We present a rigorous approach for designing a highly efficient coupling between single mode optical fibers and silicon nanophotonic waveguides based on diffractive gratings. The structures are fabricated on standard SOI wafers in a cost-effective CMOS process flow. The measured coupling efficiency reaches -1.08 dB and a record value of -0.62 dB in the 1550 nm telecommunication window using a uniform and a nonuniform grating, respectively, with a 1 dB-bandwidth larger than 40 nm.
MANFRED BERROTH AND ROLAND BOSCHAbstract-An improved method to determine the broad-band sma ll· signal equivalent circuit of field effect tra nsistors (FETs) is proposed. This method is based on an analytic solu tion of the equations for the Y parameters of the intrinsic device and allows direct determ ination of the circuit elements at any specific frequency or averaged over a frequency range. The validity of the equivalent circuit can be verified by showing the frequency independence of each element. The method can be used for the whole range of measurement frequencies and can even be applied to devices e~h ibiting sc,·ere low-frequency e!Tects.
-'l'lte.applicallon of GaAs field effect transistors in digital circuits requires a vaild description by an equivalent circuit at all possible .;ate arid dl'ain bias voltages for all frequendes from de up to lbe GHz ra4ge. This paper describes an equivalent circuit which takes into account ihe galt current of positively biased transistors as well as lbe symme~rical natun of lbe devices at low drain voltages. A fast method to determine the elements of tbe equivalent circuit from mea· sured S parameters b presented which delivers for the first time very good agreement for all ope. rating points.
The authors present a germanium on silicon p-i-n photodiode for vertical light incidence. For a Ge p-i-n photodetector with a radius of 5μm a 3dB bandwidth of 25GHz is measured at an incident wavelength of 1.55μm and zero external bias. For a modest reverse bias of 2V, the 3dB bandwidth increases to 39GHz. The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly p-doped Ge buried layer, an intrinsic absorption region, and a highly n-doped top contact layer of Ge∕Si is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the 3dB bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with 5μm mesa radius the maximum theoretical 3dB frequency is 62GHz with an intrinsic region thickness of 307nm.
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