- Buyers Guide
High Voltage, Low Cost FETs for HPA MMIC Applications
M/A-COM is a longtime supplier to the military, Navy, Army and Air Force of GaAs MMICs, employed in active phased array radars. To date, the company’s GaAs process of choice has been its Multifunction Self-aligned Gate (MSAG™) process. MSAG features selective ion implantation with noncontacting stepper lithography to provide a combination of excellent FET performance, very flexible circuit functionality, outstanding reliability and low manufacturing cost. Having been in continuous production since 1986, MSAG is a mature and well-characterized process.
The MSAG power FET produces 0.8 W/mm power density at 65 percent power-added efficiency (PAE) while operating at 14 GHz and 10 V drain voltage. Using this process, the company supplies some of the highest power GaAs MMICs available. The practical power limit for a MSAG high power amplifier (HPA) is set by the ability to implement an efficient load line matching circuit for a given amount of FET gate periphery. The load line impedance is approximately proportional to voltage, therefore, the matching problem is simplified at higher voltages for higher power applications.
Sometime ago, the Navy identified a need for more MMIC power than could be achieved using a 10 V GaAs process. This need is a part of the drive toward developing wide band-gap semiconductors, which are capable of operating at much higher voltages, with better thermal characteristics than 10 V GaAs-based devices.
M/A-COM has been engaged for several years in exploring higher operating voltages in GaAs, using derivatives of the MSAG process. For the past 18 months, the company has been under contract to ONR/MDA (N00014-020C-0453, $1.8 M) to develop and demonstrate the practicality of high voltage MSAG (HVMSAG™) devices for S-band HPA MMICs.
HVMSAG developments on the ONR/MDA program have been very positive and are leading now to the development of high voltage HPAs for commercial applications. The work is protected under M/A-COM’s US Patent 6,005,267, issued December 21, 1999, covering a MES/MIS FET with Split-gate RF Input, and US Patent 6,559,513 B1, issued May 6, 2003, for a Field Plate MESFET.
Although there is no doubt that wide band-gap semiconductors offer advantages in terms of power density compared to GaAs, the hallmark of the HVMSAG process is that no new infrastructure development is required. HVMSAG is manufactured on a mature process line, thus the manufacturing cost is low and the reliability outstanding.
The HVMSAG process is designed to operate at 24 V. The S-band power density of a typical HVMSAG FET at 3.5 GHz under CW conditions is approximately 1.5 W/mm and its PAE is 65 percent, as shown in Figures 1 and 2, respectively. Accelerated operational life testing results shown in the Arrhenius plot of Figure 3 reveal outstanding thermal robustness with an MTTF of more than 106 hours at 150°C channel temperature. The actual channel temperature depends on the HPA design and chip environment. HVMSAG is available on either 50 or 75 mm substrate thicknesses. Obviously, more care in thermal design is required when operating at 24 V versus 10 V but reliable operation of HVMSAG HPAs of much higher power than using 10 V GaAs processes is achievable. Figure 4 shows an HVMSAG HPA mounted in a test fixture.
While leveraging a transistor topology that is fabricated using the proven MSAG process, the HVMSAG high voltage FET device has been developed to provide higher power density at higher drain voltages, while maintaining all the cost, reliability and repeatability inherent in MSAG MMIC-based solutions. Table 1 compares the capability of both the MSAG and HVMSAG processes for high power S-band amplifiers. The greater power density and higher voltage of the HVMSAG process reduces chip size, lowering costs in high volume production, and halves the DC current, reducing the amount of copper in the system bus networks.
The outstanding power performance of the HVMSAG process has positioned M/A-COM as a leading contender for the Navy’s next generation S-band active phased array radars. In addition, many commercial and defense applications could benefit from this breakthrough development. HVMSAG devices are capable of more output power per area of GaAs. The resulting FETs permit the manufacture of more compact MMICs with a lower cost per watt. The resulting load line has a higher impedance for a given power and is easier to match. The new devices also simplify bias circuitry.
Figure 5 shows a typical phased array transmit element that utilizes MSAG and HVMSAG devices. The average power and PAE of the three-stage driver amplifier are shown in Figure 6. This part was eutecticly mounted in a connectorized fixture and tested at 25 percent duty cycle with a drain bias of 24 V. No matching external to the MMIC was applied in achieving these results. The small-signal gain, and input and output return losses of a single amplifier are shown in Figure 7. The IC has better than 2 dB gain flatness over a 28 percent bandwidth and excellent input VSWR. The output VSWR is as expected for a power matched application.
MMICs such as these should find use in multiple defense and commercial applications. Additional information may be obtained by contacting the company.
M/A-COM Inc., a Tyco Electronics company, Roanoke, VA (800) 366-2266.