Mass Production Established for GaN HEMTs with Industry-Highest 8 V Rated Gate VoltageGaN HEMT Issues Resolved, Wide Adoption Promoted; Contributing to Reduced Losses, Compactness in Power Conversion Applications

ROHM announced that in March 2022 it had established a system for mass production of 150 V GaN HEMTs with the industry’s highest* rated gate-source voltage of 8 V. In general, GaN devices offer low on-resistances and superior switching performance, and are anticipated to come into use as power devices for next-generation power conversion, but have had a number of issues such as low gate rated voltages. The GaN HEMTs of this announcement, in the “GNE10xxTB series (GNE1040TB)”, were developed in order to resolve such preexisting problems. We asked a ROHM engineer in charge of development about the background of this development, features of the new products, and the like.
*ROHM survey, March 23 2022

Diligent Efforts in Power Conversion Culminate in Development of Power Devices Based on Next-Generation Semiconductor Materials

ROHM has assembled lineups of various discrete components such as transistors and diodes, and in addition to Si (silicon) devices, is well known as a pioneering supplier of SiC (silicon carbide) devices. This new announcement concerns GaN (gallium nitride) devices. Could you please begin by telling me about ROHM’s efforts in the field of power devices.

ROHM has been making diligent efforts to develop devices and modules for use in power-related fields. For example, the company began mass production of power devices using SiC in 2010, and at present is the first Japanese manufacturer mass-producing SiC Schottky barrier diodes and the world’s first manufacturer of SiC MOSFETs. Currently ROHM is advancing to the 4th generation of SiC power devices, which are used in various power conversion applications in which large power levels and high efficiency are sought, such as EVs and xEVs. The graphic below lists current products for use in power-related fields.

In development of power devices, in addition to approaches involving extension of device structures and manufacturing processes, R&D on new semiconductor materials, and efforts to commercialize same, lead to breakthroughs. As previously explained, ROHM began early mass production of power devices using the semiconductor material SiC, but also conducted research on new materials in parallel with this. One such new material is GaN, and with the recent establishment of a mass production system for HEMTs (high electron mobility transistors), the company has further expanded its range of products for power applications.

What is GaN? What are GaN HEMTs?

To begin with, please explain what GaN is, and what GaN HEMTs are.

Further, because the electron saturation velocity is higher than for SiC, better switching performance can be achieved. Thus, GaN has a figure of merit at least equal to that of SiC, but crystallization and processing are more difficult than for SiC.

HEMT is an acronym for High Electron Mobility Transistor; it refers to a field effect transistor in which a high-mobility two-dimensional electron gas induced at a semiconductor heterojunction functions as a channel. In general, HEMTs are fabricated in compound semiconductors, and in addition to GaN, they also use GaAs (gallium arsenide) and other materials. As indicated in the figure, in a GaN HEMT a lateral structure forms a heterojunction (a junction of different semiconductor materials) of GaN and AlGaN (aluminum gallium nitride) on a silicon surface. The current flowing between drain and source flows in the lateral direction of the high-mobility two-dimensional electron gas channel, and electrons move at high speeds. In contrast, Si and SiC MOSFETs have a vertical structure, and the current flows through a PN junction that contains impurities, so that movement speeds of electrons are slow compared with GaN HEMTs.

GaN HEMTs with Excellent Switching Characteristics can Dramatically Reduce Switching Losses

What kinds of features do these physical properties and transistor structure result in, considered as a device?

Explanations of the physical properties of GaN and of the principles of GaN HEMTs would probably not provide a working understanding; instead, please have a look at materials that compare transistors.

The table compares the basic parameters of power devices–Si SJ (superjunction) MOSFETs, SiC MOSFETs, and GaN HEMTs. The ranges of application of GaN HEMTs are medium-voltage, medium-power ranges, but we see that switching characteristics are greatly superior. The graph compares losses in Si MOSFETs and GaN HEMTs; switching losses can be reduced by 65%.

The Most Advantageous Power Capacities and Operating Frequency Bands of Power Devices Differ Depending on the Materials and Device Structure

Upon examining the above device comparisons, I see that Si, SiC, and GaN are used selectively in their “territories” where they have advantages. But what specific applications would they be used in?

I think it will be easier to understand if you study this diagram.

As I mentioned previously, power devices, and in this case transistors, differ in their areas of advantageous use according to the semiconductor materials and device structure. This graphic maps the applicable scope and relevant applications as functions of power capacities and operating frequencies. On the right side are indicated specific details. GaN HEMTs are suited to applications with medium power and voltage levels and high frequencies. High power conversion efficiencies are demanded in various applications such as the power supplies for servers and base stations, AC adapters, OBCs (onboard chargers), DC-DC converters, and the like; in such applications, GaN HEMTs will be preferred because they are capable of switching operation at still higher frequencies than SiC devices.

Market for GaN Devices and Related Issues

I’d like to move onto to a discussion of the newly announced products. Please tell me about the background to development.

ROHM is proceeding with development of next-generation power devices, and SiC power devices are currently performing well. However, when the overall market for power devices is considered, we needed a device to complement the high-frequency region that SiC cannot cover in terms of the application range explained earlier,; here GaN devices are prime candidates.

