At present, the third-generation compound semiconductors represented by silicon carbide (SiC) and gallium nitride (GaN) are receiving increasing attention. They will use traditional silicon devices in future high-power, high-temperature, and high-voltage applications. Unachievable effect. Especially in the automobile of one of the three emerging application areas (automobile, 5G and Internet of Things), there will be very broad development prospects.
However, SiC and GaN are not the end point. Recently, gallium oxide (Ga2O3) has once again entered the field of vision. With its wider band gap than SiC and GaN, this compound semiconductor is unique in higher power applications Advantage. Therefore, the research on gallium oxide in recent years has heated up.
In fact, gallium oxide is not a very new technology. Many years ago, companies and research institutes studied its application in the field of power semiconductors. However, in terms of actual application scenarios, the past is not as wide as the applications of SiC and GaN, so Relevant research and development work has been snatched by the latter two. As the development of application requirements becomes clearer, the performance requirements of high-power devices will become higher and higher in the future, which makes people more deeply see the advantages and prospects of gallium oxide, and the corresponding research and development work has increased, and has become the United States , Japan, Germany and other countries research hotspots and competition focus. But our country is relatively lacking in this respect.
Advantages of gallium oxide
Gallium oxide is a wide band gap semiconductor with a band gap Eg = 4.9eV. It has good conductivity and light emitting characteristics. Therefore, it has broad application prospects in optoelectronic devices and is used as an insulating layer for Ga-based semiconductor materials. , And UV filters. These are the traditional application fields of gallium oxide, and its future power, especially high power application scenarios, is more worth looking forward to.
Although gallium oxide has poor thermal conductivity, its band gap (4.9eV) exceeds that of silicon carbide (about 3.4eV), gallium nitride (about 3.3eV), and silicon (1.1eV). Because the width of the forbidden band can measure the energy required to bring the electrons into a conducting state. A system made of a wide bandgap material can be thinner and lighter than a system made of a narrower bandgap material, and can handle higher power. It is expected to produce high voltage with low loss power components at low cost. In addition, the wide bandgap allows operation at higher temperatures, reducing the need for bulky cooling systems.
Relevant institutions in Japan have been leading the industry in the research of gallium oxide power devices. Earlier, research groups such as the Japan Information and Communications Research Institute (NICT) used Ga2O3 to trial-produce "MESFETs" (metal-semiconductorfield effect transistors). Although it has a very simple structure without a protective film (passivation film), the prototype has characteristics of high withstand voltage and small leakage current. When using SiC and GaN to manufacture devices with the same structure, it is very difficult to achieve characteristics such as prototypes.
In 2012, five crystal forms of Ga2O3 were identified: α, β, γ, δ, and ε. Among them, the β structure was the most stable. At that time, most of the research reports related to the crystal growth and physical properties of Ga2O3 used the β structure.
For example, β-Ga2O3 with a single crystal structure has a wide forbidden band, which makes its breakdown electric field strength very large, as shown in the figure below. The breakdown electric field strength of β-Ga2O3 is about 8MV / cm, which is more than 20 times that of Si, which is more than twice that of SiC and GaN.
As can be seen from the figure, the main advantage of β-Ga2O3 is the forbidden band width, but there are also shortcomings, mainly reflected in low mobility and thermal conductivity, especially thermal conductivity is its main short board. However, relatively speaking, these shortcomings will not have much effect on the characteristics of power devices, because the performance of power devices mainly depends on the breakdown electric field strength. In terms of β-Ga2O3, the "Baliga's figure of merit", which is a low loss index, is proportional to the third power of the breakdown electric field strength and proportional to the first power of the mobility. As a result, Barriga has a larger figure of merit, which is 10 times that of SiC and 4 times that of GaN.
Due to the higher Barriga's figure of merit of β-Ga2O3, when manufacturing unipolar power devices with the same withstand voltage, the on-resistance of the element is much lower than that of SiC or GaN. Reducing the on-resistance is helpful to reduce the power loss of the power circuit when it is on. Using β-Ga2O3 power devices can not only reduce the power loss during conduction, but also reduce the loss during switching, because unipolar components can be used in high withstand voltage applications with a withstand voltage of 1kV or more.
For example, a unipolar transistor (MOSFET) that uses a protective film to reduce electric field concentration to the gate is provided, and its withstand voltage can reach 3k to 4kV. When using silicon, a bipolar element must be used when the withstand voltage is 1kV, and even if SiC with a higher withstand voltage is used, a bipolar element must be used when the withstand voltage is 4kV. Bipolar elements use electrons and holes as carriers, so compared with unipolar elements that only use electrons as carriers, the generation and disappearance of carriers in the channel will occur during on and off switching operations. It takes time, and the loss easily increases.
