Exotic Materials Squeeze More Juice Out Of RF Power Amplifiers

Today's designers face an expanding array of transistor choices for use in RF power amplifiers in cellular communication gadgets and third-generation mobile systems. Gallium-nitride (GaN) high-electron mobility transistors (HEMTs) and silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) are mounting significant challenges to the ubiquitous lateral-diffused (LD) MOSFETs and gallium-arsenide (GaAs) HBTs.

Meanwhile, LDMOS transistors continue to make gains on the infrastructure front, where high voltages prevail, and silicon-carbide (SiC) FETs promise extra juice from smaller dies. Interestingly, even silicon bipolars are edging toward the low-voltage wireless-handset arena.

Seeing the advantages of the new compound semiconductor material in the microwave domain, several proponents of GaN RF transistors have emerged. Their dedicated efforts over the last few years are paying off. Developers have made substantial process improvements, producing high-quality material on larger substrates. Some devices have demonstrated high power, linearity, and density at high frequencies, with enough stability to make them commercially viable for wireless basestation transmitters. In short, GaN transistor technology is no longer a laboratory curiosity, but ready to roll into real production lines.

For example, Nitronex Corp. has combined the RF benefits of GaN with a novel way to fabricate HEMTs on larger, low-cost silicon wafers. Although earlier efforts focused on building these transistors on expensive 2-in. sapphire and silicon-on-insulator substrates, Nitronex has demonstrated the viability of growing GaN on mainstream silicon wafers. Toward that goal, it has developed a unique growth process called Pendeo-epitaxy, which uses the technique of proprietary metal organic chemical vapor deposition (MOCVD). This methodology has solved thermal-expansion and lattice mismatch problems, which are the major barriers to GaN's commercialization.

As a result, Nitronex has built GaN HEMTs on 4-in. silicon substrates (see "Multiple Transistor Types Vie For RF Power-Amplifier Sockets," Electronic Design, April 30, 2001, p. 56). These GaN microwave transistors exhibit 10 to 12 dB of gain with high efficiency and power density. Starting at 8 W and going up to 35 W of peak power in the 2.0-, 2.5-, and 3.0-GHz cellular bands, the devices operate from a 28-V drain voltage with ­3 to ­4 V on the gate as a pinch-off voltage.

According to the maker, the high current density and electron mobility of the transistor translates into higher output impedance. So it's easier to match these devices to 50 Ω, and they're less lossy.

Nitronex is trying to improve the reliability of metal-to-semiconductor interfaces, bond-wire connections, and packaging over the specified temperature range with respect to aging. The company hopes to take its GaN HEMTs into production by mid-2003.

Although initial HEMTs will be produced on 4-in. substrates, the developer is working toward scaling the growth of GaN by Pendeo-epitaxy on larger substrate wafers. For that, Nitronex is building a 6-in. production line in Research Triangle Park, N.C. The company also plans to create a second source for its units, but details were unavailable when this article was written.

According to Nitronex, the initial release will offer about 60-W continuous-wave (CW) power with a 28-V supply. Its power-added efficiency (PAE) will be nearly twice that offered by LDMOS transistors at the same power level. Also, the linearity will be better than LDMOS. The device will provide 50-Ω input/output matching. Although this HEMT will incorporate discrete passives inside of the package for impedance matching, the company plans to integrate passives on the die.

More Players: Recognizing GaN's benefits, and encouraged by recent improvements in GaN technology for RF applications, other companies have joined the race lately. Among them are Cree Inc., NEC Corp., and RF Micro Devices.

At last year's International Electron Devices Meeting (IEDM), Cree disclosed its GaN HEMT transistor, which hits 108-W CW at 2-GHz output. Its peak drain efficiency is 54%. Unlike Nitronex, which uses a silicon substrate, Cree's GaN microwave transistors were built on a semi-insulating SiC substrate, featuring a very high thermal conductivity (Fig. 1). Consequently, they dissipate very high levels of power in CW operation. According to Cree, with a 24-mm-gate width structure, the device has a power density of 4.5 W/mm.

"The demonstration of power levels over 100 W under CW conditions is a major step forward for this technology," says John Palmour, Cree's director of advanced devices. Still, more research is required to address the issues of reliability and repeatability for GaN RF devices, and to complete understanding of the degradation properties of the material.

As Cree continues to improve the quality of the material and the epitaxial growth, it has developed smaller HEMTs at 3.5 GHz that have the ability to deliver a CW power density of 9.3 W/mm. The pulsed power density reported for these transistors is 12.1 W/mm. Additionally, the developer has extended the reach of its GaN/AlGaN HEMTs to 10 GHz. At this frequency, it has obtained 38-W CW output power from its GaN devices on SiC substrate. The PAE for this transistor structure is 29%.

