Vertical GaN, okay?

Latest update time：2024-03-21

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Editor's note: Recently, there has been a lot of news surrounding vertical GaN, such as the collapse of NextGen system at the end of last year and the recent liquidation of Odyssey Semiconductor. This can’t help but trigger everyone’s thinking, will vertical GaN be good in the future? Let’s take a look at the prospects of this technology.

Wide bandgap materials are expected to be used as power device materials with high power efficiency. In particular, the technology development and application of GaN and silicon carbide (SiC) have made rapid progress. Lateral GaN HEMTs are already used in applications with voltages below 650V, such as ultra-small AC adapters for PCs and ultra-small smartphone chargers, while vertical SiC trench MOSFETs are already used in applications such as traction inverters (main motor drives) . It is used in electric vehicles (EV) applications with a withstand voltage of 1200V or more, and its social application is rapidly accelerating.

Although gallium nitride (GaN) is well suited as a power device material, its true potential has not been demonstrated until now due to the difficulty of producing high-quality substrates. However, Mr. Yusuke Mori, a professor at Osaka University in Japan, is working on developing high-quality, large-diameter GaN substrates to achieve innovative carbon dioxide emission reductions. Based on the results achieved so far, "preparations to unlock GaN's huge potential as a power device material are progressing steadily." “It has the potential to replace silicon carbide (SiC) and is expected to find wider applications in circuits. "

GaN’s potential exceeds SiC

If we compare the overall suitability of unipolar power devices such as MOSFETs and junction FETs (JFETs) using the Barriga index, which quantifies the overall suitability of unipolar power devices such as MOSFETs and junction FETs (JFETs), we find that, When SiC is in crystal polymorph 4H, the index is 500; while GaN is much higher at 930.

Variga quality factor is a value determined by physical properties such as electron mobility (μe), dielectric constant (ε) and dielectric breakdown strength (Ec). Originally GaN is better than SiC even in applications with a withstand voltage of 1200V or 1200V . This demonstrates its high potential as a power device material. If this potential can be tapped, it is very possible to make application equipment smaller and lighter through high-frequency operation, further improve power efficiency, and increase the output of application equipment.

In addition, when using SiC-based power devices, some people have always been worried about reliability issues and hope to apply GaN as a fundamental solution. There are more than 200 types of SiC crystals, each with a different stacking structure and the arrangement of the four closest atoms that make up the tetrahedral crystal structure. Specifically, they mainly include "3C", "4H", "6", " 15R” This concentrated structure.

Each material has different physical properties, and 4H has high mobility and is specifically used in many power devices. The concern is that when devices are used in environments where they are repeatedly heated and cooled, phase changes may occur, causing changes in device quality and leading to malfunction and failure.

Of course, when using SiC devices, we will solve the polymorphism problem by improving device structure, quality control, drive circuits, operating conditions, system configuration and other measures. But what is certain is that if the underlying factors that cause anxiety can be eliminated from the substance itself, a radical cure can be achieved that does not rely on symptomatic treatment.

For GaN, there are two different structures: hexagonal wurtzite structure and cubic sphalerite. Among them, the former is a stable phase and is used in device manufacturing; the latter is also known, but it is not a stable phase. This is why it is desirable to use GaN instead of SiC in applications requiring high reliability.

Despite this background, there is a reason why GaN devices are not currently used in applications that handle large amounts of power, such as traction inverters for electric vehicles. To handle high power, it is necessary to place the input terminals and control terminals on the front side of a semiconductor substrate similar to silicon-based MOSFETs and IGBTs (insulated gate bipolar transistors), and place the output terminals on the back side. Create a vertical device that allows large currents to flow.

At this point, realizing vertical GaN devices requires a self-supporting substrate made entirely of GaN, but with traditional substrate manufacturing techniques, many dislocations will appear that penetrate the substrate and hinder device operation. Unfortunately, the quality is not yet reach a level that can meet the requirements. Making the realization of vertical GaN devices possible.

In addition, in order to realize mass-produced vertical GaN devices, not only the substrate quality needs to be improved, but also the diameter needs to be increased, which directly leads to a reduction in device manufacturing costs. Recently, some companies have begun mass production and sales of high-quality GaN self-supporting substrates. For example, Mitsubishi Chemical has begun supplying 4-inch substrate samples made using a liquid-phase crystal growth technology called ammonothermal method, which allows high-throughput growth of high-quality GaN crystals.

