Millimeter-wave ICs Open Up New Spectrum
The market for applications that rely on millimeter-wave ICs has grown substantially in recent years. Increases in performance and reductions in cost will ensure that the range of applications continues to expand, writes Al Lawrence of TRW Ventures.
In the last decade there has been tremendous growth in applications that use the millimeter-wave region of the spectrum. Critical to this growth has been the development of low-cost, mass-producible compound semiconductor integrated circuits operating in the millimeter range (30300 GHz). Millimeter-wave ICs are currently used in a diverse set of applications including terrestrial and space-based telecom systems, scientific instruments, passive imaging and transportation products. In the future there will continue to be many drivers in the market-place motivating manufacturers to make use of this higher-frequency regime, including the demand for ever-higher bandwidth communication links, improved transportation safety systems and advanced scientific observation systems. The millimeter-wave portion of the spectrum is of interest for a variety of reasons. First, it offers new operating regions ideal for high-bandwidth, high-capacity communications systems. In addition, the millimeter-wave spectrum contains frequency bands suited to both limited-range and long-range communication links. Figure 1 illustrates the wide signal attenuation range that can be obtained with a relatively small change in operating frequency. While atmospheric attenuation is 15 dB/km at 60 GHz (V-band), the corresponding figure at 94 GHz (W-band) is only 0.4 dB/km. This makes the V-band highly suitable for short-range transmissions where interference from other nearby V-band transmitters can be minimized since signals are quickly attenuated. Conversely, the W-band is ideal for transmitting high-data-rate signals extremely long distances. For applications requiring narrow antenna beams or high spatial resolution in a compact size, millimeter-wave solutions work well thanks to their smaller wavelength (110 mm), which permits the use of small-diameter receiver and transmitter elements. The millimeter-wave portion of the spectrum also contains regions which have reasonably good propagation characteristics in all types of weather, as well as absorption lines and other properties which can be exploited for a variety of scientific studies (see ). Performance up, costs down Efforts throughout the compound semiconductor industry continue to advance millimeter-wave MMICs. Developments in materials, processing technologies and circuit-design techniques have pushed MMIC performance and reliability up while at the same time driving costs down. As an example, TRW has developed and supports a wide range of device types and processes in the pursuit of millimeter-wavelength applications. For HEMT-based circuits, TRW has production MMIC processes using 0.15 m gate-length HEMTs suitable for applications operating at up to 60 GHz, and a 0.10 m process which can be used for MMICs operating through 94 GHz. With InP, integrated circuits to 220 GHz have been produced with a 0.07 m HEMT process. For reliable, flight-qualified, high-volume HBT-based circuits, a 2.0 m GaAs MMIC and digital process suitable for applications up to 20 GHz can be used, while a 1.0 m GaAs process can support applications up to 50 GHz. A 1.0 m InP process suitable for high-speed digital and RF applications up to 200 GHz is in qualification. When they reach production GaN-based devices should play an important role in high-frequency, extremely high-power applications. Space-based telecoms flying high Space-based telecommunications was one of the first important uses of millimeter-wave ICs. GaAs and, more recently, InP have been used extensively in satellite payloads to meet size, weight, power, cost and reliability requirements. Microwave and millimeter-wave integrated circuits are used in payload hardware such as low-noise amplifiers (LNAs), receivers, frequency converters, frequency sources, transmitters, modulators, RF switches and beamforming networks. Strong growth is expected to continue in this market as numerous new systems are in production and additional ones are being planned. One significant emerging market is for new Ka-band satellite systems which have a downlink of approximately 20 GHz and an uplink of about 30 GHz, with systems addressing both the business and consumer markets. The success of these new systems relies on maximizing the capacity and minimizing the cost of the spacecraft and realizing very-low-cost millimeter-wave user terminals. Compound semiconductor ICs are key to all of these. Trends in space-based telecoms will see payloads evolving from fixed-beam systems to those with more flexible multibeam capabilities and even systems with some steerable-beam capability. There will continue to be efforts to lower the cost and enhance the capacity of transponder-type payloads. All of these will push MMIC technology toward lower-power, higher-performance, lower-cost circuits. High volumes, low costs The potential volume for the user terminal portion is projected to be in the millions. MMICs will be used extensively to realize the millimeter-wave transceiver portion of the terminal. A key component of the transceiver is a multiwatt power amplifier, with power levels ranging from 12 W for commercial applications to more than 10 W for large enterprise terminals. Initial products for this application can be implemented in GaAs. These terminals will also require very-low-noise amplifiers (< 1.5 dB) that will be critical to achieving desired antenna size GaAs and InP ICs are potential solutions for such LNAs. And in all cases, cost reduction is critical in order for them to be competitive, especially in the consumer market. In terrestrial-based telecommunications, the market for microwave and millimeter-wave digital radios has grown tremendously in the last several years. Volume production is already taking place at 40 GHz, and 60 GHz parts are ready for production. Demand is being driven by applications such as broadband wireless access, cellular backhaul, picocell mobile systems and wireless networking. The growth of these markets depends not only on system