Unlike Cree, researchers at NEC's Photonics and Wireless Devices Research Labs have developed an SiN passivated AlGaN/GaN heterojunction FET (HJFET) on a thinned sapphire substrate that boasts 113-W pulsed power at 40-V drain bias, despite the lower thermal conductivity of sapphire. The improvement comes from the passivation, which suppresses drain current dispersion and surface traps to boost the power capability of the HJFET. These results, later presented at the IEDM, were attained from a 32-mm wide passivated HJFET with a linear gain of 6.8 dB at 1.95 GHz.

RF Micro Devices accesses this advanced technology by aligning with a key developer in the field. The company acquired RF Nitro Communications, a producer of advanced materials and power transistors for broadband wireless and fiber-optic applications. RF Nitro has developed prototypes of GaN power transistors that exhibit an order-of-magnitude improvement in power density compared with conventional GaAs transistors, and two orders of magnitude better than silicon, RF Nitro claims.

In fact, RF Nitro has readied AlGaN/GaN HEMT structures on both sapphire and SiC substrates. To cut cost, it's using sapphire substrates, which deliver output power densities of up to 2 W/mm at 10 GHz.

On the other hand, SiC-based GaN transistors can dissipate more heat, affording up to 6.6-W/mm power densities at 10 GHz. Hence, using a nominal gate geometry of 0.35 mm by 1.0 mm on sapphire and 0.35 mm by 1.5 mm on SiC, these devices can deliver up to 10-W CW at 10 GHz.

Meanwhile, Cree continues extracting more juice out of SiC MESFETs for high-power RF applications. From 14 W at 1.95 GHz, Cree researchers have boosted the output power of the MESFET to over 36 W at 3.5 GHz. Recent enhancements in surface passivation and purity of the substrate material has improved power density to 5.2 W/mm and PAE to 63%. Plus, key passive components have been incorporated in the process to realize wide-bandwidth power-amplifier MMICs with the latest SiC MESFET.

Using this MESFET structure, Cree built a 50-Ω two-stage class AB power amplifier as a monolithic-microwave IC (MMIC). The device generated a pulsed power of 36.3 W with a PAE of 20.6%. The supply voltage here was 55 V.

LDMOS Keeps Improving: While new wide-bandgap semiconductor materials are being tailored to handle the RF power-amplification requirements of emerging wireless basestations, backers of silicon-based LDMOS transistors continue to overcome drawbacks and move forward. Bias-current and threshold-voltage drift issues, which have haunted this transistor technology for quite a while, have also been licked. New biasing techniques further address these issues and alleviate the problems. Because power levels, linearity, and reliability get better each year, developers can now focus on ease of use.

For instance, as the power increases, the input and output impedances of RF power transistors decrease, making impedance matching more difficult. So, suppliers have incorporated LC networks inside the package to raise the input and output impedance of the device.

Recently, Motorola took an additional step by offering input matching elements via die integration. By aligning passive elements and offering a prematched input, an LDMOS device allows room for output matching within the size constraints of the package, according to Motorola. Designed to operate in the 921- to 960-MHz GSM band, it offers a 110-W output power at 53.5% efficiency.

UltraRF Inc., a subsidiary of Cree, also is migrating to a new architecture. It leverages a proprietary planarization process with a dual-layer gold metallization system for higher RF performance and enhanced reliability. Compared to the LDMOS-7 process, the LDMOS-8 process improves W-CDMA efficiency by 10%, drain current drift by 30%, and on-resistance R_DS(ON) by 50%. Maximum operating frequency rises from 6.5 to 8.5 GHz.

The first devices to result from this development will be 2.1-GHz discrete power transistors and corresponding power modules. Power levels will range from 10 to 300 W in both standard ceramic and integrated ceramic packages, says UltraRF.

As Xemod begins to roll out second-generation LDMOS devices, it also is optimizing the structure for better drift properties and higher efficiency in the existing bands. Third-generation devices with higher cutoff frequencies are in the works too.

Meanwhile, Xicor and PMC-Sierra have developed separate chips to give these RF power transistors another shot in the arm. Xicor has created a smart bias controller for LDMOS devices, and PMC-Sierra has readied a DSP-based digital-correction technique called PALADIN, for power-amplifier linearizer and distortion inhibitor.

Using a dynamic biasing technique, Xicor's bias controller cuts drift, keeping it below 3% over the LDMOS FET's lifetime. On one chip, the device integrates all necessary analog and mixed-signal functions to automatically control the gate bias voltage of an LDMOS transistor for regulating the output power of the RF power amplifier (see "Smart Biasing Keeps RF Power Amplifier On Track," Electronic Design, Jan. 21, 2002, p. 38).

Traditional feed-forward analog techniques were employed in power amplifiers to eliminate distortion and improve spectral efficiency in wireless basestation transceivers. This method uses more power and components for a higher price. In conjunction with a high-speed DSP, PMC-Sierra's PALADIN employs a digital-correction technique to cancel distortion and squeeze more power out of the amplifier (Fig. 2). In addition, it cuts power consumption and keeps the cost low. Also, the latest member, PALADIN-15, can handle a 15-MHz input bandwidth.

