Navy electronics researchers push for new RF solid-state breakthroughs

By Edward J. Walsh

WASHINGTON — U.S. military researchers want to figure out new ways to fabricate advanced solid-state devices called wideband gap semiconductors, which experts say are necessary to design low-noise power amplifiers for future military radio-frequency (RF) systems.

Such a program, which would be a cooperative effort of the Office of Naval Research (ONR) in Washington and the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., would seek to develop cost-effective and reliable manufacturing processes for fabricating wideband gap semiconductors.

ONR experts say the program could resemble the U.S. Department of Defense Microwave/Millimeter Wave Monolithic Integrated Circuit program — better known as MIMIC — carried out from 1990 through 1995 to develop mature gallium arsenide solid-state devices

A new breed of power amplifiers using wideband gap semiconductor technology could combine with other breakthroughs in fast transistors to provide order-of-magnitude performance advances for communications, sensor, electronic warfare, and other high-performance RF systems for all the services, ONR researchers say.

The Naval Research Laboratory's Electronics Science & Technology division also is working on wideband gap semiconductors, based on gallium nitride, which NRL officials say support higher frequencies than silicon carbide.

Yet Navy solid-state research does not end with wideband gap semiconductors. ONR also is sponsoring research into transistors that support analog-to-digital converters and direct-digital synthesizers to generate signals that are free of background noise.

This research, at HRL Laboratories in Malibu, Calif., and TRW in Redondo Beach, Calif., could lead to transistors necessary for high-performance beam formers, filters, modulators, and low-noise oscillators that further refine RF signals for military systems, ONR experts say.

The Boeing Co., General Motors, and Raytheon Co jointly and equally own HRL Laboratories, formerly known as the Hughes Research Laboratories.

The HRL/TRW effort will lead to wider insertion of solid-state devices in RF systems, ONR officials say. They point out that new radars of all the services have been designed as solid-state systems. These include the Navy's multi-function radar and volume-search radar; and the high-power discriminator under development by Raytheon for future surface ships; the Army's theater high-altitude area defense system radar; and the Air Force's F-22 jet fighter radar.

In a separate but related effort, the ONR is funding a two-phase program called the advanced multi-function RF concept (AMRFC) that aims at developing devices to help integrate several different RF beams for future ships and aircraft. In this way, platform designers could eliminate electromagnetic interference and lower platform signatures by reducing the number of antenna apertures on board.

Lockheed Martin Naval Electronics & Surveillance Systems in Baltimore, Northrop Grumman's Electronic Sensor Systems Sector in Baltimore, and Raytheon Naval & Maritime Integrated Systems in Portsmouth, R.I., are participating in the phase 1 AMRFC work.

Need for vacuum electronics
The Army, however, following evaluation of competing solid-state and vacuum-based designs has selected a vacuum-electronics technology architecture for its new TYQ-47 Firefinder radar.

ONR officials point out that the transmitters of currently fielded RF systems use power amplifiers based on vacuum electronics technology such as traveling wave tubes. Yet receiver functions in these field systems are nearly all low-noise systems based on solid-state technology.

The services will continue to require solid-state and vacuum technology, insist Bobby Junker, head of the Information, Electronics, and Surveillance Department at ONR, and Gerald Borsuk superintendent of NRL's Electronics Science & Technology division, which oversees the Navy's vacuum electronics program. They say that systems designers must make decisions on the architectures of new military RF systems on considerations of cost, risk, and performance requirements.

Junker says that cost and performance tradeoffs always will be necessary to determine whether systems should adopt solid-state or vacuum electronics device technology. He adds though that ONR experts are working to "enable solid-state technology to reach a point where it is affordable" for RF components.

Both ONR and NRL officials emphasize that advanced solid-state and vacuum-electronics work aims at meeting the highly demanding performance requirements for the "front end" signal processing for military-unique RF systems.

They point out that commercial off-the-shelf (COTS) digital devices that are suitable for data-processing, analysis, and other information technology (IT) applications simply cannot meet front-end sensor requirements.

Junker says experts must distinguish between the services' needs for sensor technology on the one hand, and the computing technology under development by commercial companies on the other.

Many government and industry officials, Junker says, assume that commercial investments in electronics can meet service needs can be met by commercial investments in electronics. Furthermore, these officials often do not understand the need for investing in specialized high-performance devices for military applications.

"COTS data-processing devices are primarily available in frequencies under 1 gigahertz (GHz)," he says. "There's nothing available in COTS for RF signal processing for microwave and millimeter-wave applications. Commercial producers aren't addressing the military signal-processing requirements."

