A new approach to high power, multi-octave microwave amplifiers
using wide bandgap semiconductors is proposed. A team led by
Cornell University, including RPI and Northrop Grumman,
will develop new forms of amplifiers using unipolar GaN-based
transistors on SiC substrates. The team has related expertise
in related aspects of electrical engineering, applied physics,
materials science and industrial production. The eventual goal
of low cost production of high performance components requires
a minimum number of process steps which the proposed approach
addresses. The two classes of transistors which will be applied
to integrated amplifier circuits are the heterojunction field
effect transistor (HFET) and vertical ballistic transistor (VBT).
Both structures have been demonstrated by the team members to
exhibit world record microwave and millimeter wave operation.
At Cornell, record GaN HFET performances with ft of 40 GHz and
fmax of 99 GHz have been demonstrated in transistors with more
than 40V breakdown. Northrop Grumman SiC vertical transistors
with more than 450W UHV pulsed output power, and modules with
over 1 KW have been demonstrated. S band vertical transistors
with over 35W continuous output power have been produced with
yields beyond 50%.
The proposed amplifier approach will leverage these remarkable wide bandgap semiconductor transistor results by combining their best features. Fundamental to the high power performance of the SiC transistor is the very high thermal conductivity of the SiC substrate. The proposed GaN transistors will capitalize on this property using high resistivity SiC from Northrop Grumman for all amplifier fabrication. Further, the existing high yield vertical transistor process at Northrop Grumman will be adapted for GaN VBT fabrication. It is not clear whether the VBT or HFET will provide the best overall performance, or whether each device will find advantages of efficiency, linearity or output power over specific multi-octave frequency ranges. Both transistors will be developed and applied to amplifiers.
GaN epitaxial transistor layers will be grown by molecular beam epitaxy and organometallic vapor phase epitaxy at Cornell on bulk and epitaxial SiC from Northrop Grumman. These transistors will be compared to those grown on bulk GaN, with low defect densities, which will be grown with the support of another program. Extensive materials characterization will be directed towards eliminating sources of transistor noise and non-linearity, and boosting transistor output power and reliability.
Traveling wave circuits, with backward waves eliminated, will be implemented using unequal power splitters and combiners for each of five transistor amplifying sections, each capable of 20W at X band. Modules which demonstrate more than 100W at X band and 1KW at S band will be fabricated with optimized power added efficiency and linearity . On SiC substrates, 10-15W/mm output power will be achieved with > 100 Ohm-mm as the optimum load. Theoretical research will be based on physical concepts, and will be extended to device-modeling software. Electron transport in thin MODFET quantum wells will be jointly studied at Cornell and RPI, in addition to vertical ballistic electron transport. Technology transfer to Northrop Grumman of GaN growth, to Northrop Grumman and others in the devices and circuits area, and to anyone in the software of device and circuit models will be performed. It is anticipated that a commercial process for low cost high performance amplifiers will be the conclusion of the proposed study.
updated July 8, 1996