Congratulations go out to
Toshiba for scoring a
real technological first
with its development of a
gallium-nitride (GaN) power fieldeffect
transistor (FET), designed for
the Ku-band (12 to 18GHz) frequency
range. What’s so special
about this transistor? It achieves
an output power of 65.4W at
14.5GHz, which is the highest
level of performance yet reported
at this frequency band. Full-scale
production is expected to start in
the first quarter of 2008.
Generally speaking, technological
advances in Ku-band
microwave amplifiers focus on
replacing the electron tubes with
semiconductors and, in particular,
GaN devices. What Toshiba
did with this particular transistor
is implement a structure that’s a
high-electron mobility transistor
optimised for Ku-band application.
Via hole technology played
its part in this development, ousting
source wire bonding. This
will cut parasitic inductance and
enhance circuit design for work
at Ku-band frequencies.
Toshiba’s development has
good timing. Industry trends for
satellite microwave communications
indicate a steadily increasing
demand for GaN power FETs
for both new equipment and the
replacement of electron tubes.
The reality is that rapidly
increasing communications
demands in satellite comms systems
pushes the need for higher
output power in signal amplifying
components. The dilemma here,
though, is that increased performance
is always coupled with
heat-dissipation challenges. This
is an area where GaN devices
provide a practical advantage
over more established technologies
like gallium arsenide.
Toshiba managed to improve
performance characteristics by
optimising the composition and
thickness of the AlGaN and GaN
layers formed on the highly heatconductive
silicon-carbide (SiC)
substrate of the HEMT structure.
Also, the company applied a
shorter gate length of below
0.3μm and optimised the shape
of each electrode and element
configuration to enhance heat dissipation
and, thus, boost performance
within Ku-band frequencies.
To reduce the parasitic inductance
and improve higher frequency
performance, Toshiba also
developed a special way of forming
via holes in SiC substrate—a
particularly demanding process.
Shortening gate lengths means
that suppressing current leakage
at the gate electrode is critical. In
this case, an overcoat process is
applied around each gate electrode.
It ultimately suppresses
gate leakage to 1/30 that of
Toshiba’s conventional approaches.
An electron beam exposure
technology is applied to secure
stable processing of gate lengths
below 0.3m.
Not sitting still, the company is
preparing to develop GaN technology
for microwave frequency
applications in 18 to 30GHz frequencies
and beyond.
