Recent advances in the performance of silicon-based lateral diffused MOS (LDMOS) power transistors have given RF power-amplifier (PA) designers a viable alternative to create competitive solutions for infrastructure equipment in the 900-MHz to 2.5-GHz spectrum. Besides improvements in efficiency, linearity, peak-power capability, and cost/Watt, the developers have licked the bias current drift and aging issues that plagued this transistor for some time. Consequently, it has displaced bipolars and is going head-on against gallium-arsenide (GaAs) FETs and other heterostructures.
While suppliers continue to enhance the LDMOS transistor's performance and lower cost by going to inexpensive plastic packaging, they're further ex-panding its share on this front. And suppliers are many.
As competition gets tougher in the wireless basestation sector, many more new, exotic wide-bandgap semiconductor materials have emerged on the scene to give RF PA designers more choices in this space. For instance, on the infrastructure front, several proponents of the gallium-nitride (GaN) high-electron-mobility transistor (HEMT) have announced plans to release parts for this application. They claim that GaN HEMTs will offer high output power with higher density, better linearity, and greater efficiency in the cellular bands.
The transistor also generates higher output impedance, making it easier to match. Although reliability issues are being tackled, some samples are already in the market. Plus, makers promise to ramp up production sometime next year.
Likewise, silicon-carbide (SiC) FET developers believe that SiC could meet the needs of RF PA designers. Cree continues to refine the transistor structure and extract more juice out of the SiC metal-semiconductor FET (MESFET). Using this device, the developer has demonstrated a 50-W, two-stage, class AB PA as an MMIC.
Meanwhile, recent improvements in frequency and power levels in silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) have motivated supporters to launch RF PAs for cellular phones and wireless handsets, where power levels are moderate and voltages are lower. In this space, the SiGe transistor is going directly against the entrenched GaAs HBTs.
Aside from power performance, SiGe transistors lend themselves to integration, a major advantage over GaAs devices. Hence, developers also are leveraging this virtue to pack more RF functions on the same SiGe transistor die, further simplifying the RF PA designer's job.
The list of transistor choices for RF PAs is expanding as time goes on. Now, improvements in the silicon bipolar process have resulted in low-voltage RF bipolars for this application. Backers claim that bipolars will match the performance of GaAs devices, while delivering the cost, reliability, and flexibility advantages of silicon.
As competition heats up, GaAs power transistor makers also are growing aggressive. In addition to polishing their wares, they're migrating to larger wafers to cut cost. Concurrently, they're leveraging advances in packaging to maintain their lead.
Obviously, the designers are confronting a wide array of power transistors. But stimulating them to employ RF power transistors based on new semiconductor materials won't be simple. Beyond delivering performance and ease of use, new suppliers must ensure capacity and reliability, and the sources must be multiple.
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