[Technology Report]
Multiple Transistor Types Vie For RF Power-Amplifier Sockets
While LDMOS power devices are going to low-cost plastic packages, GaAs HBTs are migrating to larger wafers to cut cost.
With a growing market for cellular and personal communications services (PCS) and third-generation (3G) mobile systems looming on the horizon, increased attention is being given to RF power amplifiers. To make more-efficient, higher-output, and smaller-size power amplifiers, designers are using various types of power transistors, including lateral-diffused (LD) MOSFETs, gallium-arsenide (GaAs) metal-semiconductor FETs (MESFETs), GaAs/InGaP heterojunction bipolar transistors (HBTs), gallium-nitride (GaN) high-electron-mobility transistors (HEMTs), and silicon-carbide (SiC) FETs.
LD MOSFETs are beginning to replace traditional silicon bipolar transistors in infrastructure applications, while GaAs HBTs have progressed significantly to make strong inroads into the wireless handset niche. Here, they're beginning to replace the popular MESFETs as viable alternatives. Other structures, like HBTs based on GaAs/indium gallium phosphide (InGaP) and GaN HEMTs, are starting to emerge as well. Plus, more exotic material like SiC has resulted in FETs that promise to deliver high-density RF power-amplifier solutions with better thermal conductivity. In short, several power-transistor topologies are now vying for RF power-amplifier sockets in the 800-MHz to 2.5-GHz wireless communications marketplace.
Readily available voltages of 26 V and higher in basestation transmitters are making life simpler for LDMOS power amplifiers. Bias-current and threshold-voltage drift issues that previously haunted this transistor structure for quite a while have been overcome.
As developers continue improving gain, efficiency, linearity, and reliability specifications, they're adapting to low-cost plastic packages to deliver maximum power for every dollar spent.
For example, Motorola Semiconductor has expanded the use of plastic casing over a wide range of output power levels. Although low-power LDMOS transistors used as drivers were the first to adopt plastic housing, this year Motorola extended that capability to high-power output stages. In addition to ensuring high-power capability in low-cost packages, Mo-torola guarantees low intermodulation distortion (IMD). This answers the demands of newer digital modulation schemes deployed in current and next-generation cellular systems.
In line with that strategy, Motorola recently took the wraps off of a high-power plastic part. The device has a 45-W peak envelope power (PEP) rating and 18.5 dB of gain at 945 MHz. Suitable for applications of up to 1 GHz, this device achieves −31-dBc third-order IMD (IMD3) and a power-added efficiency (PAE) of 41%.
To use plastic packaging effectively requires aggressive thermal management. Aside from using better packaging materials with higher glass-transition temperatures, Motorola relies on clever temperature sensing and compensation techniques to efficiently manage tens of watts of power dissipation. Proprietary sensing and compensation techniques have been implemented on the power chip to keep the die cool.
Inside the package, Motorola in-cludes LC matching networks to raise the input and output impedance of the transistor. But the company is only interested in making it easier for the designer to increase this impedance to 50 Ω or higher. "It's up to the designer to extend that high impedance to the load impedance. That will require less complex circuitry," notes Leonard Pelletier, applications support engineer at Motorola.
Because of user demand for guaranteed specifications, Motorola has streamlined its test and manufacturing methodologies. Meanwhile, as it begins to roll device versions based on its fifth-generation HV5 process, the company is making enhancements in terms of gain flatness, drift compensation, and linearization. High-power LDMOS devices based on the the process are expected to be released sometime next year. According to Motorola, these new devices will compete directly with GaAs FET modules in the high-power arena (Table 1).
Unlike Motorola, UltraRF is exploiting the low-cost and high-density benefits of low-temperature co-fired ceramics (LTCCs). According to UltraRF, a wholly owned subsidiary of Cree Inc., LTCC is comparable in cost to plastic packaging. Yet it provides the inherent benefits of ceramic material, and a smaller size for 3D structures. (Cree acquired UltraRF late last year from Spectrian, a producer of high-power RF amplifier modules for the wireless communications industry.)
But for those applications where size isn't critical, UltraRF offers an alternative alumina-based single-layer, thick-film solution. Although power amplifiers built on alumina substrates are larger than LTCCs, they're less expensive and faster to develop, claims the maker. For this scheme, UltraRF developed a proprietary method of mounting a large ceramic piece on copper to get the best thermal performance out of the power die (Fig. 1). This allows the power module to operate in the −55°C to 155°C temperature range. Reworking techniques have also been improved to accomplish higher production yields and keep production costs low.
Additionally, UltraRF is pushing the efficiency and linearity performance of high-power LDMOS transistors. "Linearity is a key requirement in multi-channel applications," says John Quinn, vice president of marketing and new product development. "For example, the modulation envelope in wideband-CDMA or W-CDMA is a complex signal. You can't afford to distort this signal. Good linearity is extremely important to keep distortion to a minimum," he adds.
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