Targeting Handsets: Unlike infrastructure systems, cellular phones and wireless handsets run off of lower voltages and require moderate output power levels. So in this space, SiGe HBTs have emerged lately to battle en-trenched GaAs HBTs. Recent progress in frequency and power levels has motivated makers like IBM and SiGe Semiconductor to launch SiGe-based power amplifiers that can provide numerous advantages over existing devices.

For instance, IBM has readied three devices using its 0.5-µm SiGe biCMOS technology. The company believes SiGe HBTs herald a new design era in portable wireless communications devices. Beyond substantially improving thermal reliability, these amplifiers lend themselves to higher levels of integration. Consequently, unlike GaAs transistors, these devices include on-die bias control circuitry and other functions to simplify the designer's job.

Similar advances at SiGe Semiconductor have also motivated the developer to leverage the virtues of SiGe biCMOS technology for Bluetooth and wireless LAN applications. To that end, SiGe Semiconductor has developed a dual-mode, 2.4-GHz power amplifier that provides the required linear output power, high efficiency, and minimum current consumption from a miniature package (Fig. 3).

The device delivers up to 22 dBm of linear output power with an adjacent-channel power ratio of less than ­30 dB per 300 kHz. The current consumption of only 138 mA is attributed to clever power management and an efficient SiGe HBT structure that provides high thermal conductivity and a low junction temperature. Also, the amplifier offers a minimal gain variation over the temperature range. One device supports the power requirements of two different standards—Bluetooth and IEEE 802.11b.

SiGE Semiconductor is integrating the power amplifier with other front-end functions like a detector, an LNA, switches, and filters. The company is pushing the envelope to 5 GHz too.

Also in this race is the silicon bipolar transistor. Researchers at STMicroelectronics have refined a double-poly, self-aligned, RF bipolar process to generate bipolar transistors that work at up to 5 GHz. Plus, the process can embed a variety of passives and offers high breakdown voltage and high current handling capability from 900 MHz to 1.9 GHz. According to the re-searchers, trench isolation and optimized die layout minimize parasitic effects, resulting in a high power efficiency. The product matches the performance of GaAs devices with the cost, reliability, and flexibility of silicon, asserts the developer.

STMicroelectronics has demonstrated the viability of this technology by building a three-stage power-amplifier module that delivers an output power of 33.2 dBm with PAE of 50% at 1.8 GHz and 3.4 V_CC. In fact, exploiting this success, the supplier is now prepping a power-amplifier module for dual DCS1800/PCS1900 cellular bands with built-in 50-Ω input/output matching. Temperature-compensated bias control is included on-chip too. Now, the company is extending this technology to build a triple-band power amplifier, expected later in the year.

Meanwhile, Motorola continues to expand its portfolio of GaAs devices. In conjunction with process refinements for higher performance, Motorola engineers have crafted a design methodology called the high-impedance integrated power amplifier (HIIPA) for its GaAs-based enhancement mode heterostructure insulated-gate FET (HIGFET). By adding passive components onto a pc board along with the power-amplifier chip, the HIIPA offers a complete 50-Ω solution without the need for additional passives (Fig. 4). Additionally, the enhancement-mode HIGFET suppresses off-state leakage and eliminates the use of a negative voltage generator, resulting in a single-supply power-amplifier solution.

The first result of this concept is a quad-band, single-supply RF power-amplifier module for GSM, DCS, PCS, and GPRS handheld radios. In fact, the input-matching capacitors are integrated on this GaAs die, whereas the inductors are implemented using wire bonds of variable lengths. This allows the power-amplifier module to be housed in a 7- by 7-mm package with a profile of less than 1.11 mm. Motorola believes that this package sets a new standard in volumetric efficiency for RF power-amplifier modules.

To pack more functionality in a power-amplifier module, Motorola also is pursuing LTCC packaging technology, which should be unveiled later this year. Concurrently, the developer is shrinking geometries and modifying the process to push the frequency scale and achieve higher linearity and power efficiency.

Meanwhile, to maintain their lead, traditional GaAs HBT backers continue to push the performance envelope and lower cost by migrating to larger wafers. For instance, Raytheon RF Components has implemented an adaptive bias control technique, called PowerEdge, to maximize efficiency. By automatically adjusting amplifier bias current in accordance to the signal strength, PowerEdge minimizes power consumption and maximizes efficiency.

Like others, Anadigics also is tapping the cost benefits of 6-in. wafers for its GaAs/InGaP HBTs. Plus, it's exploiting these devices to build compact multi-mode RF power-amplifier modules for GSM/DCS applications.

Called PowerPlexer, these modules pack two GaAs/InGaP HBT power amplifiers, RF switches, integrated passives, and a CMOS bias controller chip to deliver one RF transmit engine for multiple bands. Although these modules presently employ discrete passives and bias control circuitry, the trend is to integrate these components on-chip and offer an integrated solution.