NEW ORLEANS — Lateral DMOS appears to be gaining on gallium arsenide (GaAs) and is substantially ahead of silicon germanium (SiGe) as the technology of choice for RF modules in cellular basestations. Ericsson's Components Group (Morgan Hill, Calif.) stressed the benefits of LDMOS this week at Wireless '99, the Cellular Telephone Industries Association's annual convention.
Motorola Inc. has been touting LDMOS since the Microwave Technology and Techniques Symposium last June. Philips Semiconductors (Sunnyvale, Calif.) is expected to make announcements on LDMOS technology at the Penton Wireless Symposium/Portable by Design conference this month.
LDMOS FETs, in which hot carriers are injected into a D-shaped tub of base oxide, are slowly replacing bipolar RF transmitters and receivers in the cellular basestations built by Ericsson, said Bengt Ahl, RF-power expert at Ericsson's Radio Access Labs (Gaevle, Sweden). The lateral structure of LDMOS — with its side-to-side (rather than vertical) current flow — offers better linearity and higher efficiency in the 1- and 2-GHz region than bipolar provides, according to Ahl. The reliance on gold-plated transistor contacts is reflected in Ericsson's name for its LDMOS devices: GoldMOS.
The power level for RF transmitters can very from 10 or 20 W to several hundred watts, Ahl said, and the low-noise receivers can consume up to 1 kW in their effort to capture weak signals. "We operate them 10 dB down, to give us some headroom for peak signals," he said. LDMOS appears to be more efficient than both bipolar and GaAs devices at those power levels.
"I'm letting our 'customer' speak for us," said Henrik Hoyer, vice president for RF power products in Morgan Hill. Ericsson's Radio Access Labs are a major consumer for the LDMOS devices, but the customer roster includes all the major basestation suppliers, Hoyer said.
Ahl explained that his choice of RF devices for cellular basestations embodied a trade-off among breakdown voltage, gain and efficiency. A higher breakdown voltage also tends to inhibit frequency response. Despite the popularity of GaAs devices in cellular handsets, their gain and efficiency do not appear to be as high as LDMOS' at basestation power levels. Further, Ahl said, GaAs power devices are considerably more expensive, and vertical DMOS transistors are only useful up to several hundred MHz.
Clifford Vaughan, director of marketing for high-speed products at GaAs supplier Fujitsu Components (San Jose, Calif.), confirmed that LDMOS is nosing out GaAs in cellular basestations operating at 800, 900 and, in some cases, 1,800 MHz.
"GaAs had the market [for 20- to 200-W devices] but lost it," he said. "The reason is simple: LDMOS is cheaper than GaAs. On a cost-per-watt basis, at 1-GHz LDMOS is one-half to one-third cheaper than GaAs."
And because LDMOS runs on single supply voltages, Vaughan said, "it is easier to handle."
But as the frequency ranges of digital cellular systems — especially PCS systems — extend to 1.8 GHz and beyond, the cost difference between LDMOS and GaAs begins to collapse, Vaughan said. Furthermore, LDMOS has more distortion than GaAs at frequencies above 2 GHz. And because of their high efficiency (especially with battery power supplies), GaAs seems better equipped to support the trend toward 5- and 10-W micro-basestations, Vaughan said.
The choice of GaAs or LDMOS may ultimately depend on the modulation scheme used by the basestation. LDMOS may support time-division multiple access and advanced mobile-phone service (AMPS); but more complex modulation schemes, such as code-division multiple access, require "super-linear amplifiers," said Vaughan. Achieving a 20- to 25-W linear transmitter amplifier in LDMOS for CDMA requires that the engineer build a 200-W amplifier and "then back it off," or operate it at lower power levels. To build a 20- to 25-W amplifier in GaAs, engineers must design for 60 W.
Thus, GaAs represents a better solution for third-generation (3G) systems operating in the 1.8- to 2.2-GHz region.
Both Fujitsu's Vaughan and Ericsson's Ahl acknowledged leakage-current issues with LDMOS. The open-base breakdown voltage on the LDMOS devices Ahl uses is lower than the supply voltage (24 V). Consequently, he used a low-value resistor both to protect the devices and to compensate for leakage current.
Ericsson hopes to address the problem of LDMOS power degradation. Because of the hot-carrier injection into the base oxide, the current-carrying ability tends to degrade over time, so that more voltage is required to deliver the same current. There is also a problem with the field strengths near the drain area of the FET. The solution may be expansion of the depletion area under the gate.
