency, and ∼λ/20 of the higher limit frequency. Our measurements and simulated results indicate that this AFSS absorber can be thin and achieve a broad bandwidth simultaneously.

The electrical properties of n-Ge metal-oxide-semiconductor (MOS) capacitors with HfO2/LaON or HfO2/La2O3 stacked gate dielectric (LaON or La2O3 as interlayer) are investigated. It is found that better electrical performances, including lower interface-state density, smaller gate leakage current, smaller capacitance equivalent thickness, larger k value, and negligible C-V frequency dispersion, can be achieved for the MOS device with LaON interlayer. The involved mechanism lies in that the LaON interlayer can effectively block the interdiffusions of Ge, O, and Hf, thus suppressing the growth of unstable GeOx interlayer and improving the dielectric/Ge interface quality.

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A convenient fabrication process about the ultrathin broadband composite microwave absorption material (MAM), which involves a frequency selected surface (FSS) sandwiched between two different magnetic sheets, is presented in this paper. The FSS consists of an array of elements with the geometry of square-loop having a split in each edge (denoted as Edge-split Square-loop FSS). The top and bottom magnetic sheets were prepared based on silicon rubber filled with sphere-shaped and flake-shaped carbonyl iron powders, respectively. It is revealed by measurement and simulation results that the composite MAM with the FSS which induces multiple resonances has a broad operating bandwidth compared with the traditional magnetic MAM without FSS. And the low-frequency resonance can be manipulated by adjusting the size of the split S, while the high-frequency resonance is generally invariable. The composite MAM with the thickness of 2.2 mm has an operating bandwidth with the reflectivity below -10 dB in the frequency range of 4-18 GHz, which is nearly 2.7 times the bandwidth of the traditional magnetic MAM.Index Terms-Frequency selective surface (FSS), microwave absorbing materials, reflectivity.