LDMOS has been derived from mainstream CMOS technology and has proven to be the technology of choice for base station, broadcast and radar applications for more than one decade. Read the article for today’s LDMOS technology for cellular infrastructure.

LDMOS (laterally diffused metal oxide semiconduc to r ) t echno logy was introduced into the RF Power market more than 10 years ago by both Motorola-Freescale and Philips-NXP as replacement

of bipolar devices. Nowadays LDMOS is considered as technology of choice for many RF Power applications like base station, broadcast and radar-microwave. LDMOS performance is continuously pushed to its limits realizing superb power and efficiency levels at ever increasing frequencies. In parallel research and development efforts are put into GaN technology. GaN has outstanding material parameters and is therefore seen as the RF Power technology of the future. GaN is already used in niche applications and may gradually develop as the technology to support advanced high efficiency power amplifier concepts.LDMOS has been derived from mainstream CMOS technology and has proven to be the technology of choice for base station, broadcast and radar applications for more than one decade. LDMOS is very reliable, rugged and cost effective in combination with best-in-class RF performance. There exist 3 LDMOS technology flavors optimized for operation at supply voltages of 28V, 42V and 50V respectively. For power applications below 1.5 GHz (VHF, UHF, ISM, Broadcast) the 42V and 50V LDMOS are the best choices; 42V LDMOS gives the highest broadband efficiency while 50V LDMOS gives highest power, both in combination with extremely good ruggedness. The 28V LDMOS is applied in products for frequencies up to 4 GHz and is mostly used for GSM-EDGE, W-CDMA, WiMax and S-band radar applications. New generations are every few years introduced on the

market (currently NXP is rolling out the 7th generation) with best in class RF performance. Efficiency is the predominant technology roadmap driver. Furthermore, the maximum operating frequency of LDMOS will continuously be increased, allowing more and more applications (e.g. the recently introduced WiMax bands) to be addressed with LDMOS. GaN is a wide band gap material and has superb material parameters, but still needs to mature as technology like CMOS to become well accepted and establish a reliable alternative. In their research programs, companies build upon proven technology features and application knowledge of Si LDMOS technology to work out how to make reliable and rugged GaN devices. Consortia are set up between universities are semiconductor companies to co-develop new GaN technology, e.g. NXP has set up a GaN program together with IAF and UMS, forming a European platform for wide band gap expertise. GaN’s large band gap enables power densities greater than for Si LDMOS technologies. The maximum operation frequency also exceeds LDMOS allowing operation far above 4 GHz.

LDMOS is today’s technology of choice for 3G applications, LTE, WiMax Base stations, Broadcast applications, ISM, and radar applications. Today’s LDMOS technology is produced in deep sub-micron fabs with gate lengths down to 300 nm, utilizing advanced CMOS equipment. Three LDMOS technologies are available for operation at supply voltages of 28V, 42V and 50V, respectively. LDMOS devices are mostly used for class AB and

Today’s LDMOS technology for cellular infrastructure

DR. STEVEN THEEUWEN, NXP SEMICONDUCTORS

LDMOS Transistors for cellular technologies and infrastructures

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(n-way) Doherty architectures (see Figure 1).

Doherty architectures are suited to realize high

efficiency levels for modern modulated signal types

with high peak to average ratios. New LDMOS generations (like NXP 7th generation) are optimized for Doherty operation and efficiency levels of 50% have been realized for W-CDMA signals (see Figure 2). Digital Pre-Distortion equipment is used to meet the system linearity requirements. LDMOS devices are very well suited to achieve the tough linearity requirements (see Figure 3 and 4) due to their stable frequency and temperature behavior. Mismatch conditions can be tolerated for LDMOS without degradation of performance (i.e. excellent ruggedness). For LDMOS several best in class home built models are available for large signal simulation. These models can be used for a large range of device sizes. A thermal node is included in these models as well. Good heat removal is essential for power devices since it can limit the power level of the device and can badly affect reliability. LDMOS makes use of the good conducting Si substrates and thinning of substrates, improved thermal flange materials, optimized die attach and thermally optimized layouts. A whole MMIC family has been developed with extra functionality like bias and temperature control. In general, for each transistor generation an equivalent MMIC product generation is released. In each subsequent generation both the transistor performance and the passive components are improved. The multi layer metal CMOS back-end is leveraged in LDMOS processes to produce high quality passives.

The quest for ever-increasing efficiencies results in

“Green” LDMOS Doherty amplifiers

Fig. 1. The Doherty reference design, with two BLC7G22LS-130 devices.

Fig. 4. The memory effect of the Doherty amplifier is measured up to an IF bandwidth of 30 MHz. The figure shows that the magnitude level of the residual IM3 is very low, which enhances pre-distortion capabilities.

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Fig. 2. CW power sweep at 2.14 GHz and Vds = 28 V. NXP's Gen7 technology improves the efficiency and gain of the Doherty amplifier, which makes the overall line-up more efficient.

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Fig. 3. Pre-distorted spectrum of the Doherty amplifier at Vds = 28 V.

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less lost power, which means "greener" amplifiers. Higher efficiency does not only mean better use of RF Power, but also reduces the energy necessary to remove the dissipated energy (cooling systems). Hence, improved efficiency helps twice, on amplifier- and system level, to reduce overall power consumption. NXP is one of the technology frontiers delivering both LDMOS and GaN devices to the RF power market: it’s LDMOS devices show state of the art efficiency values in combination with best in class ruggedness and reliability. Furthermore NXP is pioneering new high efficiency concepts for cellular infrastructure, like the 3-way Doherty concept and the fully integrated Doherty amplifier. All these advances at the technology and amplifier level contribute to the realization of a new generation of “green” amplifiers.

[1] F. van Rijs, S.J.C.H. Theeuwen, “Efficiency improvement of LDMOS transistors for base stations: towards the theoretical limit”, IEDM2006, pp. 205-208 (2006).[2] F. van Rijs, “Status and trends of silicon LDMOS base station PA technologies to go beyond 2.5 GHz applications”, RWS 2008, pp. 69-72 (2008).

References

[3] S.J.C.H. Theeuwen, H. Mollee, “S-Band Radar LDMOS Transistors “, Proceedings of the 4rd european Microwave Integrated Circuits Conference, EuMIC 2009, pp. xx-xx (2009). [4] S.J.C.H. Theeuwen, W.J.A.M. Sneijers, J.G.E. Klappe, J.A.M. de Boet, “High Voltage RF LDMOS Technology for Broadcast Applications”, Proceedings of the 3rd european Microwave Integrated Circuits Conference, EuMIC 2008, pp. 24-27 (2008).[5] S.J.C.H. Theeuwen, J.A.M. de Boet, V.J. Bloem, W.J.A.M. Sneijers, “LDMOS Ruggedness Reliability”, Microwave Journal, technical feature, April 2009, pp. 96-104 (2009).[6] R. Quay, F. van Raay, J. Kuehn, R. Kiefer, P. Waltereit, M. Zorcic, M. Musser, W. Bronner, M. Dammann, M. Seelmann-Eggebert , M. Schlechtweg, M. Mikulla, O. Ambacher, J. Thorpe, K. Riepe, F. van Rijs, S. Murad, L. Harm, and T. Roedle, “Efficient AlGaN/GaN HEMT Power Amplifiers”, Proceedings of the 3rd european Microwave Integrated Circuits Conference, EuMA 2008, pp. 87-90 (2008).

Dr. Steven Theeuwen, Device physicist. Device & Technology Innovation BU MMS / BL AMS /

PL RF Power- Base stations, NXP Semiconductors.

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