New GaN Technology Simplifies Driving GaN-Based HEMTs
GaN-based lateral transistors can operate at high switching frequencies, allowing for a very compact design.
Among the different structures available for GaN transistors, the lateral one is currently the most widely used. Gallium nitride high-electron–mobility transistors (HEMTs) are lateral devices that use a two-dimensional electron gas as a transistor channel. These devices allow power circuits, such as converters, to achieve high efficiency and high-power density. With a very low specific on-state resistance, GaN-based lateral transistors can operate at high switching frequencies, allowing for a very compact design. Normally off transistors are usually preferred by designers, as they can guarantee safe operation in power electronics systems.
Although it might seem simpler at first, the gate driver circuit for these devices requires a careful design. First of all, a normally off GaN-based HEMT requires a negative voltage for switching it off and for maintaining it in the off state, avoiding undesired turn-on. That is the reason why this feature is often integrated in commercial gate drivers available on the market. Another reason why a negative voltage is needed is that GaN HEMT transistors have a very low threshold voltage (1.2–1.5 V), and undesired turn-on is quite common, especially if the layout is not optimized. Moreover, these devices have a maximum gate voltage of about 6.0–6.5 V, and therefore, proper clamping circuits are needed to keep this voltage below the threshold limit, thus avoiding potential failures or even damage. In this article, we will see how a novel technology can overcome those limitations, simplifying the gate drive circuit required by GaN transistors.
ICeGaN technology
Cambridge GaN Devices (CGD) is a company founded in 2016 by CEO Giorgia Longobardi and CTO Florin Udrea after almost a decade of leading-edge research and development at Cambridge University. CGD can count on a team of professionals with wide and long experience with power semiconductors, including design and reliability aspects of GaN-based devices.
In an interview with Power Electronics News, Andrea Bricconi, vice president of business development at CGD, said, “Our company is focused on two objectives: contributing to improving the way the world uses energy by delivering gallium nitride solutions that are easy to use and reliable.”
Besides developing innovative devices, packaging, and power electronics boards, CGD also leads international projects, such as GaNext, an excellent example of international cooperation. With a group of 13 partners coordinated by CGD, this European-founded project has the sole aim of creating prototypes of GaN power modules addressing low- and high-power applications.
“The GaN market is booming today, starting from consumer segments below 1 kW. The GaNext project is aimed at demonstrating that GaN-based power modules can be successfully conceived and adopted in a variety of low and high-power applications such as on-board chargers (OBC), LED drivers for lighting, photovoltaic inverters, and, in the long term, traction inverters; the ultimate target is to enable a giant leap in power density across the board, “said Bricconi.
The roadmap drawn up by CGD foresees a commercialization phase of its GaN-based devices in 2022, with the entry of 650-V/750-V power transistors into the market. ICeGaN technology consists of a power transistor with monolithic integration of smart interfaces for sensing/protection, ease of use, and enhanced gate reliability. With a threshold voltage of 3 V, these enhanced-mode single-chip transistors can be driven with a gate voltage of up to 20 V, just as if they were common MOSFETs. In achieving this task, CGD will be supported by TSMC (the world’s largest semiconductor foundry), a big plus for a startup.
According to Bricconi, GaN-on-silicon offers simpler integration than both silicon and silicon carbide due to its lateral structure. This, in turn, allows designers to integrate more features (such as multi-chip and logic) into the same chip.
CGD’s ICeGaN application diagram is shown in Figure 1. The advantages of this technology can be summarized as follows:
- It enables use of standard MOSFET drivers and/or controllers.
- Uncompromised GaN switching performance or RDS (on)
- Additional smart features
- Opportunity for application optimization
- It has DV/DT control.
It still requires being driven by an external IC.
The goal of ICeGaN technology is to integrate the logic (the green box in Figure 1) into a robust, state-of-the-art enhanced-mode GaN HEMT. That brings two benefits for the user:
- Ease of use: CGD’s GaN HEMTs can be interfaced to drivers and controllers with minimal effort.
- Cost savings: There is no need for additional components, voltage clamping, or costly gate drivers.
A comparison between a standard GaN HEMT and ICeGaN HEMT is shown in Figure 2. With standard GaN devices, gate driving is difficult, time-consuming, and costly. Moreover, the gate voltage is limited to a maximum value of about 6-7 V, after which the forbidden area begins. The other way around, CGD’s GaN HEMT features a threshold voltage high enough, and gate voltage can be extended up to about 20 V without compromising the device reliability.
With an average gate threshold voltage of about 1.4 V, conventional GaN devices require a typical gate source voltage (Vgs) of –3 V in the off state and between 4 V and 6 V in the on state. CGD ICeGaN devices require a Vgs of 0 V (with built-in Miller clamp) in the off state and between 8 V and 20 V in the on state, compatible with any gate driver and controller.
As Bricconi pointed out, CGD thinks the monolithic integration of the gate driver is very good at low power but could be hardly scalable to high power.
“Our ICeGaN technology allows us to add new features, such as integrated current sensing,” said Bricconi. “The user can replace external sense resistors, thus connecting the power device to ground and improving the cooling path greatly.”
At the moment, CGD is working on the consumer market and partly on the industrial one (datacom and telecom applications with a power level above 1 kW). The former offers fewer barriers but puts a lot of pressure on costs. Regarding automotives, Bricconi said that this market has design cycles with a duration of approximately several years. “The first cars with GaN transistors will come out no earlier than 2024 -2025.
Even though a high degree of reliability is required, it has been demonstrated that the power density of an OBC can be doubled using GaN.
Datacom
Cambridge GaN Devices has announced the beginning of ICeData, a project funded by the UK Department for Business, Energy and Industry Strategy (BEIS) aimed at developing and commercializing a high-efficiency GaN-based integrated circuit (IC) for use in data center server power supplies. Data centers presently consume roughly 2% of global energy, with an annual power usage of 400 TWh in 2018, expected to double by the end of the decade. GaN’s combination of better efficiency and higher power density has the potential to save up to 10% on investment and running expenses.
The goal of this project is to develop GaN power IC technology to enhance data center server power supply efficiency to over 98%. IceData will help CGD deliver GaN solutions specifically designed for switch-mode power supplies for data centers and telecommunication servers, which will help save more than 8 megatons of CO2 annually by 2030.
CGD estimates that the global Power GaN market will grow to more than $1 billion by 2026, driven by demand for lighter, more efficient power supplies and more compact and powerful OBCs for electric vehicles and hybrid electric vehicles. CGD’s first product line with ICeGaN technology will be released in the first half of 2022.
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