INTEGRATED MISSION SOLUTIONS DD(X ... - Raytheon

CHIP TECHNOLOGY (continued)
Figure 4. Comparison of PHEMT, MHEMT and InP Materials
Starting in the early 1990’s with MESFET,
RRFC has migrated to virtually all PHEMT
for its present production. While PHEMT is
the present production process, RRFC has
been developing an advanced process
called metamorphic, or MHEMT. Figure 4
shows a comparison of PHEMT, indium
phosphide (InP) and MHEMT material structures.
InP devices get their outstanding
electrical properties from the high percentage
of indium (>50%) in the channel.
Typical PHEMT devices are limited to about
20% indium. The MHEMT device uses a
graded buffer layer to compensate the
strain caused by different lattice constants
between a high indium content channel
and a GaAs substrate. The result is a device
with indium phosphide performance on a
low-cost GaAs wafer. The performance of a
3-stage K-band LNA is shown in Figure 5.
Figure 5. Measured Results on 3-Stage MHEMT LNA
18 summer 2003
Raytheon is currently in the final throes of
bringing its metamorphic HEMT or MHEMT
technology to production status. The use of
an MHEMT device allows low noise amplifiers
to have approximately 0.5 dB less
noise figure at X-band than their PHEMT
counterparts. This improvement in noise
figure translates directly to improvement
in receiver sensitivity, which can improve
range and detectability for a given phased
array system.
Even as the MHEMT device is being
brought into production, RRFC is working
on the next generation of device for use in
major systems in the 2010 time period.
Figure 6 shows a multifunction circuit that
integrates digital circuitry with microwave
circuitry on the same wafer. This process,
called E/DpHEMT, uses multiple etch stops
to set the depth of gates for enhancement
and depletion mode FET devices. The
resulting MMIC chips can integrate several
disparate functions onto the same piece of
GaAs, greatly reducing the parts count and
assembly touch labor at the T/R module
assembly level. A type of device that is even
farther out in development time is the
gallium nitride device shown in Figure 7.
This new type of device will use different
materials other than gallium arsenide and
will be what is known as a wide band gap
semiconductor. Wide band gap semiconductor
devices can support much higher
AT25 pHEMT attenuator with
digital control logic
Microwave
Circuitry
Digital Circuitry 50%
area reduction possible
using E/D pHEMT
Figure 6. E/D pHEMT Multifunction chip
300 Å i-Al 0.2 Ga 0.8 N
i-AlGaN Spacer
0.3 µm i-GaN
0.1 µm AlN Buffer
SiC Substrate
high thermal conductivity
high power handling
Figure 7. GaN HEMT Device
large bandgap
large critical field
high breakdown voltage
high voltage operation
high saturation velocity
high drain current
bias voltages than GaAs and therefore are
capable of delivering much higher transmit
levels than present devices.
Raytheon RF Components continues today
to develop the technologies needed for
future defense systems built by Raytheon.
Using the semiconductor devices developed
at RRFC, Raytheon has the technology
capability to go from chips to ships.
- David Laighton