Keywords: GaN ultraviolet photodetector, metal-semiconductor-metal, periodic trapezoid column-shaped patterned sapphire substrate, responsivity, UV-to-visible rejection ratio A GaN ultraviolet (UV) photodetector with a metal-semiconductor-metal structure is grown on a periodic trapezoid-column patterned sapphire substrate by metalorganic chemical vapor deposition. Under 5 V reverse bias, the photodetector fabricated on such a patterned sapphire substrate exhibits a lower dark current, a higher photocurrent, a much larger UV-to-visible rejection ratio, and a 476% enhancement in the maximum responsivity than those of the photodetector fabricated on a conventional flat sapphire substrate. These phenomena may all be attributed to the reduction in threading dislocation density and the greater number of photogenerated carriers caused by the improved quality of the GaN film, as well as the reflection and/or scattering of unabsorbed photons on the interface between the GaN film and the periodic trapezoid-column pattern of the substrate.
power level at 4.7 GHz is 10 dBm. The output spectrum of the modulator is shown in Figure 9.The measured unwanted sideband suppression is 22.9 dB. The typical amplitude error and phase error of the IF divider at 70 MHz is 0.42 dB and 0.19Њ. It shows the same result of the sideband suppression shown in Figure 3.The measured conversion gain of the modulator at 4.7 GHz is Ϫ11 dB, and the LO to RF isolations is 44 dB.
CONCLUSIONThis paper analyzes the influence of amplitude and phase error of different path on SSB used as an upconverter. Error variables equations for maximum output sideband suppression is also deduced. Simulation results show that higher sideband suppression for the SSB can be obtained when the error variables satisfy certain relationship as well as when they are highly consistent. The completed modulator shows the validity of the deduced expression. 2. A.H. Baree, I.D. Robertson, and J.S. Bharj, MMIC SSB frequency translators with image-rejection for satellite transponder applications,
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