review research Navy unveils novel HBT
RESEARCHERS at the Naval Research Laboratory in Washington DC claim to have produced the first InAlAsSb/InGaSb DHBTs with an InAsSb emitter and sub-collector.
Introduction of this ternary emitter has led to significant improvements in DC and RF performance. This transistor, which is based on materials with a 6.2 Angstrom lattice constant, has a collector current density of 1.9 x 105
A/cm2 , a breakdown voltage in
excess of 2.5 V, and values for the cut-off frequency and maximum oscillation frequency of 59 GHz and 34 GHz, respectively.
According to corresponding author James Champlain, these results show that this particular DHBT could serve many applications demanding low powers, high frequency performance, or both. Examples include deployment in A-to-D and D-to-A converters; ultra-linear low-noise amplifiers; and radar and imaging systems, especially those operating at terahertz frequencies.
One of the strengths of the team’s transistor is its low power consumption, which is a major plus point in battery-powered, portable applications.
Solid-source MBE was used to form the DBHT epistructures that included a complex buffer (see figure for details). Standard processing and e-beam lithography techniques formed transistors from these epiwafers with a 2 x 10 µm2
emitter.
Characterization revealed base and collector ideality factors of 1.5 and 1.0, respectively. Series resistance was relatively low, thanks to the introduction of the InAsSb emitter and sub-collector layers.
Champlain says that the team will now work on improving DHBT performance through modifications to the device’s design layout and its material structure. This could involve reducing the area of the device, which should
“Currently the device’s design includes a quaternary collector,” explains Champlain. “Classically, mixed alloys, such as ternaries and quaternaries, have poor thermal conductivity compared to binary alloys. Therefore, alternative collector designs and/or process technologies, such as substrate transfer, may be needed to capture the full benefit of these devices.”
J.G. Champlain et al. Elec. Lett. 1333 46 (2010)
Ammonothermal yields high-quality semi-polar GaN
POLISH GaN substrate manufacturer Ammono has unveiled characteristics of its semi-polar (2021) substrates. This cut of GaN is a promising candidate for the production of green lasers. Last summer, engineers at Sumitomo produced a 531 nm edge-emitter by exploiting the relatively high indium incorporation in InGaN quantum wells grown on this plane, plus the built-in electric fields that push emission to longer wavelengths.
Working in partnership with Wroclaw University of Technology, Poland, Ammono has employed X-ray diffraction to probe its semi-polar material that is produced in a high-pressure ammonia solution. X-ray diffraction rocking curves on pieces of (2021) GaN, which has a typical dislocation density of 5 x 103
cm-2 and a
radius of curvature in excess of 100 m, produce a full width at half maximum of just 17 and 21 arcsec for the (2021) and (2020) peaks. “The best crystallographic properties and the lowest dislocation
Ammono is pioneering ammonothermal growth of semi-polar and polar GaN
density may suggest the best semi-polar GaN ever produced,” says Ammono president Robert Dwilinski.
Contactless electroreflectance has also been used to study semi-polar (2021) GaN. “With this technique, instead of measuring
the optical reflectance of the material, the derivative with respect to a modulating electric field is evaluated,” explains Dwilinski. This measurement yielded a sharp, strong resonance peak at 3.4 eV, indicating that the sample had both good optical properties and a good surface.
The piece of semi-polar GaN studied by the Polish researchers had dimensions of 9 mm by 12 mm. But far larger sizes should be possible, given that Ammono has already produced 1-inch GaN non-polar crystals that can yield semi-polar substrates of at least that size. Today the Polish company sells 10 mm x 10 mm, 10 mm x 20 mm and 13 mm x 15 mm substrates. “We will work to increasing the size of our semi-polar substrates to 1-inch in 2011,” says Dwilinski.
“This size is not available on the commercial market, and is hardly achievable by HVPE.”
R. Kucharski et al. Appl. Phys Express 3 101001 (2010)
November / December 2010
www.compoundsemiconductor.net 53
“Reduction of battery weight and extended lifetime are critical in applications where weight and space are limited, such as space-based applications,” says Champlain. “Alternatively, in large distributed systems, such as some large phased array or imaging applications where power must be distributed to each cell of the system, reduced power consumption is critical in achieving practical applications.”
Figure caption: The DHBTS incorporate InAs0.66
Sb0.34 layers that have “superb”
transport properties, and can provide an extremely low contact resistance when used for n-type contacts.
cut capacitance. Optimizing the selection of III-V layers offers another opportunity to increase transistor performance.
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