Monolithic microwave integrated circuits (MMICs) based on gal l ium arsenide (GaAs) have been a part of this industry for a quarter century. And Freescale Semiconductor, the former Motorola Semiconductor, has enjoyed a long history of product development in analog and digital integrated circuits (ICs) and high-power RF transistors. But, although the firm has not lacked for the technological capability, it has not been a part of the GaAs MMIC marketplaceuntil now. The company's bold entry into this competitive market comes with four initial GaAs MMICs and a commitment to grow this portfolio with additional devices covering different frequencies and delivering different power levels.
Although Freescale is principally known today for its exceptional position in silicon laterally diffused metaloxide- semiconductor (LDMOS) RF power transistors, its experience with compound semiconductor materials such as GaAs ranges back into the early 1990s, when GaAs field-effect transistors (FETs) and MMICs entered widespread commercial availability for use in RF and microwave circuits (see the sidebar, "Freescale and GaAs: A Long-Term Relationship," on p. 134). In 2007, the company introduced a line of "general-purpose amplifiers" based on InGaP heterojunction bipolar transistors (HBTs) and GaAs heterostructure field-effect transistor (HFETs) that has proven successful in spite of the dense competition in this arena.
As its first MMIC products, the company chose two performance-critical segments in wireless infrastructure, namely the low-noise amplifier (LNA) in the receiver and the power amplifiers in the transmit chain (Fig. 1). Target applications range from traditional macro base stations to repeaters and the femto cells that are likely to proliferate with the widespread adoption of very-high-data-rate technologies such as HSPA+ and LTE. Both technologies require excellent signal coverage in order to achieve their full potential.
The MMA20312B is a two-stage InGaP HBT power amplifier that is designed to cover 1800 to 2200 MHz, and deliver output power at 1-dB compression (P1dB) of +31 dBm at 2140 MHz, small-signal gain of 26 dB, and output third-order output intercept point (OIP3) of +45 dBm. It is designed for use in standard wireless base stations as well as repeaters and femto cells. This device has bandwidth broadband enough to be used in other wireless applications such as smart grid transceivers as well.
The MML09211H is an enhancement- mode pseudomorphic-high-electron- mobility-transistor (pHEMT) MMIC LNA designed to meet the need in receivers for exceptional noise performance in all types of wireless systems operating within the range of 400 to 1400 MHz. The device has a noise figure of 0.6 dB (Fig. 2), small-signal gain of 20 dB at 900 MHz, P1dB of +21 dBm, adjacent-channel power ratio (ACPR) of -40 dBc (at a 5-MHz channel offset), and OIP3 of +32 dBm at 900 MHz. Its frequency range makes the MML09211H well suited for applications ranging from wideband-code-division-multiple-access (WCDMA) base stations as well as in high-data-rate, LTE-based networks currently being implemented in the newly allocated 700-MHz band. Noise figure, often considered the most critical performance parameter for an LNA, is measured in a matched 50- test fixture so its specified performance may be achievable in the final application. Other ways to specify the noise figure parameter, such as device-level values that do not take circuit effects (parasitic from bond wireless and interconnections, for example) into account, result in low levels of noise figure that cannot be realized in practice, and are thus of limited benefit to the system designer.
The two other broadband MMIC amplifiers are equally suited for use as driver amplifiers in the transmit chain or as second-stage LNAs in the receive chain. The MMG15241H is a pHEMT device that covers 500 to 2500 MHz, with a noise figure of 1.6 dB at 2140 MHz, P1dB of +24 dBm, smallsignal gain of 19 dB, and OIP3 of +39 dBm. Device performance in specific parameters such as OIP3, noise figure, and return loss, can be optimized for specific applications with the use of external matching elements. Finally, the MMG20271H LNA covers 1500 to 2400 MHz, and has a noise figure of 1.8 dB at 2140 MHz, P1dB of +28 dBm, small-signal gain of 15 dB, and OIP3 of +42.5 dBm. Both devices exhibit excellent linearity with lower current consumption levels than typical HBT solutions. Detailed specifications for all four devices are shown in the table.
In conjunction with the launch of the four MMICs, Freescale is also supplementing its existing general-purposeamplifier (GPA) product solutions with a set of application boards and RF characterization data to showcase the product capabilities in the context of a comprehensive base station transmitter line-up. Specifically, the MMG3004NT1, MMG3005NT1, and MMG3006NT1 GPAs have been characterized for linearity as a function of output power for a variety of bias points. The result of these efforts shows that current consumption can be reduced by almost 50 percent from the initial data sheet conditions with little penalty in linearity. Output Intercept Point (OIP3) of the MMG3004NT1 GPA at various levels of drive current is shown in Fig. 3.
