Semiconductor refers to a material of conductivity between the conductor and the insulator at room temperature. As the cornerstone of the information age, the semiconductor industry supports the development and progress of information technology in society as a whole and further changes the way of life, production, communication and thinking. In general, the first-generation semiconductor materials specifically refer to silicon materials and the second-generation semiconductor materials specifically refer to GaAs materials. In recent years, the third-generation semiconductor materials represented by GaN/SiC wide energy gap semiconductors (energy gap is an important characteristic parameter of semiconductors whose value depends mainly on the semiconductor's energy band structure, that is, related to the crystal structure and the binding property of atoms) have attracted the attention of governments, industries and research communities all over the world and achieved rapid development. Third-generation semiconductor materials have such superior performance including high voltage resistance, high frequency, high efficiency, high temperature resistance and high radiation resistance, that they look to be the "core" of a new generation of information technology, energy saving and smart manufacturing, can be used to produce high voltage, high power devices and have broad application prospects in many fields such as consumer electronics, industrial drives, agricultural machinery, power transmission, rail transit and military applications. China attaches great importance to the research on third-generation semiconductors, and GaN and SiC semiconductor devices have been included in the "new generation information functional materials and devices" and "strategic emerging industries" with key support in the Outline of the National Program for Long- and Medium-Term Scientific and Technological Development (2006-2020) and the Outline of the Twelfth Five-Year Plan.

Hongyu Yu, a professor from the Department of Electrical and Electronic Engineering of SUSTech, is an expert in the micro-nano electronic device field. He joined SUSTech as one member of the first batch of the national "Young Overseas High-level Talents Plan" in 2011. Combined with the needs of specific industries in China, particularly in Shenzhen, he established the "Shenzhen Key Laboratory of Third Generation Semiconductor Device", key infrastructure in which he actively carried out research in GaN power devices, GaN sensors and multiplayer ceramic chip capacitors (MLCC).

Hefty power adapter can also be "compact"

Power semiconductor devices are the basis of power electronics technology. Power devices usually operate under high voltage and high current and have a wide range of applications. They are another major branch of the semiconductor industry, second only to very-large-scale integrated circuits (VLSI). Research and development of new energy-efficient power semiconductor devices can promote the overall upgrading of the industry, reduce power consumption of devices and realize energy saving, emission reduction and green carbon development. GaN-based switching power devices have higher breakdown voltages and lower on-resistance (resistance across the device when powered) than Si and are ideal materials for advanced power electronic elements.

The team led by Hongyu Yu has conducted a lot of research in the GaN power devices and packaging technology and made a series of creative achievements. At present, a 600V D-mode HEMT (High Electron Mobility Transistor) device has been developed on a Complementary Metal-Oxide-Semiconductor (CMOS) compatible process platform (FIGS. 1-3); and based on the HEMT devices, a 65W compact power adapter with dimensions of 54 mm × 29 mm × 22 mm (only 28% of the volume of its kind) and up to 93% efficiency (Figure 4) has been developed.

Among GaN power devices, AlGaN/GaN HEMTs can best embody the advantages of GaN materials. The high electron mobility of GaN HEMTs originates from the 2DEG (meaning that the electron gas is free to move in a two-dimensional direction and is limited in the third dimension) at the AlGaN/GaN interface due to polarization. The presence of 2DEG can significantly reduce the device’s on-resistance and increase the operating frequency of the device, but it is precisely because of the presence of 2DEG that the channel has been in the on state, which is not conducive to practical applications. Therefore, how to fabricate an enhanced (normally-off) GaN HEMT has become an urgent issue to be solved in the current GaN HEMT industry. The Team led by Hongyu Yu focuses on research of enhanced (normally-off) GaN HMET devices and uses innovative device structures to achieve the purpose of the positive threshold value of the device, and PCT patents have been applied for.

Figure 1. Photo of 6-inch GaN on Si HEMT device

Figure 2. (a) MIS-HEMT structure; (b), (c) single-finger/multi-finger device optical microscopic image

Figure 3. Typical output characteristics and transfer characteristics of a 600 V Si-based GaN HEMT device with a channel width of 0.25 mm

Figure 4. 65W compact power adapter

Exhaust gas, harmful gas and poison gas detection

IOT technology is an important application of information technology, and it cannot do without sensor technology. Gas sensors have a wide range of applications in daily life and industry, from detecting car exhausts and harmful gases arising from decoration to that of exhaust gas residue produced in industrial production, laboratory gas leaks, and measurement of gas concentration in the field of scientific research. Therefore, gas sensors have a high research value and a huge market.

