LDMOS Expands Its Niche To ISM And Beyond

In the semiconductor industry, each process technology is known for its own strengths and weaknesses. As a result, every technology targets different applications and often develops niches. RF laterally diffused metal oxide semiconductor (LDMOS), for example, is currently the dominant device technology used in high-power RF transistors and power-amplifier (PA) applications from 1 MHz to beyond 3.5 GHz. In a 12-page white paper, Freescale Semiconductor, Inc. emphasizes how the technology's strengths can be more widely leveraged.

The document is titled, "50V RF LDMOS: An ideal RF power technology for ISM, broadcast, and commercial aerospace applications." Its mission is to explain how the very same attributes that helped LDMOS replace bipolar transistors for cellular-infrastructure applications can now lead to its usage in the broadband RF power market. The paper asserts that enhanced-ruggedness LDMOS devices can displace the vertical MOS (VMOS) transistors and vacuum-tube devices that are still used in some applications in the following markets: Industrial, Scientific, and Medical; broadcast; and commercial aerospace. Specifically, the document delves into the firm's high-voltage sixth-generation (VHV6) platform, describing is device structure and enhanced-ruggedness variant.

In the RF-power market, the primary competitors are silicon vertical MOS andto a lesser extent28-V LDMOS. A performance comparison is provided for those technologies and 50-V LDMOS. LDMOS is shown to have superior gain and efficiency as well as device structure advantages. For example, deep-submicron self-aligned gates and shields reduce feedback capacitance.

LDMOS devices also offer thermal-resistance benefits, thanks to the fact that their backside source can be connected directly to the thermally and electrically conductive package flange (which is directly mounted to the heatsink). In contrast, typical VMOS devices have the drain on the wafer's backside. As a result, an electrically isolating flange material must be attached to the die to increase the structure's effective thermal resistance. Thanks to their superior thermal conductivity, 50-V LDMOS-packaged products also achieve significantly higher power density compared to the 28-V variant. In addition, 50-V LDMOS usually has 35% less output capacitance per Watt than competing 50-V silicon technologies.

Among the trends supporting a move by LDMOS into other markets is its increased frequency of operationranging to 3.8 GHz for 28 V and exceeding 3 GHz for 50 V. Also coming into play is the release of high-power multi-stage integrated circuits (ICs)discrete devices with integrated input and output matching networks. Finally, over-molded plastic has been adopted for high-power RF applications. This packaging is the lowest-cost option currently available, according to Freescale. Should readers have any doubts over whether the firm's LDMOS really provides the ruggedness promised, the paper closes by explaining how it tests and qualifies the 50-V LDMOS devices.

TAGS: Technologies
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