Power Electronics ♦ news digest
This trimming of the defect density is revealed in cross-sectional images of the lower part of the epitaxial stack: The 50 nm-thick AlN nucleation layer, 200 nm-thick GaN and AlN stress mitigation layers, and the GaN buffer. The nucleation layer and lower GaN layer are riddled with defects, but many dislocations terminate at the interface between the second AlN layer and the GaN buffer, and a substantial proportion of those that propagate into this second buffer bend and interact within the first few hundred nanometres.
Estimations based on X-ray diffraction analysis suggest that the density of screw-type dislocations in the GaN buffer falls from 7.7 x 109 cm-2 to 2.1 x 109 cm-2 when the buffer thickness is increased from 0.9 µm to 1.7 µm. Resistance mapping of the HEMT epiwafers, which have a 28 nm-thick Al0.25 Ga0.75N barrier and a 2 nm-thick GaN cap deposited on the GaN buffer, show an average sheet resistance of 368 Ω /square.
Meanwhile, room-temperature Hall measurements reveal that the carrier density and mobility of the two-dimensional electron gas are 1.2 x 1013 cm-2 and 1350 cm2/Vs. Cool the sample to 90K, and mobility rises to 4290 cm2/Vs.
To determine the electrical characteristics of the buffer layer, engineers formed test structures with two ohmic contacts with a gap of 5 µm. A structure with a 1.7 µm-thick buffer produced a buffer leakage current of 2.6 x 10-4 mA/mm at 20 V and had a ratio between on-current and off-current of 7.3 x 106.
HEMTs with 0.3 µm T-shaped gates were formed on high-resistivity silicon. These transistors delivered a peak drain current of 768 mA/mm, produced a maximum transconductance of 190 mS/mm and exhibited a threshold voltage of -4.53 V.
Further details of this work have been published in the paper, “ Demonstration of AlGaN/GaN High- Electron-Mobility Transistors on 100-mm-Diameter Si(111) by Ammonia Molecular Beam Epitaxy”, by N. Dharmarasu et al in Applied Physics Express 5 091003 (2012). DOI:10.1143/APEX.5.091003
TriQuint wins GaN contract to triple RF PA performance
The firm will use its gallium nitride on silicon carbide experience to improve its power amplifier devices
TriQuint Semiconductor has received a $2.7 million contract from the Defense Advanced Research Projects Agency (DARPA) to triple the power handling performance of GaN circuits.
The Near Junction Thermal Transport (NJTT) effort will build on TriQuint’s advanced GaN-on-SiC technology and the reliability of its state-of-the-art RF integrated circuits.
“We are very pleased that DARPA selected TriQuint to develop this critical technology. Like other programs we have supported, NJTT will set the stage for substantial MMIC performance enhancements including reduced size, weight and power consumption while increasing reliability and output power,” says TriQuint Vice President and General Manager for Infrastructure and Defence Products, James L. Klein.
The NJTT initiative is the latest in DARPA’s overarching Thermal Management Technologies program. NJTT focuses on thermal resistance at the ‘near junction’ of the transistor die as well as the device substrate. These areas can be responsible for more than 50 percent of operational temperature increases. By combining its GaN-on-SiC process technology with diamond substrates and new thermal handling processes, TriQuint seeks to significantly reduce heat build-up to enable GaN devices that can generate much more power.
TriQuint’s partners in the program include the University of Bristol in the United Kingdom, Group4 Labs and Lockheed Martin. The University of Bristol is recognised for its leadership in thermal testing, modeling and micro Raman thermography. Group4 Labs is a pioneer in the use of diamond substrates and has worked with TriQuint to demonstrate diamond’s potential as a substrate material. Lockheed Martin will evaluate the results of the program for its projected impact on future defence systems.
TriQuint has pioneered GaN technology since 1999 and is currently working on multiple process and
October 2012
www.compoundsemiconductor.net 107
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