However, industry insiders believe that increasing the diameter of a substrate using ammonothermal methods will be limited to about 4 inches due to the characteristics of crystal growth. A technological breakthrough is needed to create high-quality, free-standing GaN substrates with larger diameters.

Vertical GaN, going well

The research and development of vertical gallium nitride (GaN) semiconductors is going smoothly and is moving towards practical use.

Starting in 2022, Panasonic HD and Toyoda Gosei have developed the following two vertical GaN power semiconductors planned to be used in EV inverters. One is the vertical Junction FET (JFET) developed by Panasonic HD. This product not only helps achieve miniaturization and lightweight of high-frequency operating equipment, but also has a p-GaN gate structure. The second is a vertical channel MOSFET developed by Toyoda Gosei that is expected to achieve extremely high versatility.

Figure 1: Concept diagram of the process for manufacturing large-diameter, high-quality GaN seed crystals by combining Na flux method and multi-point seeding method (left), and (right) 6-inch GaN seed crystal manufactured.

It is understood that in this project, Toyoda Gosei introduced a technology for manufacturing high-quality, large-diameter GaN substrates, which combines the "Na flux method" and "multi-point seed crystal method" developed by Osaka University (pictured) 1), develop a device that can grow large-diameter crystals of more than 8 inches, and produce GaN seed crystals with large diameter and defect density of 104/cm2.

The Na flux method was invented by Professor Hisanori Yamane of Tohoku University in Japan in 1996. It is a technology for growing high-quality GaN single crystals by dissolving gallium (Ga) and nitrogen (N) into liquid sodium (Na). Since it is grown in a liquid phase, it is suitable for producing high-quality crystals.

On the other hand, the multi-point seed crystal method is a technology that pre-distributes many small GaN seed crystals on a large sapphire substrate and fuses the growing crystals together during the crystal growth process. Using this technology, large-diameter single crystals can be produced. Combining the characteristics of the two methods, it will be possible to produce high-quality, large-diameter GaN seed crystals.

The technology of manufacturing GaN crystals on sapphire substrates is the technical basis for applying the multi-point seed crystal method, and is a technology matured by Toyoda Gosei as an optical device technology. Therefore, manufacturing technology can be improved at the mass production level.

In addition, "the combination of the Na flux method and the multi-point seed crystal method is very compatible, and there are no special obstacles to further increasing the diameter. Currently, we are using 8-inch seed crystals to produce 6-inch seed crystals." We are continuing to develop this technology , in the future we will also consider using 12-inch sapphire substrates to produce 10-inch GaN seed crystals. "Toyoda Gosei said.

According to reports, both of the above-mentioned devices can withstand 650V high voltage and 20A current, and they plan to use GaN wafers made by the "OVPE (Oxide Vapor Phase Epitaxy) method" in the future. The "OVPE method" was jointly developed by Japan's Osaka University, Panasonic HD, and Toyoda Gosei. The ON resistance of GaN devices made using this method is one order of magnitude lower than SiC.

Figure 2: 4-inch GaN substrate manufactured using OVPE method

对于用于电动汽车逆变器等的垂直GaN器件，如果可以降低衬底的电阻，则可以降低器件的导通电阻，从而提高功率效率。通过在晶体中添加硅（Si）或氧（O）等元素可以降低GaN的电阻，但通过OVPE方法，可以生长添加更多的GaN晶体。可以形成电阻值低至10 ^-4 Ωcm 、超过SiC（10 ^-3 Ωcm）、位错密度低至10 4 /cm ² 的GaN晶体（图3）。

图3：使用OVPE方法制造的超低电阻GaN衬底可以制造比SiC导通电阻更低的器件

Panasonic HD 表示：“我们已经确认，可以使用 OVPE 方法在 2 英寸高质量 GaN 籽晶上制造超低电阻衬底。我们正在向丰田合成和我们的公司提供衬底。”作为环境部项目的一部分，我们正在开发垂直 GaN 器件。此外，在保持质量的同时，与传统方法相比，我们使用 OVPE 方法将晶体生长速率提高了一倍。”