The company is also trending toward focusing on a comprehensive lineup of devices in an amplifier (for example), rather than looking at individual device performance. This is being done in response to customer requests to speed up development times and to leverage RF vendor system capabilities. For example, a typical lineup for a 2100-MHz asymmetrical Doherty amplifier includes an MMG3014N predriver GPA with 15 dB gain, followed by an MW7IC2240NB multistage driver RFIC with 30 dB gain. The driver stage is followed by an eighthgeneration (HV8) MRF8S21120H and MRF8S21240H LDMOS asymmetric Doherty amplifier, for total system gain of 59 dB. These reference designs are characterized with devices on the same circuit board, and are supported by evaluation boards, models, and other tools required to implement the complete transmit section.
The four new MMICs represent the first in what is expected to become a broad range of MMIC devices covering popular wireless bands at power levels that are well suited to systems ranging from macro base stations, to repeaters, femto cells, and other wireless infrastructure equipment. These four devices are expected to be available for general sampling by June 2010. Freescale Semiconductor, 2100 E Elliot Rd., Tempe, Arizona 85284; Internet: www.freescale.com/rfpower.
Editor's Note: Freescale, the Freescale logo are trademarks of Freescale Semiconductor, Inc., Registered with the United States Patent and Trademark Office. All other product or service names are the property of their respective owners. 2010 Freescale Semiconductor, Inc.
Freescale and GaAs: A Long-Term Relationship
Freescale's entry into the GaAs MMIC marketplace might come as a surprise since the company is known for its silicon LDMOS RF power transistors. To gain some perspective, Microwaves & RF spoke with Jim Norling, Director of Product Management at Freescale's Radio Frequency Division.
MRF: The obvious first question is probably "why GaAs," and the second is "why MMICs?"
Norling: It's hardly surprising that people in the RF and microwave industry associate Freescale with silicon LDMOS, since we've taken this technology from its creation by Motorola technologists in 1993 to near universal use in base station transceivers throughout the world. However, development of GaAs devices has been a major focus here for more than 20 years, and in fact our line of general-purpose amplifiers is based principally on GaAs HBTs. Our MMICs are a logical extension of this technology into devices that serve other parts of the wireless infrastructure transmit and receive chain.
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MRF: Tell us about development of GaAs at Freescale.
Norling: As your veteran readers probably know, our activities here in Arizona were once the Semiconductor Products Sector (SPS) of Motorola, Inc. Motorola spun off its semiconductor activities as a separate company in April 2004, including not just our RF products but other product technologies today offered by Freescale. Our GaAs development began when this compound semiconductor technology was being commercialized for high-frequency applications.
In 1991, we opened a 30,000 square-foot Class 10 GaAs foundry at our facility on the corner of Price and Elliot roads in Tempe, AZ, creatively naming it Compound Semiconductor One or CS1. We began with 4-in. wafers producing mostly low-power, lowvoltage metal-epitaxial-semiconductor field-effect-transistor (MESFET) devices for the nascent cellular phone market. As that market grew so did CS1's capabilities, which soon included GaAs heterojunction bipolar transistors (HBTs) and pseudomorphic high-electron-mobility transistors (pHEMTs).
MRF: Did development keep the same pace over the years?
Norling: CS1 grew with the cellular industry, and we added an enhanced pHEMT process and higher-breakdown-voltage (12 and 28 V) FETs. We ultimately were able to design and fabricate heterostructure field-effect transistor (HFETs), radio-frequency integrated circuits (RFICs) with integrated-passive-device (IPD) matching structures, PIN-diode switching arrays, and Darlington pair self-biasing general-purposes amplifiers (GPAs).
MRF: What achievements can you attribute to CS1?
Norling: More than 100 GaAs process and development patents can be attributed to the work accomplished at CS1. At its peak, it was one of the industry's largest GaAs foundries and included more than 30,000 square feet of wafer process area, employed more than 250 people, and performed more than 1500 six-inch wafer starts a week.
MRF: With all this capability, why is Freescale known mostly for silicon LDMOS?
Norling: I think there are several reasons. First, we have devoted significant resources to improving our LDMOS process that is currently in their eighth generation. LDMOS is well matched to the needs of base station transceivers, especially in the area of RF power generation, and our success has paralleled that of the wireless industry in general. The second reason has nothing to do with our technological capabilities in GaAs and everything to do with marketing. Freescale's introduction of GPAs in 2007 was really the first time that the company's GaAs capabilities came to light, and since then this product line has become extremely important for us. Introduction of our MMIC devices really puts our GaAs capabilities front and center.
MRF: I understand that the CS1 facility was closed in 2008. How has that affected your GaAs activities?
Norling: When Freescale exited the handset device market in 2008, we felt it best to close CS1 and use an external foundry. The main designers within CS1 were transferred to the RF Division, where they help in GaAs device development.