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Komljenovic T.,University of Zagreb | Babic D.,Group4 Labs Inc. | Sipus Z.,University of Zagreb
Journal of Lightwave Technology | Year: 2011

We propose and theoretically analyze a novel transmitter emitting m wavelength-division multiplexed and independently modulated optical signals. The transmitter is based on an extended cavity formed by m wavelength-agnostic reflective semiconductor optical amplifiers (one for each wavelength) on one end and a single modulation-averaging reflector on the other end of the cavity. We show that optimally designed modulation-averaging reflectors can efficiently change the intensity probability density function from bound bit-stream modulation to a normal distribution and thereby improve signal-to-noise ratio of all emitted optical signals. This improvement may potentially allow the realization of cost-efficient wavelength division multiplexing passive optical networks (WDM-PON). © 2011 IEEE.

Babic D.I.,University of Zagreb | Babic D.I.,Group4 Labs Inc.
Journal of Heat Transfer | Year: 2013

Thermal analysis of planar and near-square semiconductor device chips employing angular Fourier-series (AFS) expansion is presented for the first time. The determination of the device peak temperature using AFS requires only a single two-dimensional computation, while full three-dimensional temperature distribution can be obtained, if desired, by successively adding higher-order Fourier terms, each of which requires a separate 2D computation. The AFS method is used to compare the heat spreading characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on silicon, silicon carbide, and synthetic diamond. We show that AlGaN/GaN HEMTs built using GaN/diamond technology can offer better than half the thermal resistance of GaN/SiC HEMTs under worst-case cooling conditions. Furthermore, we show that, if left unmanaged, an inherent and non-negligible thermal boundary resistance due to the integration of semiconductor epilayers with non-native substrates will dampen the benefits of highly conductive substrates such as SiC and diamond. Copyright © 2013 by ASME.

A method for integrating wide-gap semiconductors, and specifically, gallium nitride epilayers, with synthetic diamond substrates is disclosed. Diamond substrates are created by depositing synthetic diamond onto a nucleating layer deposited or formed on a layered structure that comprises at least one layer of gallium nitride. Methods for manufacturing GaN-on-diamond wafers with low bow and high crystalline quality are disclosed along with preferred choices for manufacturing GaN-on-diamond wafers and chips tailored to specific applications.

Chabak K.D.,Air Force Research Lab | Gillespie J.K.,Air Force Research Lab | Miller V.,Air Force Research Lab | Crespo A.,Air Force Research Lab | And 11 more authors.
IEEE Electron Device Letters | Year: 2010

We report on electrical characterization and uniformity measurements of the first conventionally processed AlGaN/GaN high electron mobility transistors (HEMTs) on free-standing chemical-vapor-deposited (CVD) diamond substrate wafers. DC and RF device performance is reported on HEMTs fabricated on ∼130-μm-thick and 30-mm round CVD diamond substrates without mechanical carrying wafers. A measured fT LG product of 12.5 GHz μ m is the best reported data for all GaN-on-diamond technology. X-band power performance of AlGaN/GaN HEMTs on diamond is reported to be 2.08 W/mm and 44.1% power added efficiency. This letter demonstrates the potential for GaN HEMTs to be fabricated on CVD diamond substrates utilizing contact lithography process techniques. Further optimization of the epitaxy and diamond substrate attachment process could provide for improvements in thermal spreading while preserving the electrical properties. © 2009 IEEE.

Cho J.,Stanford University | Li Z.,Stanford University | Bozorg-Grayeli E.,Stanford University | Kodama T.,Stanford University | And 5 more authors.
IEEE Transactions on Components, Packaging and Manufacturing Technology | Year: 2013

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates, including the high-thermal-conductivity diamond, are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of thermal resistances at GaN-diamond interfaces for two generations (first and second) of GaN-on-diamond substrates, using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the second generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN/AlN transition layer thickness, result in a strongly reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices. © 2011-2012 IEEE.

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