technology GaN electronics
rhombus4
Building on UltraGaN that feature diamond, an incredibly efficient heat spreader.
“We got excellent results in UltraGaN. It was a major Some progress has already been made, including the
success,” says Sylvain Delage, a researcher from the deposition of relatively smooth epitaxial layers directly on
Alcatel-Thales III-V Laboratory in France who is now all three orientations of the material.
leading the MORGaN project. “In America they are now
launching many projects using AlInN, based on all these Sensing the environment
results we obtained in UltraGaN.” He claims that one of Other partners in MORGaN are developing sensors that
the highlights of this project was the demonstration of an employ an AlInN HEMT to measure values of pH or
AlInN/GaN HEMT working at 1000
o
C, which is 200
o
C pressure. HEMTs can reveal levels of acidity, because
higher than an AlGaN-based equivalent. And another liquids impinging on the surface alter the chemical
success was the realization of a continuous-wave output potential within the device, leading to a variation in current
of more than 10 W/mm at 10 GHz, alongside a power- flow through the transistor. And measurements of
added efficiency of 56 percent. pressure are possible, because the piezoelectric nature of
the material means that strain impacts the concentration
These results were produced with a very simple, normally- of the 2DEG. Variations can be determined with great
on transistor architecture that is now being employed in precision by connecting this HEMT to three others in a
the MORGaN project. By lattice-matching the AlInN/GaN Wheatstone bridge configuration.
heterostructure, it is possible to form a 2DEG with
properties that are independent of the thickness of the One company that’s leading the development of both
ternary layer. “That means that we can make a HEMT with types of sensors is a start-up called MicroGaN, which
about 3 nm of AlIN,” claims Delage. This allows a metal spun out of the University of Ulm in 2002. The company
gate to change the surface potential of the material, and has already developed GaN-on-silicon cantilever
opens the door to the fabrication of a normally-off structures that are 200-400 µm wide, and 1 mm in length,
transistor, because the Schottky contact removes the and GaN actors, where cantilevers can be moved by
2DEG below it. electrical stimulation.
Below and
below right: Producing AlInN/GaN heterostructures is challenging, The new cantilever design will provide superior
F-cubic can because the ternary tends to phase separate and it is performance at high temperatures, according to
produce various difficult to form a high-quality interface between the two MicroGaN’s Ulrich Heinle. MicroGaN and its partners will
3-D structures materials. So to overcome these barriers to high-quality need to develop new processing steps for these elevated
with a high- AlInN growth, four strong epitaxial partners were set loose temperatures. “Traditional ohmic contacts are based on
precision, ink- on these issues: the MOCVD toolmaker Aixtron, titanium and aluminum, and these are not practical for
jet printing Germany; EPFL (Ecole Polytechnique Fédérale de temperatures above 500
o
C,” says Heinle, who explains
technique. This Lausanne), Switzerland; the Alcatel-Thales III-V lab, that a partner is working on copper-based contacts.
includes a set France; and FORTH (Foundation for Research and Another issue is that the current produced by this type of
of chess Technology – Hellas), Greece. Between them, they have sensor is not stable at 600
o
C, and researchers are
pieces, and one MBE reactor, which has provided excellent results for working to try to understand the degradation mechanisms
packages for AlN growth to demonstrate an AlN/GaN HEMT, and three and devise ways to address them.
housing AlInN- MOCVD tools, which have produced the better
based devices InAlN/GaN device results. The UltraGaN project used The GaN-on-silicon cantilevers that MicroGaN has made
operating at sapphire substrate for heterostructure optimization, while tend to bend slightly under no external pressure, due to
very high best power demonstrations were obtained on SiC
temperatures. substrates thanks to their superior high thermal
Credit: fcubic conductivity.
These teams - plus the University
of Bath, England - are now
focusing on the
development of
growth
technologies for
realizing high-
quality InAlN/GaN
heterostructures
on a variety of
platforms. These
include composites
28
www.compoundsemiconductor.net November / December 2009
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