High Voltage, Low Cost FETS For HPA MMIC Applications

By Cobham

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 wellcharacterized process.

The MSAG power FET produces 0.8 W/mm power density at 65 percent poweradded 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 bandgap 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.

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Reprinted with permission of Microwave Journal from the December 2004 issue.