What is the difference between super junction MOSFET and MOSFET? Why can RDSon be reduced?

The power electronics industry is experiencing rapid innovation as high-power electronic products such as data centers and electric vehicles become more and more popular. To meet the growing demand for high power and high efficiency, many new technologies such as wide bandgap (WBG) semiconductors are becoming increasingly important.

One technology showing promise in these high-power applications is the superjunction MOSFET, an improvement over conventional technology that offers higher efficiency and smaller size.

This week, Alpha & Omega (A&O) announced two new superjunction MOSFETs aimed directly at these high-power applications.

In this article, we’ll discuss superjunction technology—why it’s critical for data centers, and A&O’s solution.

MOSFET on-resistance challenges at high voltages

For MOSFETs in power switching applications, one of the most critical device parameters is the on-resistance RDS(on).

RDS(on) measures the effective resistance of the transistor and this specification directly determines the power efficiency of the MOSFET.

One of the biggest factors affecting the RDS(on) resistance of a FET is the epitaxial (epi) layer of the device, which is the main voltage-sustaining region of the device.

Layout of a conventional planar MOSFET.

As the voltage increases, the epitaxial layer also needs to increase in thickness and become more lightly doped to help block the high voltage.

However, this has the undesirable effect of increasing the resistance of the epitaxial layer, thereby increasing the overall RDS(on) of the MOSFET.

For a MOSFET rated at 600 V, more than 95% of the device resistance comes directly from the epitaxial layers. Specifically, it is estimated that for every doubling of the rated voltage, the area required to maintain the previous RDS(on) increases fivefold.

Data center high voltage applications

When designing power FETs for high voltage applications, there is a trade-off between the size of the transistor, its voltage isolation capability, and its RDS(on).

Specifically, in high-voltage applications in data centers, designers need power FETs that can achieve high efficiency and a slim form factor to fit into increasingly thinner systems, such as 1 U or 0.5 U. For these applications, superjunction MOSFETs have become a viable choice.

The structure of a planar power MOSFET (left) and a superjunction MOSFET (right).

Super junction MOSFETs are developed by using techniques such as deep trench filling to overcome the resistance of the epitaxial layer.

The superjunction field effect transistor adopts a trench structure and multiple vertical PN junctions are arranged, which effectively reduces the resistivity of the epitaxial layer while maintaining a high blocking voltage.

Therefore, superjunction MOSFETs can achieve extremely low RDS(on) while maintaining small size and high blocking voltage. This feature makes the device a popular choice for high-power applications such as data centers.

A&O targets data centers

As mentioned earlier, this week A&O announced the release of two new superjunction MOSFETs targeted at data center applications.

The two new products, AONV110A60 and AONV140A60, are 600 V superjunction FETs designed to have very low RDS(on) in a small package.

AONV140A60 on-resistance vs. drain current and gate voltage. Image courtesy of A&O

According to the datasheet, the two devices have an RDS(on) of 0.11 Ω and 0.14 Ω respectively, and both are available in an 8 mm x 8 mm x 0.9 mm DFN package.

In addition to data centers, A&O claims these products are well suited for a variety of other applications, including fast charging, solar inverters, and industrial power supplies.