According to Yole Developpement's report "Power GaN 2012," GaN power devices have a huge market potential. The semiconductor power device market was approximately $17.7 billion in 2011, and it is projected to increase by 8.1% to $35.7 billion by 2020. The power supply market using GaN power devices is expected to take off in 2014, followed by a period of rapid development, with sales excluding defense budgets potentially reaching $2 billion by 2020.

Currently, 50% of power device production lines are 6 inches in size, and many factories are shifting to 8-inch production lines. In 2011, Infineon became the first factory to introduce a 12-inch production line. GaN power devices have also entered a period of development. In addition to new companies focusing on GaN (such as EPC, Transphorm, and Micro GaN), the world's top power semiconductor companies have also entered the GaN power device market. These include companies that previously worked with silicon (such as IR, Furukawa, Toshiba, and Sanken), companies that worked with compound semiconductors (such as Infineon, RFMD, Fujitsu, and NXP), and companies that produce both LEDs and power devices (such as Panasonic, Samsung, LG, and Sharp). For GaN power device suppliers, the IDM (Integrated Device Manufacturer) model has become the mainstream business model, with companies like IR, Panasonic, Sanken, and Transphorm being IDM enterprises. Currently, investment in GaN power device companies continues to grow. In July 2012, AZZURRO raised €2.6 million to develop 8-inch GaN-on-Si epitaxial wafers. In October of the same year, Transphorm raised $35 million to develop GaN power devices. In May of this year, the UK government funded NXP with £2 million to develop GaN power devices in Hazel Grove.

Currently, 4-inch and 6-inch GaN-on-Si wafers have been commercialized, and several research institutions and companies have successively reported research results on 8-inch GaN-on-Si wafers [2, 3]. In 2012, Singapore's IMRE reported a 200mm AlGaN/GaN-on–Si(111) wafer. In the same year, Singapore's Institute of Microelectronics and NXP in the Netherlands announced a collaboration to develop 200mm GaN-on-Si wafers and power device technology. Companies such as IMEC in Belgium, IR in the USA, IQE in the USA, Dowa in Japan, and Azzurro in Germany are also developing 200mm GaN-on-Si epitaxial technology.

The growth of GaN epitaxial layers on large-diameter silicon substrates of 4 inches and above is rapidly developing and will eventually mature. The main challenges currently faced are as follows: First, there is a mismatch issue, with lattice constant mismatch, thermal expansion coefficient mismatch, and crystal structure mismatch between the silicon substrate and GaN. Second, there is a polarity issue, as Si atoms form pure covalent bonds and are non-polar semiconductors, while GaN atoms form polar bonds and are polar semiconductors. Third, there is the issue of Si atom diffusion on the silicon substrate, which degrades the crystal quality of the epitaxial layer.

Theoretically, under the same breakdown voltage, GaN power devices have lower on-resistance than Si and SiC power devices. However, their current performance is far from the theoretical value. Research has found that the main reason is leakage current through the longitudinal GaN buffer layer between the device's source and drain, along the interface between the Si substrate and the GaN buffer layer [4]. Therefore, the current solutions to improve device breakdown voltage mainly focus on the following three directions: (1) improving substrate structure; (2) improving buffer layer structure; (3) improving device structure.

Devices based on the AlGaN/GaN structure are depletion-type (normally-on) devices. Enhancement-type (normally-off) power devices with positive threshold voltage can ensure the safety of power electronic systems, reduce system cost and complexity, and are the preferred devices in power systems. Therefore, the realization of enhancement-type devices is a highly concerned issue for GaN power devices. Currently, internationally, methods such as recessed gate, p-GaN gate, and fluorine ion implantation are widely used to directly achieve enhancement-type devices. Additionally, Cascode cascading technology is used to indirectly achieve normally-off devices.

The main methods to suppress current collapse are as follows: (1) Surface passivation, which has the issue of complex passivation processes and low repeatability, and cannot completely eliminate the current collapse effect. It also affects the device's gate leakage current and cutoff frequency, increasing the device's heat dissipation problem. (2) Field plate, in 2011, HRL in the USA used a triple field plate structure combined with SiN passivation to achieve high-voltage, low dynamic resistance Si-based GaN power devices. Under a switching speed of 5us, the ratio of dynamic to static Ron was 1.2 at 350V and 1.6 at 600V [5]. (3) Cap layer growth, such as using a p-type GaN cap layer to form a negative space charge layer with ionized acceptor impurities, shielding the influence of surface potential fluctuations on channel electrons. This method has a relatively simple material growth process and is easy to control, but it increases process difficulty, such as a more complex gate fabrication process. (4) Barrier layer doping, which increases the channel electron concentration or reduces the surface state density of the barrier layer. Generally, such devices grow a thin layer of undoped GaN or AlGaN cap layer.

The manufacturing process of GaN power devices is compatible with existing Si manufacturing processes, which is an important factor in promoting the industrialization and widespread application of GaN power devices. The key to developing a GaN power device manufacturing process compatible with existing Si manufacturing processes lies in the development of a metal-free process. In 2012, at the ISPSD conference, IMEC reported the fabrication of enhanced GaN power transistors on 8-inch GaN-on-Si wafers using a CMOS-compatible metal-free process combined with a recessed gate process [6]. In 2012, at the ISPSD conference, IMB-CNM-CSIC reported the fabrication of MIS-HEMT and i-HEMT on 4-inch Si using a CMOS-compatible metal-free process [7]. The development of metal-free processes has received significant attention from both academia and industry in recent years and is an important way to reduce costs for mass production and large-scale commercial applications.

Forming independent and complete functional modules that directly face end applications, including GaN power core devices, device drivers, protection circuits, and peripheral passive devices, is the current development direction of GaN power devices. Highly integrated GaN intelligent power integration technology will achieve high performance, high operating safety, high speed, and high-temperature endurance that traditional Si power chip technology cannot reach. Based on the development of GaN power device technology, the development of power integration technology is gradually becoming another hot spot in GaN research in recent years. In 2008, US-based IR released a GaN POL converter based on a Si substrate, with an input voltage of 12V, an output voltage of 1.2V under a 12A load current, and an operating frequency of 6MHz. In 2009, MIT in the USA reported the preparation of Si-GaN-Si wafers using wafer bonding and selective etching [8]. In 2009, Chen Wanjun et al. reported a GaN-on-Si switch-mode Boost converter, and K Y Wong et al. successfully achieved monolithic integration of high-voltage power devices and peripheral low-voltage devices [9]. In 2010, Transphorm released 800KHz 220-400V Boost converters based on AlGaN/GaN-on-Si and Si Sj-MOSFET.

With the continuous progress of various device technologies, GaN devices have gradually shifted from laboratories to the industry, and reliability has become a concern for all parties. Compared to silicon power device technology, the research on the reliability and stability of GaN power devices is relatively lagging behind. Although there are some research reports on device degradation laws, failure mechanisms and modes, and methods to enhance reliability, they are far from meeting the needs of devices entering large-scale practical application stages.

The reasons affecting the reliability of GaN power devices are relatively complex, including material quality, device structure, and device process. According to the working mode characteristics and working environment of power devices, the key points of reliability research for GaN power devices mainly include the following: (1) Gate