他们还已经证实，可以找到在使用OVPE法的晶体生长过程中，通过组合Na熔剂法和点晶种法制造的籽晶中残留的夹杂物不破裂的条件。然而，“在实际量产中，有可能要求夹杂物为零。”我们将考虑解决这些问题的方法，我们还将在结合氨热法制造的基板上使用OVPE。“我们还在试验晶体生长技术，”Panasonic HD 说道。

日本名古屋大学有效利用上述试作品，并综合考虑各试作品的静态特性、动态特性，目前正在研发一款输出功率为50kW的逆变器，该逆变器具有线路合理、工作条件和结构规格均出色的特点。其目标是在2025年应用于EV。未来，名古屋大学还将进一步提升输出功率，同时，也在通过调整器件本身的结构，以研发出可耐1200V高压的元件。

下一步是挑战1200V的产品

据了解，由松下HD研发的p-GaN栅（Gate）结构的垂直型JFET特点如下，通过将栅（Gate）周边调整为p-GaN/AlGaN/GaN，使常关闭（Normally-off）和低ON阻值成为了可能（下图4）。电流流经路径的一部分会形成与HEMT通道（Chanel）类似的二维电子气（2D EG）。因此，易于降低ON阻值，更适用于高速工作。通过将此类垂直型JFET应用于高频开关电源线路、电机驱动线路，不仅有利于提升功率，有利于实现电路中零部件的小型化（如线圈等）。

图 4：Panasonic HD 开发的具有 p-GaN 栅极结构的垂直JFET

图5：丰田合成开发的垂直沟槽MOSFET

此外，松下HD 还利用栅部分的p-GaN来减轻电流阻挡层端部的电场，以降低电流的泄露。此外，10A试作品的实际验证成果如下，阈值电压为1.5V、RonA= 1.7mΩcm²、击穿耐压为600V以上。

由于GaN器件不会像Si、SiC一样可通过热氧化形成高质量的半导体和绝缘层界面，因此很难形成MOSFET结构，仅从这一点就阻碍了GaN器件的生产制程。但是，松下HD研发的GaN为JFET结构，不需要形成氧化膜。

如今，用于PC方向超小型AC适配器等设备的横向型HEMT结构的GaN功率器件也由同样的工艺制成，因此，可在现有的GaN器件的工厂内生产。此外，不同于栅极由电压驱动的MOSFET，JFET由电流驱动。虽然驱动IC等周边电路的配置不同于Si、SiC，但就这一点而言，有利于横向型器件的技术积累。（松下HD）

为尽快实现实际应用，松下HD还评价了其研发的器件的可靠性和热阻。为进一步改善此次试做的器件，松下HD还进行了短路耐受测试、连续开关测试，并找到了一些课题。利用OVPE法形成的GaN层虽然可以降低电阻、减少发热，但也存在热阻较高，不易散热的问题。为了发挥OVPE法应用的优势，需要考虑更改器件的设计、或针对封装（Package）开发出新的散热手段，以避免对器件的工作造成不良影响。

松下HD还在研发可耐1200V以上高压的垂直型GaN器件，并讨论了可降低碳（C，此处为随机进入漂移（Drift）层，并补充施体（Donor）的碳）浓度的结晶生长条件，同时还发现了可将碳浓度控制在5×10 ¹⁵ /cm ³ 以下的漂移（Drift）层的生长条件，以促进GaN器件的制作。为了今后稳定生产高耐压垂直型GaN器件，还需要研发出可进一步降低碳浓度的结晶生长技术。

丰田合成研发的沟槽（Trench）MOSFET基本上采用了与Si基、SiC基垂直型沟槽MOSFET同样的结构（下图4）。丰田合成的工艺如下，在GaN晶圆上同时外延生长出漂移层（n-GaN层）、Body层（pGaN层）、源接触层（Source Contact，n+GaN层）。丰田合成的工艺不使用离子注入来制作pGaN层，因此制程相对简单。

随后，用干蚀刻加工了接触区域的凹槽（Recess）、栅极沟槽（Gate Trench）。丰田合成特意采用了原子层沉积法，以使形成栅极沟槽的栅极绝缘膜的厚度、性质更均一。其MOSFET特性如下，一颗芯片排列有数十万个六角形的MOSFET单元。当需要对应较大的电流时，可通过增加单元数量来满足。

