Toshiba Announces Gallium Nitride Power FET With World's Highest Output Power in X-band

Toshiba Corporation today announced development of a gallium
nitride (GaN) power field effect transistor (FET) that far surpasses
the operating performance of gallium arsenide (GaAs) FET widely used
in microwave solid-state amplifiers for radar and satellite microwave
communications in the 8GHz to 12GHz X-band frequency range. The new
transistor achieves an output power of 81.3W at 9.5GHz, the highest
level of performance yet reported at this frequency.

Toshiba realized this breakthrough performance enhancement by
optimizing the epitaxial layer and chip structures for X-band
operation. The result is a GaN power FET with six times the power
density of a GaAs FET and the world's highest output power at the
9.5GHz frequency level.

Toshiba has established manufacturing technology for GaN power FET
in the range of 50W and started to release samples. The company
expects to start mass production within the next six months.

Full details of the new GaN power FET and its technology will be
presented at the IEEE Compound Semiconductor IC Symposium (CSISC),
from November 12 to 15 (US time) in San Antonio, Texas, U.S.A.

Background and development aims

Ever increasing communications flows are driving demand for higher
output power in the amplifying devices used in radar and satellite
microwave communications. Until now, Toshiba has met this demand with
GaAs-based FETs offering 90W output power at 6GHz frequency and 30W at
14GHz.

However, balancing heat dissipation and performance
characteristics in high frequencies is a critical issue with GaAs, and
is reaching to the point that the material is fast approaching the
upper limits.

GaN shows great promise for application in high output power
amplifiers that support higher frequencies above the microwave
frequency band, since it offers higher saturation electron velocity,
higher dielectric breakdown voltage and a higher operating temperature
range than GaAs.

Toshiba initially directed its efforts at developing a GaN power
FET for the 4GHz to 8GHz (C-band), and last year announced a GaN power
FET with a power output of 174W in the 6GHz band. The company has now
built on its success in that area with structural optimization that
has achieved a device supporting higher X-band frequencies and that
achieves the highest power output yet obtained by a GaN power FET
operating at 9.5GHz. This progress will allow for integration of
devices for solid-state amplifiers and allow for their downsizing,
even while they deliver higher output power.

Toshiba is confident that this breakthrough opens the way even
higher frequency levels in the 12GHz to 18GHz (Ku-band). The company
will continue its development activities toward this.

Key features

1. Epitaxial layer structure

The FET adopts a High Electron Mobility Transistor (HEMT)
structure. By optimizing conditions of the composition and the
thickness of the AlGaN and GaN layers, Toshiba has achieved
outstanding performance.

2. Chip structure

Working with the epitaxial layer structure, Toshiba processed and
optimized the FET unit structure, including gate length and the
distance between the source and drain electrodes. This not only
assures heat dissipation but also high performance in the X-band
frequencies (9.5GHz).

3. Process and package

Toshiba's heat treatment technology achieves low contact
resistance at the source and drain electrodes, allowing maximization
of the GaN material characteristics. In order to produce high
performance at X-band, the FET requires a gate electrode under 0.5
micron meter. As a high voltage is applied, suppression of current
leakage at the gate electrode is essential for achieving high level
performance. A unique gate electrode structure and overcoat process
contributes to suppressing gate leakage to 1/30 that of today's
conventional technology.

4. Chip uniformity and output merge

The conventional approach to boosting output power in GaN devices
has been to fabricate a large chip with high power capacity in a large
package. Such devices ran very hot, which could cause components to
deteriorate and also damage the amplifiers into which the devices were
integrated. GaN is a difficult material to work with and achieve
uniformity of chip characteristics, and tends to suffer power losses
when combining the power of multiple chips in a package.

Toshiba's unique process technology achieves chip uniformity
across the wafer, while the company's advanced power management
reduces power dissipation inside the package. These approaches
successfully disperse heat and reduce potential component
deterioration, and support achievement of a high power output of over
80W in the X-band.

5. Stepper lithography

While electron beam exposure technology has been commonly used in
a lithography process for GaN power FET as the C-band and higher
frequencies, Toshiba has adopted stepper exposure that is better
suited to mass production for X-band FET, which require a gate length
of less than 0.5 microns.