In terms of thermal conductivity, if the thermal conductivity is low, it is difficult for power devices to work at high temperatures. However, the working temperature in practical applications generally does not exceed 250 ° C, so no major problems will arise in practical applications. In addition, the heat-resistant temperature of the packaging materials, wiring, solder, sealing resin, etc. used in the modules and power circuits that encapsulate power devices is only 250 ° C. Therefore, the operating temperature of power devices must be controlled below this level.
Research progress
High-quality β-Ga2O3 crystal
For a long time, China's research on β-Ga2O3 crystal materials and devices is relatively backward, especially for power devices. The key reason is that it is limited by the availability of large-size, high-quality β-Ga2O3 crystals.
In August 2017, the team of Associate Professor Tang Huili and Professor Xu Jun of the School of Physical Science and Engineering of Tongji University in China successfully adopted the guided mode method of independent intellectual property rights to successfully prepare a 2-inch high-quality β-Ga2O3 single crystal. The obtained high-quality β-Ga2O3 single crystal, X-ray double crystal rocking curve with a full width at half maximum of 27 ″, a dislocation density of 3.2 × 104cm-2, and a surface roughness of less than 5A. Development of devices and detection devices.
α-Ga2O3
In early 2018, Denso and FLOSFIA decided to jointly develop a new generation of gallium oxide power semiconductor materials for automotive applications-α-Ga2O3.
α-Ga2O3 is the world's first single-crystal synthetic material developed by Professor Shizuo Fujita of Kyoto University. It can be used in converters for electric vehicles. It can achieve low power consumption, low cost, and small size and weight.
FLOSFIA is a joint venture company founded by Kyoto University in 2011. Committed to the development of α-Ga2O3 power semiconductors. In 2015, the world's smallest on-resistance 0.1mΩcm2 SBD (Schottky Barrier Diode) trial production data was published. In 2016, a new type P-type semiconductor α-Ir2O3 was successfully developed.
Gallium oxide MOSFET
Earlier this year, Dr. Uttam Singisetti, an associate professor of electrical engineering at the University of Buffalo (UB) School of Engineering and Applied Sciences, and his students made a 5 micron-thick GaN MOSFET.
The researchers said that the transistor's breakdown voltage was 1,850 V, more than double the record for gallium oxide semiconductors. Breakdown voltage is the amount of electricity required to convert a material, in this case gallium oxide, from an insulator to a conductor. The higher the breakdown voltage, the higher the power the device can handle.
Singisetti said that because transistors are relatively large, they are not suitable for smartphones and other small devices. But it may help regulate energy flows in large-scale operations, such as power plants that harvest solar and wind energy, as well as electric cars, trains, and airplanes.
However, this research needs to be further explored to address its shortcomings of poor thermal conductivity.
Vertical Ga2O3 Power Device
Recently, the Japanese Institute of Information and Communications Technology (NICT) and Tokyo University of Agriculture and Technology (TUAT) demonstrated a "vertical" gallium oxide MOSFET that uses an "all-ion-implanted" process for N-type and P-type Doping, paving the way for low-cost, high-manufacturability Ga2O3 power electronics.
For the past few years, the development of Ga2O3 transistors has focused on the study of lateral geometry. However, due to the large area, reliability issues caused by heat generation, and surface instability, lateral devices cannot easily withstand the high current and high voltage tests required by many applications.
In contrast, the vertical geometry can be driven at higher currents without having to increase chip size, which simplifies thermal management. The characteristics of vertical transistor switches are designed by introducing two kinds of impurities (dopants) into the semiconductor. When the switch is "on", the N-type doping provides mobile carriers (electrons) for carrying current; when the switch is "off", the P-type doping will initiate voltage blocking.
The NICT research team led by Masataka Higashiwaki was the first to use silicon as an N-type dopant in Ga2O3 devices, but the scientific community has long been working to find a suitable P-type dopant. Earlier this year, the same research group announced the feasibility of using nitrogen (N) as a P-type dopant. Their latest achievements include the first high-energy dopant introduction process, the so-called "ion implantation", integrating silicon and nitrogen doping to design a Ga2O3 transistor.
It is reported that the vertical power device can achieve a current of more than 100A and a voltage of more than 1kV. Such a combination is required by many applications, especially the power industry and automotive power systems.
Research on Thermal Management Methods
Recently, researchers at the University of Florida, the United States Naval Research Laboratory, and the University of Korea are also studying gallium oxide MOSFETs. Stephen Pearton, a professor of materials science and engineering at the University of Florida, said they are studying the potential of gallium oxide as a MOSFET. Traditionally, these miniature electronic switches are made of silicon and are used in laptops, smartphones and other electronics. For systems like electric vehicle charging stations, we need MOSFETs that can operate at higher power levels than silicon-based devices, and gallium oxide may be the solution. To implement these advanced MOSFETs, the team identified a need for improved gate dielectrics and more effective thermal management methods to release heat from the device.