Wireless Systems Design

Cool RF Transistors Pack A Mean Punch

This device family guarantees much cooler base-station temperatures while decreasing cost.

Power-Amplifier (PA) and base-station manufacturers are faced with a series of similar problems. They both have a common interest in keeping base stations cool and minimizing cost. And they each have a desire for access to more than one independent provider of RF power transistors. Sure, these problems may sound miniscule in the grand scheme of things. But the truth is that until now, these goals have been difficult to meet in any substantial way. Traditionally, these base-station manufacturers have been serviced primarily by a single transistor provider.

Thanks to an announcement by Agere Systems, though, this picture may finally change. In fact, it could be altered in a way that's more dramatic than one might first think. The company has come to market with a family of 21 different silicon (Si) laterally diffused metal-oxide-semiconductor (LDMOS) RF power transistors (FIG. 1). This product family is best characterized by increased thermal and RF performance and minimized cost. As a result of this introduction, Agere is now a viable source for components manufacturing. PA and base-station manufacturers alike can benefit from this competitive offering that targets every band, every possible wireless standard, and a broad range of power levels from the outset. Undeniably, the result could only be faster time to market at a lower overall BOM cost.

These product offerings target 3G, 2.5G, and 2G base-station equipment. They enable wireless base stations that are cooler, smaller, more reliable, and less expensive than would be possible using any other RF power-transistor technology. Plus, they deliver lower capital and operating expenses for wireless service providers. As a result, Agere claims that its new product line may begin to save wireless service providers billions of dollars annually.

In addition to the cost savings realized, this product family could accelerate the industry's trend toward shrinking the size and shifting the location of typical base stations. Most base stations are currently about the size of a backyard tool shed. They are installed on the ground. In contrast, Agere's transistors may eventually give way to base stations that are the size of a suitcase and installed above the ground on wireless antenna towers.

This company has always had a strong commitment to this marketplace. "Over the course of the past two years, we have been in a stealth development mode, intent on improving Si device technology. Our main goal was to come to market with state-of-the-art silicon LDMOS power-transistor solutions," says Ozzie Lopez, Product Line Director at Agere Systems.

The company didn't want to just replicate what was already available, however. Instead, it sought to provide a more comprehensive, substantially improved offering. Its enthusiasm was matched by the actual technological breakthroughs that it was able to achieve in the process. Among these breakthroughs are Enhanced Ther-mal Resistance (ETR) and High Density Source (HDS).

ETR is a proprietary wafer-scale technique for thinning die (FIG. 2). It is both low in cost and high in yield. In addition, it eliminates the defects that have traditionally occurred when making ultra-thin silicon wafers. The resulting chips are not only thinner, but cooler as well. According to Peter Gammel, Chief Technology Officer of RF Power Products Processing in Agere Systems' Aggregation and Switching Business Unit, "ETR enables a die thickness of 40 µm. That's 50% thinner than all competing SiRF power transistors. This die thickness translates into a junction-temperature reduction of 10° to 15°C." ETR also promotes die lengths that are 30% shorter than other competing products. Best of all, the technique is scalable from 4- to 8-in. wafer diameters. Nor does it require new and expensive tooling to implement.

With the chips becoming so thin, heat dissipation gets much easier. Thinner, shorter transistors get rid of heat more effectively. This fact is crucial because the traditional method of removing heat—using fans—is not reliable. Often, it contributes significantly to noise pollution. Thanks to ETR, which lowers operating temperature by 10% to 15% over competing solutions, the use of fans may soon be a thing of the past. Manufacturers can now look to alternate cooling methods, such as natural convection.

The second breakthrough brought to the table by Agere is HDS, which is a technique for shrinking the source contact (FIG. 3). Rather than using a diffused source contact, the company sought to apply a unique interconnect technology to realize this structure. As Gammel explains, the result of this application was a "high-density source contact that increases the performance and power density of LDMOS. Agere's source contact is a factor of 10 smaller than is typical in the industry. It is independent of the properties of the EPI layer. Both gain and efficiency were increased using this technology. This source contact, combined with ETR, could allow Agere to one day design LDMOS power transistors with up to a 35% increase in power density over current offerings."

Combined with Si LDMOS technology, ETR and HDS enable the family of RF power transistors to achieve performance levels comparable to SiC and GaN semiconductor technologies. At the same time, they retain the cost, reliability, and maturity of established Si technology. ETR and HDS also effectively extend the life of LDMOS. The combination of these technologies allowed Agere to create transistors with reduced resistance and parasitic capacitance. With the addition of ETR and HDS, LDMOS technology shows no apparent signs of limitations with regard to wireless-infrastructure applications. This conclusion has significant cost implications, as it will allow the migration to new wireless standards to occur much more quickly. Also, keep in mind that LDMOS is a very mature technology. All necessary tooling is already in place. As a result, it will probably become too costly for people to not keep using it.

One of the key products in this product family is the 125-W AGR21125E. This high-voltage, gold-metallized transistor targets W-CDMA single- and multicarrier-class AB wireless base-station PA applications. As an N-channel, E-mode lateral MOSFET device, it operates in the 2.110-to-2.170-GHz range.

For a two-carrier, 3GPP W-CDMA system, its typical performance can be measured using F1 = 2135 Mz and F2 = 2145 MHz with a 3.84-MHz channel BW. The adjacent-channel BW is 3.84 MHz at F1 − 5 MHz and F2 + 5 MHz. The third-order distortion is measured over 3.84-MHz BW at F1 − 10 MHz and F2 + 10 MHz. The product's typical P/A ratio is 8.5 dB at 0.01% (probability) CCDF. Under these conditions, the device boasts an output power of 28 W, a power gain of 14 dB, an efficiency of 28%, and 125-W continuous-wave output power. Under the 3GPP test conditions, additional characteristics of this part are an IM3 of −36 dBc, an ACPR of −39 dBc, and a return loss of −10 dB.

The AGR21125E transistor is internally matched and comes with ESD protection. It boasts a low HCI-induced bias drift over 20 years. It is available in either a surface-mount or flanged package.

Agere's transistors are currently sampling and under evaluation by more than 20 companies. Production quantities are expected to be available in the third quarter of this year. In unit quantities of 10,000, pricing for the transistor products will range from $12 to $207.

Agere Systems
Room 10A-301C, 1110 American Parkway NE, Lehigh Valley Central Campus, Allentown, PA 18109; (610) 712-4323, FAX: (610) 712-4106, www.agere.com.

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