A breakthrough for the bidirectional switch

By exceeding a blocking voltage of 5 kV while keeping conduction loss and reliability in check, a new generation of monolithic GaN bidirectional switches promise efficient, more compact medium-voltage AC power conversion.

BY YUAN QIN FROM UNIVERSITY OF SCIENCE AND TECHNOLOGY OF CHINA, MING XIAO FROM XIDIAN UNIVERSITY, KAI CHENG FROM ENKRIS SEMICONDUCTOR, AND YUHAO ZHANG FROM THE UNIVERSITY OF HONG KONG

With minimal fanfare, power electronics is becoming the enabling layer for a number of converging transitions: electrified transport and industry, the build-out of renewable generation and storage, and the expansion of energy-intensive digital infrastructure. For all these cases, higher efficiency and a higher power density are no longer optional – instead, they directly translate into smaller passive components, simplified thermal management and a lower lifetime cost. The introduction of wide-bandgap semiconductors has already shifted the landscape, with GaN’s success story told through unidirectional HEMTs, powering compact, high-frequency converters from 30 V to 400 V.

But many of the next frontiers for GaN are inherently AC-facing, so switches will have to handle current reversal and bipolar off-state voltages. While it is workable to implement that bidirectional capability with anti-series devices, that solution scales poorly as voltage rises: chip area grows, parasitic capacitances stack, and gate-drive and layout complexity multiply.

A better approach is to attack this problem at its root, using a monolithic bidirectional switch. This form of switch shares the high-voltage region inside one device, and preserves efficiency while simplifying the circuit.

Taking the performance of these switches to a new realm, by delivering a hike in the voltage-handling capability, is our team from University of Science and Technology of China, Xidian University, Enkris Semiconductor and the University of Hong Kong. Our breakthrough promises to open the door to the deployment of this device in medium-voltage AC applications.

Motivation for our efforts has come from considering three questions: Why does bidirectional switching matter in real converters? Why do monolithic implementations offer such a strong advantage? And how is it possible to reach a high voltage with a bidirectional switch?

Why bidirectional?
The applications most in need of a bidirectional switch are those that involve a converter having to connect and disconnect an AC node efficiently. Matrix converters and current-source inverters are classic examples. In these circuits, the switch must handle current in both directions, as the load current and the instantaneous line voltage are not necessarily aligned. In addition, the same device must often sustain positive and negative blocking voltages, as switching states change over a line cycle.

Bidirectional switches are also valuable in solid-state transformers. They are a critical component in Nvidia’s next-generation 800 V power delivery architecture in AI data centres, as well as in medium-voltage motor drives, and emerging AC distribution and protection concepts, where fast, controllable bidirectional elements can replace bulky passive components. For all these examples there tends to be a common economic driver: if a bidirectional function can be implemented with fewer devices, the overall system can be smaller, faster and more efficient.

Figure 1. Conceptual comparison between a bidirectional switch realised using two discrete GaN transistors in anti-series and a single monolithic bidirectional switch sharing one drift region.

At medium voltages, strong opportunity for the deployment of the bidirectional switch is in the AC-DC front end, used in data centres, rail traction and industrial power supplies. Here, design engineers are using three-phase active rectifiers and solid-state transformer stages to increasingly target higher switching frequencies and shrink magnetics.

In these systems, the bidirectional function is often realised with anti-series device pairs, a compromised solution that increases the active device count and stacks parasitic capacitances and stored charge. With this approach there’s a higher commutation energy penalty on every transition, making it harder to push up the switching frequency without sacrificing efficiency.

Additional examples associated with the potential deployment of bidirectional switches are medium-voltage motor drives and variable-speed compressors. In these cases, introducing bidirectional switches enables matrix-converter and current-source topologies that remove the bulky DC-link electrolytic capacitor. Eliminating or downsizing the DC link improves lifetime and power density. However, to ensure success, the bidirectional switch must block bipolar voltages safely and repeatedly under fast switching, elevated temperature and rapidly changing voltages.

The monolithic advantage
When a brute-force anti-series architecture is adopted, this doubles the number of active devices. That’s not an elegant solution – as each device contributes roughly half the total on-resistance, there is a potential quadrupling of chip area to realise the same conduction loss as a single unidirectional transistor. At the system level, the penalty can be even more severe, with extra devices introducing additional capacitance, which must be charged and discharged for every switching cycle.

Introducing monolithic bidirectional switches reduces this overhead by placing two gated channels on one shared drift region – this avoids duplication in the high-voltage region (see Figure 1). At lower voltages, monolithic GaN bidirectional switches are already appearing as engineering samples in industry, pointing to a path for adoption.

Figure 2. Device schematic and equivalent circuit model of multi-channel GaN monolithic bidirectional switch demonstrated at IEDM 2025.