capabilities, but also on reducing the cost of the devices. These objectives are being pursued through advances in processing equipment and techniques, while new processes are also being used to reduce die size, simplify overall transceiver architecture and increase yields. In addition, the need for higher data rates will continue to drive not only the development of higher-linearity ICs, but also the use of more complex modulation schemes. Higher data rates can also be achieved by increasing operational frequencies above 40 GHz, and GaAs is well suited for bands up to 94 GHz. Furthermore, the enhanced capability of InP parts can be used to realize even higher operating frequencies, or can be traded off against other system requirements. The outstanding characteristics which InP-based HEMTs are able to deliver have been discussed previously (see Compound Semiconductor April 2000, p28), demonstrating their suitability for next-generation high-frequency systems. For example, power MMICs can deliver 200 mW with 40% PAE and 1000 mW with 25% PAE for 60 GHz operation, while 480 mW is obtained at 95 GHz with 20% PAE. When these InP HEMTs are inserted into an eight-way combiner (see ), a total output power of 2.2 W at 94 GHz operation is achieved, with a PAE of 9.8% and associated gain of 19.5 dB. The development of InP HBT technology has led to new options for designers of digital radios. InP HBTs show exceptional linearity and provide the best linearity performance for amplifiers operating at up to 44 GHz, achieving record IP3:DC power ratios. These HBTs are also well suited to the fabrication of very-low-noise VCOs operating over the 20100 GHz range. InP HBT device characteristics also allow extremely high-efficiency power amplifiers. Operating at 28 GHz, these devices exhibit a PAE of 60%, and when operated at 2.4 GHz a PAE of 94% can be obtained. In addition, InP HBTs are capable of operating at higher current densities than either GaAs HEMTs or GaAs HBTs, which enables the fabrication of smaller die-size ICs for a given power requirement, thereby lowering the overall cost per die. Instruments of observation Millimeter-wave technologies are also widely used in scientific instrumentation. A good example is in the implementation of Earth observation systems, which use microwave and millimeter-wave sensor electronics in devices such as radiometers, where imaging is achieved by exploiting the differences in the reflectance and emission properties of different materials. As an example, TRW has fabricated a cryogenic radiometer suitable for the range 77106 GHz. When operated at 20 K, the InP HEMT MMIC module delivers 32 dB of gain with an exceptionally low noise figure of 0.4 dB. shows an LNA MMIC that was designed by the Jet Propulsion Laboratory (JPL) and manufactured in an InP HEMT technology at TRW. The six-stage coplanar waveguide MMIC has a bandwidth of 65 GHz, and is employed in applications such as space-based Earth observation systems. Getting a clearer picture Passive millimeter-wave imaging exploits the differences in absorption and reflection of different materials to construct an image. As shown in , there is a minimum in the atmospheric absorption curve at around 94 GHz, making this an optimum frequency for seeing through fog, heavy rain and smoke. One example of a passive millimeter-wave imaging system is an all-weather aircraft camera system built by TRW. This first-generation millimeter-wave camera consists of a focal-plane array with 1040 receiver cards (see ), each containing 40 receiver modules which in turn hold four GaAs receiver MMICs. The seven-stage LNA MMICs were implemented in a GaAs PHEMT technology, providing 40 dB of gain with an 8 dB noise figure at 94 GHz. A second-generation camera is now being developed employing InP HEMTs to improve performance. The effectiveness of a millimeter-wave camera is shown in , in which a view of an airfield on a clear day is compared with the visual image on a foggy day when the visibility has been reduced to 50 m. When viewed through the millimeter-wave camera, the fog-shrouded airfield is as clear as the fog-free image. As suggests, take-off and landing systems in all-weather aircraft are a major application of this technology; others include television imaging through fog, rain and smoke; enhanced vision for fire-fighters in smoky environments; and land, air and sea navigational aids. Homing in on the transport market Transportation is potentially a huge market for microwave and millimeter-wave devices. Applications for vehicle safety and convenience could include collision warning, obstacle-detection systems, intelligent cruise control and predictive crash sensing, which in turn would allow air-bag inflation to be tailored to better protect the passengers. These devices could also be used in infrastructure applications and intelligent vehicle highway systems, including vehicle-to-vehicle and vehicle-to-infrastructure communications, as well as roadway sensing and obstacle detection. While the basic concept of automotive radar has been around since the 1960s, development did not begin until the 1990s when the costs of sensor technology and processing hardware began to drop substantially. MMIC-based millimeter-wave sensors offer a very effective solution for many of these applications; they are rugged, reliable and can be produced at low cost and in high volumes. In addition, the small wavelength at millimeter-wave frequencies minimizes antenna size and allows for an extremely compact sensor. Systems operating at 60 and 77 GHz are now being produced, with 94 GHz and higher-frequency versions expected in the future to further reduce the sensor size. Typical forward-looking radar systems need to operate over a 100 m range, and need about 1 resolution, which translates into a single lane width at 100 m. In addition, they require about an 8 field of view to be able to view adjacent lanes. Figure 6 shows a prototype radar sensor developed at TRW, which employs W-band transceiver MMICs. Clearly there exists a broad range of applications that can be satisfied through the use of millimeter-wave ICs. As the demand for these devices continues to grow, and costs are driven down, it is expected that even more potential applications will arise.