How Are Super-Junction MOSFETs Different from Common D-MOS?

Power semiconductor devices are central to modern power conversion systems. Among them, the common double-diffused MOSFET (D-MOS) has long been the workhorse for many applications. In recent years, however, super-junction (SJ) MOSFETs have emerged as a superior alternative for high-voltage applications. This article explains the fundamental differences between these two types of MOSFETs—from their unique structures to the performance benefits they offer—and how these differences impact real-world applications.

Overview of D-MOS (Double-Diffused MOSFETs

Conventional D-MOS devices, sometimes simply called DMOS, are vertical power MOSFETs whose structure is defined by a lightly doped drift region that supports high voltages. Key characteristics include:

- Planar Doping Profile: The drift region’s doping is optimized through a double diffusion process. This method, however, hits the so-called “silicon limit,” where increased voltage ratings force the drift region to be thicker and more lightly doped, leading to higher on-resistance.

- Non-uniform Electric Field: In a standard D-MOS, the electric field peaks at the p–n junction interface, setting a practical limit on breakdown voltage.

- Trade-off Between Voltage and R_DS(on): As the breakdown voltage increases, the conduction losses (or on-resistance) tend to increase significantly.

Introduction to Super-Junction MOSFETs

Super-junction MOSFETs were developed to overcome the inherent limitations of conventional D-MOS devices. They incorporate a unique design that allows for higher doping in the drift region without sacrificing voltage blocking capability. Key points include:

- Charge Compensation Structure: SJ-MOSFETs use alternating columns or pillars of p-type and n-type material in the drift region. This “super-junction” structure evenly distributes the electric field across the device when blocking high voltage.

- Reduced On-Resistance: By enabling a higher doping concentration in the drift region while still maintaining high breakdown voltage, SJ-MOSFETs achieve significantly lower on-resistance compared to D-MOS devices of the same voltage rating.

- Improved Switching Performance: The more uniform electric field and lower parasitic capacitances in super-junction devices lead to faster switching speeds and lower switching losses.

Structural Differences

D-MOS Structure

- Single-Layer Drift Region: In a conventional D-MOS, the drift region is uniformly doped at a low level to support the desired breakdown voltage.

- Limitation Due to the “Silicon Limit”: To maintain high voltage capability, the drift region must be thicker and less heavily doped, which increases the device’s resistance and switching losses.

Super-Junction Structure

- Alternating P/N Columns: The key innovation in SJ-MOSFETs is the incorporation of multiple alternating p-type and n-type regions in the drift area. These columns are precisely engineered to balance charge (charge compensation), resulting in a nearly uniform electric field.

- Optimized Drift Region: Because the electric field is spread uniformly, designers can use a higher doping concentration in the drift region without compromising the breakdown voltage. This design directly translates into lower on-resistance.

- Enhanced Capacitance Characteristics: The super-junction structure also results in reduced parasitic capacitance, which benefits switching performance by minimizing energy losses during transitions.

For example, Infineon’s CoolMOS™ CE devices demonstrate how the SJ structure achieves a dramatically lower R_DS(on) compared to conventional MOSFETs while still meeting high-voltage requirements.

Performance Comparison

On-Resistance and Efficiency

- D-MOS: The on-resistance in conventional D-MOS devices increases sharply with voltage. The need for a thick, lightly doped drift region leads to higher conduction losses.

- Super-Junction MOSFETs: Thanks to the charge compensation technique, SJ-MOSFETs have a much lower on-resistance. This efficiency improvement means that for the same voltage rating, an SJ-MOSFET will dissipate less power as heat, which is critical for applications like power converters and motor drives.

Switching Speed

- Lower Parasitics: SJ-MOSFETs benefit from reduced input and output capacitances due to the optimized drift region structure. This reduction allows for faster rise and fall times, enabling more efficient high-frequency operation.

- Improved dv/dt: The more uniform electric field in SJ devices contributes to improved switching characteristics with lower energy loss during transitions.

Breakdown Voltage

- Uniform Electric Field Distribution: In D-MOS devices, the electric field concentrates at a single junction, which can limit the breakdown voltage. In contrast, the SJ structure distributes the electric field evenly across the drift region, allowing for higher breakdown voltages without sacrificing on-resistance.

These performance advantages of SJ-MOSFETs have made them particularly attractive in applications where efficiency and thermal management are critical.

Applications and Trade-Offs

Applications

Super-junction MOSFETs are commonly used in:

- High-Voltage Power Converters: Lower conduction and switching losses make them ideal for switched-mode power supplies, inverters, and DC-DC converters.

- Automotive Electronics: Their efficiency and thermal benefits are crucial in automotive power management and electric vehicle systems.

- Industrial Motor Drives: Improved efficiency leads to better performance in motor control and industrial drive systems.

Trade-Offs

While SJ-MOSFETs offer clear advantages in efficiency and switching performance, they can also exhibit:

- Higher Reverse Recovery Currents: In some cases, the increased p–n junction area in the SJ structure may lead to larger reverse recovery currents. However, design optimizations have largely mitigated these issues.

- Cost and Complexity: The fabrication process for super-junction devices is more complex than that of conventional D-MOS, which can sometimes lead to higher manufacturing costs. Yet, as production volumes increase, the cost differential continues to shrink.

Conclusion

Super-junction MOSFETs fundamentally differ from common D-MOS devices by rethinking the drift region design. Instead of a uniform low-doped layer, SJ-MOSFETs use a charge compensation structure with alternating p-type and n-type columns to achieve a more uniform electric field. This innovation allows for higher doping levels, dramatically reducing on-resistance while maintaining high breakdown voltage. The result is a device that not only improves conduction and switching efficiency but also supports higher power density and reliability in demanding applications.

For designers aiming for greater efficiency in high-voltage power applications, super-junction MOSFETs represent a significant step forward compared to traditional D-MOS technology.