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LDMOS (Laterally Diffused Metal Oxide Semiconductor) is a type of power transistor specifically designed for high-frequency applications, especially in the 900MHz cellular phone market. As the demand for wireless communication grows, LDMOS technology has matured and become more cost-effective, making it a strong competitor to traditional bipolar transistors. In many cases, LDMOS is expected to replace bipolar transistors due to its superior performance.
One of the key advantages of LDMOS is its higher gain, which can reach over 14dB compared to 5–6dB for bipolar transistors. This means that fewer devices are needed to achieve the same output power, increasing system reliability. Additionally, LDMOS can handle three times the VSWR (Voltage Standing Wave Ratio) of a bipolar transistor, allowing it to operate at higher reflected power without damage. This makes it ideal for digital signal transmission, where input signals can be unpredictable.
The gain curve of LDMOS is smoother, enabling better multi-carrier signal amplification with less distortion. It also exhibits low and stable intermodulation levels, unlike bipolar transistors, which show increased intermodulation as power increases. This leads to better linearity and allows LDMOS to deliver twice the power of a bipolar transistor while maintaining signal integrity.
Another major benefit is its excellent thermal stability. LDMOS has a negative temperature coefficient, which prevents hot spots from forming during operation. This results in minimal amplitude variation—only 0.1 dB—compared to 0.5–0.6 dB for bipolar transistors, which often require temperature compensation circuits.
LDMOS is compatible with CMOS processes, making it easier to integrate into modern semiconductor manufacturing. The structure of an LDMOS device includes a dual-diffusion region, where arsenic and boron are implanted at different concentrations to form a gradient channel. A drift region between the active area and drain helps increase breakdown voltage by reducing impurity concentration, allowing the device to withstand higher voltages.
The field plate on the polysilicon layer further enhances breakdown voltage by weakening the electric field in the drift region. Proper design of the field plate length and SiO2 thickness is crucial for optimal performance.
In terms of manufacturing, LDMOS combines BPT (Bipolar Power Transistor) and GaAs (Gallium Arsenide) techniques, improving thermal conductivity and extending device lifespan. Unlike bipolar transistors, LDMOS does not suffer from localized heating or hot spots, making it more reliable under load mismatch or overdrive conditions.
LDMOS also offers simpler biasing and lower noise, making it ideal for RF signal amplification in both analog and digital TV systems. Its linear behavior in small-signal operation reduces intermodulation distortion, simplifying correction circuits. Additionally, the DC gate current is nearly zero, eliminating the need for complex biasing networks.
Key parameters such as epitaxial layer thickness, doping concentration, and drift region length determine the performance of LDMOS. A longer drift region increases breakdown voltage but also raises on-resistance, requiring a balance between these factors for optimal efficiency.
LDMOS excels in several areas: thermal stability, frequency stability, higher gain, improved durability, lower noise, reduced feedback capacitance, simpler bias circuits, consistent input impedance, better IMD (Intermodulation Distortion) performance, lower thermal resistance, and enhanced AGC (Automatic Gain Control) capability. These features make it well-suited for applications like CDMA, W-CDMA, TETRA, and digital terrestrial television.
Originally developed for RF power amplifiers in mobile base stations, LDMOS is now used in HF, VHF, UHF broadcast transmitters, microwave radar, and navigation systems. With continuous improvements in power density, linearity, and efficiency, LDMOS is becoming the preferred choice for next-generation baseband amplifiers.
Philips has continuously advanced LDMOS technology, introducing new generations with improved performance. Their third-generation 0.8-micron ultra-low distortion LDMOS improves linearity by 5–8 dB, offering better performance for 3G base stations. The fourth generation brings a 50% increase in power density and a 6–8% improvement in W-CDMA efficiency. The fifth generation pushes performance even further, achieving over 30% efficiency for W-CDMA and 17dB gain for PCS/DCS, while reducing thermal resistance to 0.5 K/W.
These advancements have enabled LDMOS to support high-power applications up to 150W CW in a single-ended package, making it a critical component in modern wireless infrastructure.