review research TriQuint ups GaN-on-silicon HEMT efficiency
TriQuint claims that it has broken the power added efficiency (PAE) record for GaN-on- silicon HEMTs operating at 10 GHz.
Transistors fabricated in the labs of this US chipmaker have delivered a PAE of 65 percent, a value that is very similar to that produced by the company’s commercial, state-of-the-art process on the superior platform of SiC.
Engineers at TriQuint point to improvements in epitaxy and device architecture for closing the PAE gap between GaN HEMTs on silicon and on SiC.
“Epitaxial material was designed to maximize the drain-source current while keeping a low leakage current and a high breakdown voltage,” revealed corresponding author Deep Dumka. “Buffer growth was optimized to minimize RF loss.”
Dumka told Compound Semiconductor that the device fabrication process was similar to
the company’s production process for GaN- on-SiC HEMTs. The GaN-on-silicon HEMTs were created by first growing a
GaN/Al0.26Ga0.74N/GaN stack on 4-inch silicon (111). Reactive ion etching (RIE) created AlGaN/GaN mesa patterns, and evaporation and rapid thermal annealing formed Ti/Al-based ohmic contacts.
E-beam lithography, a RIE-based low- damage nitride-etch process and evaporation of a Pt/Au metal stack defined 0.25 µm, T-shaped gates that inherently formed a field plate. A second field plate was added on top via optical lithography
UCSB reveals green laser secrets
Engineers at the University of California, Santa Barbara, (UCSB) have shown that AlGaN barriers hold the key to the growth of a high-quality active region for a green semi- polar laser.
The team’s investigation focused on devices grown on the (2021) plane of GaN, a cut that Sumitomo employed last summer to win the race for the first green laser.
While Sumitomo has said very little about the architecture of its active region, UCSB, in partnership with substrate supplier Mitsubishi Chemical, is now revealing some important findings about this light-emitting region on semi-polar GaN .
This partnership’s recent study involved a comparison of the material quality and device performance of green lasers built with three different types of active region.
Variants fabricated with 10 nm-thick barriers
made from GaN, Al0.05Ga0.95N and In0.03Ga0.97N, all produced a spontaneous peak emission wavelength between 520 nm and 540 nm.
All three laser designs featured 4.5 nm-thick InGaN wells, a 1.2 mm-long cavity and high- reflectivity, distributed Bragg reflectors. These mirrors that were formed by sputtering provide reflectivity at the front and rear facets of 97 percent and 99 percent, respectively.
Hioraki Ohta from UCSB admitted that the use of AlGaN causes inferior optical confinement due to its lower refractive index. “However, material quality, and in turn internal quantum efficiency, was much better.”
Fluorescence microscopy exposed the superior material quality stemming from the AlGaN barrier. Structures with GaN and InGaN barriers featured many non- luminescent triangles with sides of 100 µm or more, which are presumed to contain many non-radiative recombination centers. These triangles were absent in the structure with AlGaN barriers.
A laser with the AlGaN barrier produced 516.3 nm emission when driven in pulsed mode with a 0.01 percent duty cycle.
and Ti/Pt/Au metallization.
This pair of field plates cuts the peak electric field between drain and gate, leading to improved HEMT performance at high voltages.
At an input power of 20.8 dBm, TriQuint’s GaN-on-silicon transistor delivered a peak PAE of 65.6 percent, an output power of 33.9 dBm (6.1 W/mm) and a gain of 13.1 dB. This HEMT is also capable of producing 34.5 dBm (7.0W/mm), but this increase in output power comes at the expense of a reduction in PAE to just over 60 percent.
TriQuint’s GaN-on-silicon process is being developed with commercial interests in mind. “However, there is no firm date yet for production transfer of this process, since our GaN-on-SiC technology is meeting present demands” says Dumka.
D.C. Dumka et al. Electron. Lett. 46 946 (2010)
Researchers at UCSB have fabricated an 8 mW laser emitting at 516 nm. Credit: UCSB
Threshold current density was 30 kA/cm2, and the device delivered 8 mW at a drive current of nearly 1A and a 40 V operating voltage.
Ohta says that the team will now focus on further improvement of the active region through optimization of the structure and the growth process.
Y.-D. Lin et al. Appl. Phys. Express 3 082001 (2010)
August / September 2010
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