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LDMOS (Laterally Diffused Metal Oxide Semiconductor) is a key technology designed for 900MHz cellular communication systems. As the demand for wireless connectivity grows, LDMOS transistors are becoming more prevalent due to their improving performance and decreasing costs. In the future, they are expected to replace bipolar transistors in most applications. One of the main advantages of LDMOS is its higher gain—up to 14dB compared to 5–6dB for bipolar transistors. This allows PA modules using LDMOS to achieve up to 60dB of gain, reducing the number of components needed and increasing system reliability.
LDMOS also has superior robustness. It can handle three times the VSWR of bipolar transistors and operate at higher reflected power without damage. Additionally, it can tolerate input signal overdrive, making it ideal for digital signal transmission. The smoother gain curve of LDMOS supports multi-carrier amplification with less distortion. Its low intermodulation distortion, even near saturation, gives it better linearity than bipolar transistors. Moreover, LDMOS has a negative temperature coefficient, which helps prevent thermal runaway and ensures stable performance, with amplitude variation as low as 0.1dB, compared to 0.5–0.6dB for bipolar devices.
The structure of an LDMOS device is based on a dual-diffusion process. Arsenic and boron are implanted into the same source/drain region at different concentrations. After high-temperature annealing, the boron diffuses further under the gate, forming a graded channel. A drift region is placed between the active area and the drain to increase breakdown voltage. The drift region's low doping concentration allows it to withstand high voltages. A polysilicon field plate above the drift region helps reduce surface electric fields, further enhancing the breakdown voltage.
LDMOS manufacturing integrates BPT and GaAs processes, offering improved thermal conductivity and longer lifespan. Unlike bipolar transistors, LDMOS does not create hot spots when heated, making it more reliable under mismatched loads. Its gentle input-output curve at the 1dB compression point expands the dynamic range, beneficial for both analog and digital TV RF amplification. LDMOS is also linear in small-signal operation, minimizing distortion and simplifying correction circuits. Its zero DC gate current makes biasing easier, eliminating the need for complex temperature-compensated circuits.
Key parameters like epitaxial layer thickness, doping concentration, and drift region length determine LDMOS performance. Longer drift regions increase breakdown voltage but raise on-resistance and chip size. Balancing these factors is crucial for optimal performance. LDMOS excels in thermal stability, frequency stability, gain, durability, noise reduction, and linearity. It is especially suited for CDMA, W-CDMA, TETRA, and digital terrestrial TV applications.
Originally developed for mobile base stations, LDMOS is now used in HF, VHF, UHF broadcast transmitters, radar, and navigation systems. It offers high PAR, gain, and linearity, enabling faster data transfer for multimedia services. Over the years, Philips has continuously improved LDMOS technology. Their third-generation 0.8-micron ultra-low distortion LDMOS boosts linearity by 5–8dB, while the fourth-generation 0.6-micron process increases power density by 50% and W-CDMA efficiency by 6–8%. The fifth generation brings even greater improvements, achieving over 30% efficiency for W-CDMA and 17dB gain for PCS/DCS. With reduced thermal resistance and higher power density, it enables 150W CW operation in single-ended packages. These advancements make LDMOS a leading choice for next-generation RF amplifiers.