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 VCC. 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.