RFMD® GaN Technology Meets Future Wireless
Market Demands - Today
By David Aichele, Director Business Development and Jay Martin, Design Engineering Manager, RFMD®

Introduction
Every so often, along comes a semiconductor technology that will leap onto the market and provide invaluable solutions to end customers not readily available in present (or future generation) incumbent semiconductor processes. Gallium Nitride (GaN) on Silicon Carbide (SiC) is such a semiconductor technology that offers improved performance based on superior material properties and better device parameters. GaN high power amplifiers (HPA) provide benefits of higher efficiency, greater bandwidth and lower thermal dissipation to end customers looking to incorporate the technology into current and next generation wireless platforms.

RFMD®, a leader in high volume III-V GaAs wafer fabrication, introduced a family of cellular wireless GaN high electron mobility transistors (HEMTs) power amplifiers for sampling in June 2006 and continues to expand its offering of GaN power amplifiers. Most recently, RFMD has developed a family of GaN unmatched power transistors consisting of RF3930-RF3934, which range in output power from 10W to 120W at 48V operation. This product family targets a broad customer and application base that can utilize the devices under constant envelope or even linear digital modulation techniques.

Market Perspective
The versatile RF393X (see Figure 1) GaN unmatched power transistor family addresses many of the growing semiconductor amplifier demands of today's wireless market. Applications include: Mission critical communications - public mobile radios (PMR), radar, jammers, military communications, cellular infrastructure, industrial scientific and medical (ISM), digital video broadcast (DVB) and general purpose amplifiers. Some of the demands the GaN unmatched power transistor family addresses include:

Greater Bandwidth: High-speed data rate transfers, single equipment platforms for multiple users and multiband operations requirements are driving development of new technologies such as ultra-wideband (UWB) and software defined radios (SDR). Original equipment manufacturers (OEMs) are seeking single RF devices with multi-octave and potentially decade bandwidth performance that would replace multiple RF devices. RF devices that offer tunable bandwidth at the board level will improve manufacturing logistics, provide economies of scale benefits and minimize inventory carrying costs for mass production and replacement spares.

Higher Efficiency: Increasing efficiency is an ongoing requirement from the market for applications such as cellular wireless, DVB and ISM. End users are looking to the OEMs to provide solutions that reduce both CAPEX and OPEX costs through goals such as 1) reducing size and site acquisition, 2) reducing battery backup, 3) reducing electricity demands. System architects are balancing the need to maintain/improve efficiency and still meet more difficult linear power specifications for modulation techniques like orthogonal frequency division multiplexing (OFDM) and wideband code-division multiple access (W-CDMA). This demand is placing an emphasis on identifying RF output amplifiers technology that will operate more efficiently in conventional or more complex linear architectures.

Higher Thermal Operations: Cellular base stations (BTS), tower mounted amplifiers (TMA), military communications and radar applications are driving the need for RF power amplifiers that will operate at higher junction temperatures greater than incumbent semiconductor processes. Improvements to the operating junction temperature will enable reductions in air conditioning requirements, capability to operate in non-cooled environments and greater margin for increased reliability, which will provide additional cost improvements to CAPEX and OPEX reductions.

All of these parameters and more are motivating technologists to consider better suited emerging RF semiconductor devices for today's and future system platforms.

Product Performance
The RF393X family of GaN high power unmatched power transistor products is poised to replace incumbent technologies such as LDMOS in some applications, simplify system designs for other applications, and also create entirely new opportunities to insert transistor technologies into system functions that could not be addressed by earlier technologies. This wide variety of prospects has been created by the outstanding performance of GaN on SiC. A few key parameters which demonstrate the strength of RFMD's GaN portfolio are high efficiency, wide bandwidth, very high power densities, 48V operating voltage and good linearity. Measurement results for a few of the device types that will populate RFMD's RF393X family are discussed below.

Bandwidth: Basic S-Parameter characterization of the GaN devices demonstrates high frequency operation with Ft's of 11 GHz and Fmax of 18 GHz as depicted in Figure 2. This range of operation allows each device within the RF393X family to address numerous customary bands of operation see example in Figure 1. In order to demonstrate this capability, these devices are being characterized at 900 MHz, 2.14 GHz, 2.5 GHz, and 3.5 GHz - a sampling of which is included here.

This wide range of frequencies at which RFMD's GaN devices can operate also opens up the possibility of creating switched or dynamically matched and filtered transmitter solutions for multiband operation, or even software defined radio solutions.

Output Power and Efficiency: Initial power and efficiency results as obtained from load pull characterization at 900 MHz and 2.14 GHz are summarized in Table 1 and Table 2, respectively. The devices were biased with 48V on the drain and were source and load pulled using a Maury automatic tuner system to determine the optimum operating impedances at each frequency. It is important to note that these results were obtained under CW (continuous wave) operation, so any deleterious effects of self-heating are included. Pulsed waveforms, depending on the specifics of the modulation scheme, are expected to improve these characteristics further.

Illustrating the excellent efficiencies achieved by these devices, the RF3933 is an 80 watt device which reaches 63% peak drain efficiencies at 900 MHz. The same device is capable of 74 watt operation and 67% peak drain efficiencies at 2.14 GHz. This compares very well to LDMOS alternatives, the best of which typically offer maximum efficiency in the low 50% range. The very healthy linear gain of these devices, which is greater than 22 dB at 900 MHz and greater than 13.5 dB at 2.14 GHz, allow for respectable power added efficiency (PAE) performance.

Power Density and Linearity: One hallmark of GaN devices is very high RF power densities. These GaN high power unmatched power transistors are no exception. The results shown in Table 2 represent power densities of more than 4 watts/mm.

These amplifiers also show good linearity with third order intermodulation distortion (IM3) results typically less than -30 dBc, even up to and beyond 10 MHz tone spacing, over most of the output power range. An example of this is shown in Figure 3, illustrating the RF3930 IM3 of less than -30 dBc up to 37.7 dBm output power (5.9 watts).

Summary
Early characterization results for RFMD's new GaN high power unmatched power transistors demonstrate much of the performance advantage promised by GaN technology for some time. The RF393X product family demonstrates high operating powers from 10 to 120 watts, excellent drain efficiency typically 5 or more percentage points higher than competing technologies, wide bandwidths and high operating temperatures. These characteristics combine to make these devices ideally suited to allow replacement of incumbent technologies for existing applications, simplification of transmitter topologies, and creation of new and unique transmitter concepts and form factors.

RFMD
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