On the other hand, in the GaN power device market, new and powerful semiconductor device manufacturers have begun to release products, and markets have emerged, but a number of problems have emerged as well. Issues that are in especially urgent need of attention include the low rated values of the gate-source voltage VGS, and the difficulty in handling the packages that are being used. In response to these issues, ROHM has developed technology to resolve such problems, and has initially established a system for mass production of the GEN10xxTB series of GaN HEMTs, in order to promote the widespread use of GaN power devices with superior characteristics.

GaN HEMTs with a Rated Voltage of 150 V: Resolving Problems through the GNE10xxTB Series

You mentioned that problems with GaN HEMTs include the low V_GS rated voltage, and the difficulty in handling the packages used; how were these issues resolved?

Boosting the V_GS rated voltage to 8 V through a proprietary structure

Standard GaN HEMTs with a rated voltage of 200 V or lower have a rated gate-source voltage V_GS of 6 V, for a gate driving voltage of 5 V, due to the device structure. That is, there is a gate driving voltage margin of only 1 V. The rated voltage is in essence a value that must not be exceeded; if it were to be exceeded, operation anomalies and degradation, and in the worst case, device destruction could result. Hence advanced control is necessary to ensure that the gate driving volage does not exceed the rated value for V_GS; this has been a problem impeding the widespread adoption of GaN HEMTs.

The GNE10xxTB series adopts a proprietary structure to resolve this issue, and succeeds in raising the rated V_GS from the general value of 6 V to an industry-highest 8 V. With this, the margin of the gate driving voltage increases from 1 V to 3 V, and it is possible to relax constraint conditions such as voltage overshoot during switching operation.

Highly versatile DFN packages capable of handling large currents and with excellent heat dissipation adopted

GNE10xxTB Series: High Efficiencies of 96.5% and Up Achievable Even in High-Frequency Operation

When a GNE10xxTB series product is used in a power supply circuit, what kind of efficiency can be expected?

In addition to the fast-switching characteristics that are a feature of GaN HEMTs, the GNE10xxTB series products feature low losses even during high-frequency switching due to optimized gate driving through expanded V_GS ratings and adoption of a low-inductance package. In high-frequency switching power supplies operating at 1 MHz, conversion efficiencies of 96.5% and above are possible. The efficiency curve below is an example when using a half-bridge circuit to step down from 48 V to 12 V.

150 V GaN HEMTs: GNE10xxTB Series Product Summaries and Applications

Perhaps this should have come earlier, but please summarize the products for me.

Certainly. This is a simple summary. The product features are as I have already explained, but it should be noted that the V_DS ratings are for the 150 V series. At present, the data sheet for the GNE1040TB can be obtained from the ROHM website. Interested persons should ask regarding samples and the like. The GNE1015TB and GNE1007TB are currently in development.

＜GaN HEMTs: GNE10xxT Series＞

＊: Under development

With a voltage rating of 150 V, what kinds of applications are anticipated?

In essence, the applicable scope is as indicated previously; more specifically, however, possible applications will include:

• ・48 V input step-down converter circuits for data centers, base stations, etc.
• ・Step-up converter circuits in the power amplifier sections of base stations
• ・LiDAR driving circuits for industrial uses
• ・Class D audio amplifiers

Below are brief illustrations of circuits, which may be used as reference.

Lineup of GaN Devices Contributing to Energy Savings and Compactness Introduced as “EcoGaN™”, to Potentially Include 650 V Products

Well then, could you please give us a concluding overall summary.

The GNE10xxTB series is 150V GaN HEMTs with the industry’s highest gate-to-source voltage rating of 8V, which has been a challenge for general GaN HEMTs, and are packaged in an easy-to-handle DFN. GaN HEMTs are expected to be one of the next-generation power devices because of their extremely fast switching speed and the ability to significantly reduce switching losses compared to SiC MOSFETs, which also feature fast switching. ROHM has established a mass production system for the GNE10xxTB series that reflects user needs in order to promote the widespread use of GaN devices.

The GNE10xxTB series is intended for power supply applications in the medium voltage range, and will contribute to improved efficiency and smaller designs of power supplies mainly for use in base stations, data centers and the like, as well as in various industrial equipment and communication devices.

And what about future plans?

At ROHM, GaN devices contributing to power efficiency and compactness are being organized into an “EcoGaN™” lineup. In addition, as indicated in the “roadmap” below, Nano Pulse Control™ and other analog power supply technologies will be exploited in the development of control ICs and modules incorporating the latter, with further plans to develop 650 V products in addition to these 150 V devices.

＊EcoGaN™ and Nano Pulse Control™ are trademarks of ROHM Co., Ltd.

Thank you very much.

Related Information

• ◆ News release: ROHM starts Production of 150V GaN HEMTs: Featuring Breakthrough 8V Withstand Gate Voltage
  https://www.rohm.com/news-detail?news-title=150v-gan-hemts&defaultGroupId=false
• ◆ New release product introduction（PDF 3.1MB）
  https://www.rohm.com/documents/11303/9973405/Introduction_NewRelease150VGaNHEMT-GNE10xxTB_e.pdf/958c01a7-06c6-122f-16cc-3a7b453239e0?t=1668656122007