图6：丰田合成研发的垂直型沟槽MOSFET。

MOSFET的特点在于它比JFET更容易实现常关动作（Normally-off），这对确保信赖性十分重要。此外，MOSFET的另一个优势是，可基于更微缩化的技术实现更高的性能。另外，MOSFET需要满足以下应用要求，如在某些应用中（如汽车等），需要满足与传统器件的兼容性，同时采用传统的器件的结构、或采用类似于现有技术的线路布局，而不是单纯的提升性能。

如上文所述，丰田合成已经制成了可评价650V、20A基本性能的测试样品。“即使不使用基于OPVE法制成的超低电阻GaN晶圆，也可获得性能不低于SiC的垂直型GaN MOSFET。未来，可基于OPVE法进一步提升性能。（丰田合成）”日本环境省的项目中提到，在2025年实现50Kw级别的逆变器，且计划将650V耐压器件的额定电流提升至60A。如果今后需要将输出功率提升至100Kw，则需要考虑研发1200V耐压的器件。

商业化很快了？

Although the research and development results are "pleasant", the road to mass production seems to be relatively long. According to Toyoda Gosei, there is a correlation between the flatness of the wafer (SFQR: Site Front least sQuare Range) and the yield of the process. Toyoda Gosei chose wafers with better flatness, which not only enabled the larger size of the device itself, but also enabled the device to meet larger currents. It is reported that Toyoda Gosei tried to make a vertical wafer that can withstand 650V high voltage and 20A current. type trench MOSFET. Toyoda Gosei's R&D project has obvious advantages, namely, timely feedback of verification results to wafer R&D, which increases the speed of R&D from wafer to chip.

In addition, Nagoya University in Japan discovered the following problem after testing the above-mentioned samples. During switching operation, the ON resistance increased sharply. After investigation by Toyoda Gosei, it was found that the concentration of magnesium (Mg) in pGaN is related to the problem pointed out by Nagoya University, and the above problem is especially obvious under high current load conditions. Since it needs to maintain a normally-off operation, the concentration of magnesium (Mg) cannot be adjusted significantly. Toyoda Gosei will develop other solutions in the future.

In the research and development work to achieve a withstand voltage of more than 1200V, Panasonic HD's JFET faces the same problem, that is, it is difficult to control the concentration of impurities in the drift layer.

On the one hand, many companies are developing vertical GaN devices that can compete with SiC devices. At the same time, many companies are developing technologies that enable lateral GaN FETs to operate at higher voltages. It is reported that Toyoda Gosei is developing a technology called "Polarization Super Junction (PSJ)", which is based on the HEMT structure and forms GaN/AlGaN/GaN heterogeneous links in the channel area. It is reported that using PSJ technology, it is expected to obtain devices that can withstand high voltages of 10,000 volts, and ultra-high voltage resistance is usually achieved through SiC MOSFETs and Si IGBTs.

Compared with the HEMT structure, the switching frequency of the PSJ structure GaN FET is lower, but it can still support operation of several MHz. The withstand voltage level not only exceeds that of SiC devices, but the switching frequency is also much higher than that of SiC MOSFET (the upper limit is hundreds of kHz) , Si IGBT (tens of kHz). If sapphire wafers (GaN on Sapphire), which are more economical and have better insulation than GaN self-supporting wafers, are used, lateral devices are expected to be launched earlier than vertical GaN.

Of course, if the technology developed in the future for low-resistance, high-quality GaN self-supporting wafers for vertical GaN can be applied to insulating wafers, then "GaN on GaN" PSJ devices with high withstand voltage and high performance The market demand is expected. Toyoda Gosei pointed out: "With the advancement of the epitaxial layer (epi) and process technology of the PSJ structure, the performance and reliability of GaN on Sapphire will be much higher than originally imagined. In the steady advancement of GaN self-supporting wafers, vertical GaN While conducting research and development, we expect to verify the characteristics and practicality of PSJ devices based on GaN on Sapphire and evaluate the possibility of entering the GaN power semiconductor market as soon as possible.”

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