However, the conventional Si-based sensor can only be used below 350℃. GaN, with a high energy gap, can withstand higher temperatures and work in harsher environments. At the same time, 2DEG can be formed by depositing an AlGaN layer on the GaN substrate, which improves the output current and thus increases the detection limit and sensitivity of the sensor. Therefore, the team led by Hongyu Yu together with Professor Fei Wang at Electrical and Electronic Engineering, collaborated with other world-class research teams and companies to jointly carry out research on GaN HEMT high temperature gas sensors, and the research is closely linked to market demand and social green development needs and therefore, has good prospects.

Figure 5. Structure drawing and principles of a GaN-based gas sensor

Figure 6. Time response curve of GaN-based gas sensor

A core element of iPhone X and Tesla

Multi-layers Ceramic Capacitor (MLCC) is a new, miniaturized and chip-scale high-precision capacitor and is the core component among chip capacitive sensing elements (chip components are leadless or short-lead new tiny components) with the greatest usage and the fastest development. As one of the most important electronic components in the electronic information industry, MLCCs are widely used in surface mount technology (SMT) assembly of printed circuit boards (PCBs) and hybrid integrated circuits (HICs), realize miniaturized, digital, multi-functional and high-performance electronic products and complete machines, and have a decisive role in upgrading the core technology of the information industry.

From the market application of the MLCC all over the world in 2016, the mobile phone field occupies the highest proportion, i.e., 29.97%, followed computers, home appliances and automobiles. For example, each iPhone X is estimated to consume at least 1,000 MLCCs, while the automotive electronics makes the MLCCs consumed by electric vehicle reach up to 5,000, and the latest unmanned Tesla electric vehicle consumes up to 12,000 MLCCs. In recent years, MLCC has been widely used in DC/DC converter circuit and charge pump converter circuit. The operating frequency of these power circuits has reached 1 ~ 3MHz, MLCC in this frequency range has excellent performance of low energy loss, low ripple voltage, high conversion frequency, small size and low cost and can replace tantalum electrolytic capacitors, which is conducive to realizing miniaturized and lightweight portable products. As an important gathering place for electronic products in China and even the world, Shenzhen has gathered a large number of tech giants at home and abroad in the electronic information industry: Huawei, ZTE and IBM. At the same time, it owns extremely developed components and parts supply and trade business such as SEG market to provide a relatively mature supporting supply chain for MLCC upstream and downstream electronic products.

Break through the siege to achieve independent research and development

Despite the promising market prospect, the market is still dominated by a few large foreign companies such as Japan's Murata, TDK, Kyocera, Taiyo Yuden and South Korea's Samsung. Due to the key materials for manufacturing high-performance MLCC, high-end equipment for manufacturing miniaturized products need to be imported, and the technical blockade of international counterparts make the MLCC industry chain in China very weak and greatly restricted. Due to weak competitiveness in domestic and overseas markets, China cannot get better market opportunities and economic benefits. According to statistics, from 2011 to 2015, the average annual foreign trade deficit of MLCC in China exceeded 4 billion U.S. dollars.

The research focuses Hongyu Yu’s team in this project include MLCC formula powder (including high purity nano-barium titanate base powder), ultra-thin layer casting process/equipment and BME-MLCC low temperature sintering with an aim to realize import substitution of high-capacity and ultra-thin MLCC products and related raw materials. In the aspect of powder preparation, the barium titanate powder of a small size (100 nm) and high tetragonality (c/a 1.008) is synthesized in the improved sanding solid/hydrothermal method. The ceramic samples achieved a dielectric constant of ~ 3000, with a dielectric loss ~ 4%, resistivity 1012 Ω · cm or more and an excellent performance, and it has the potential to catch up with the international advanced level. In the aspect of tape casting: they have developed single ethanol, water-based and other environment-friendly ultra-thin layer (20 μm) cast films, and independently developed the supporting molding equipment, forming a process with independent intellectual property rights.

Strive for mutual assistance and win-win relationships through university-enterprise cooperation

The team led by Hongyu Yu has established close links with  Shenzhen Founder Microelectronics, Suzhou Enkris and other companies specialized in this field. Taking cooperation with Founder Microelectronics as an example, the team is responsible for formulating the design scheme and technical route for GaN devices, GaN devices design, process simulation and single-step process experiments. Shenzhen Founder Microelectronics established GaN power device R & D lines based on the needs of the laboratory of SUSTech, and is responsible for the single process development, integrated process development, the actual device tape-out, and so on. In addition, the two parties will work together on device testing and failure analysis to optimize the stability and uniformity of the device parameters, thus ensuring yield improvements. Through industry-university-research cooperation, the research results of GaN can be successfully industrialized and scientific research put into practice to bring about economic growth. 

Source: Hongyu Yu research group

Website：http://www.sustc.edu.cn/en/info_focus/2266