ONR has funded several different collaborative efforts with companies and university laboratories that aim at increasing the performance of solid-state devices. In the mid-1990s ONR established a program at the University of California-Santa Barbara (UCSB) to increase the switching speed of new transistors — referred to as heterojunction bipolar transistors (HBTs) — by reducing the capacitance caused by the semiconductor metal used in fabricating the device.

In 1995 UCSB scientists achieved a critical breakthrough when a researcher mistakenly left a device in an etching machine longer than planned. The process removed a portion of the semiconductor material on the device, which resulted in a dramatic increase in switching speed. In 1999 the program achieved a speed of 1,200 GHz, or 1.2 Terahertz (THz), and now is expected to reach 2 THz.

Silicon transistors will achieve a speed of 100 GHz in 2006, Junker points out, citing a study at the Semiconductor Industry Association in San Jose, Calif. Currently, he says, silicon-based direct-digital synthesizers are available commercially at speeds in the range of 300 MHz. DARPA, in a program now completed, achieved a speed of 900 MHz using gallium arsenide — the primary device material used in the MIMIC program.

The ONR-funded work has demonstrated a logic speed for HBTs of 82 GHz, says Max Yoder, director of the ONR's Electronics Division. The highest logic speed achievable with silicon is 1.4 GHz.

Junker stresses that the HBT program aims at developing a high-power, low-noise transistor for front-end signal processing components.

These components could include analog-to-digital converters — direct-digital synthesizers — that generate signals, as well as beam-formers, oscillators, modulators, and filters, for new military sensor, communications, and electronic warfare systems. He stresses, though, that military systems designers require such highly specialized devices in limited numbers.

The services will continue to require vacuum-electronics-based amplifiers for some time, because advanced devices such as the wideband-gap semiconductors required for low-noise amplifiers are not yet economically available, ONR officials say.

The cost advantage of vacuum devices is "hard to beat," Yoder says — particularly for applications such as mechanical steering of a single RF beam. Vacuum technology will continue to cost less than solid-state, he says, for the foreseeable future.

Junker says though that in terms of performance, solid-state devices will be superior to vacuum, even for single-beam applications. High-power vacuum-based amplifiers, Yoder adds, have been required for some transmitter applications because of inadequate receiver performance. He says that the introduction of more sensitive solid-state receivers that reduce noise levels while maintaining required operating capabilities, will permit reduced levels of transmit power. This, he says, will eliminate the need for the high-power vacuum-based amplifiers.

For future transmitter architectures, moreover, wideband gap amplifier modules fabricated with gallium nitride will result in lower costs because the system will require fewer modules. These modules are capable of generating eight to 10 times the power of gallium arsenide-based modules.

The systems approach for decisions on either solid-state or vacuum should be based on a development of a "programmable system," according to Junker. Although vacuum electronics devices generate more power then transistors, the key factor is overall system power and performance, not individual device power.

He says that while solid-state is not currently affordable for single functions, it will provide extremely high performance levels for all front-end components. The future solid-state architecture, consisting of the wideband gap and HBT technologies, will support not only communications, electronic warfare, and radar, but also guidance, target-illumination systems, and other RF systems.

In the commercial arena, ONR officials say also that commercial manufacturers of cellular telephones expect in coming years to introduce the capability to transmit as many as 100 separate signals on single beams.

Yoder says that such a capability, referred to as code-division multiple access or wideband code multiple access, requires high-power amplifiers based on solid-state wideband gap semiconductor technology to manage the highly linear discrimination among signals and avoid problems of cross-modulation. No wireless company, he says, is considering vacuum electronics for such applications.

The ONR-DARPA collaborative program under discussion will focus initially on cost-effective ways of producing new materials — primarily gallium nitride — for use in the wide-band gap semiconductors. Early development wideband gap devices built from gallium nitride have generated six watts per millimeter (total power per transistor) compared to 0.8 watt for gallium arsenide a large-area devices.

The program would build on work already underway through the ONR-led AMRFC phase 1 effort. The three AMRFC phase-1 participants — Lockheed Martin, Northrop Grumman, and Raytheon — now are developing gallium arsenide devices that would be used to integrate communications and electronic warfare functions.

The companies currently are working toward a phase 1 demonstration scheduled for late 2003. Phase 2 aims at using new wideband-gap semiconductors to build direct digital synthesizers, and would incorporate radar capabilities.


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