【World Express】FLOSFIA Solved the Problem of the P-layer of Gallium Oxide

For the first time in the world, the barrier effect of P-type semiconductor gallium oxide has been successfully confirmed.

Key points of this research

    The improvement measures, taken by FLOSFIA for the three environmental (energy, processes, materials) loads generated by the rapid development of semiconductors, are named "Semiconductor Ecology" ^※1 and FLOSFIA takes it as the ultimate goal, which aims to promote the popularization of the new power semiconductor^※2 gallium oxide (Ga₂O₃)^※3, developed by Kyoto University. The semiconductor ecology can maximize the physical parameters of gallium oxide. FLOSFIA believe it  indispensable for the application of high-quality P-type semiconductor combined with gallium oxide.

Figure 1. Comparison of devices made with only gallium oxide and devices made with P-type semiconductors.

    Since its cooperation with Kyoto University in 2016, FLOSFIA has successfully developed a new P-type semiconductor called iridium oxide (α-Ir₂O₃) with the same crystal structure as gallium oxide, which further promotes the device verification of the new P-type semiconductor layer. This time, FLOSFIA employs the most advanced JBS structure^※4 , which is used in the silicon carbide (SiC) diode, to gallium oxide devices, and uses the irium oxide [α-(IrGa)₂O₃]^※5 film as a P-type semiconductor layer, which in the world firstly successfully proved that the effect of the barrier effectively suppressed leakage current (FIG. 2, FIG. 3).

    The iridium gallium oxide used in this study is an ultra-wide band gap semiconductor^※6, with a large band gap of about 5 eV and a high hole concentration of 1x10¹⁹ cm^–3, indicating that it can be applied to a wide range of device designs based on high electric fields. Gallium oxide devices can be combined with this iridium oxide to maximize their characteristics as various power devices (e.g., SBD, MOSFET, and IGBT), allowing large current to flow on small chips. In addition, we believe that significant progress can be made in achieving semiconductor ecology by reducing equipment costs and so on.

Research Achievements

    First, a groove structure is made in a part of the gallium oxide n-layer, in which the new P-type semiconductor is embedded and crystal grows (Figure 2). The Mist Dry method^※7 proprietary to FLOSFIA is used for crystal growth. The size of the chip is about 0.9mm, and the line and space size (L\S) are 1 μ m each.

    Later, the made JBS construction chip is placed under the reverse voltage to check the effect of the embedded iridium gallium oxide structure on leakage current inhibition (for comparison, the SBD made from the same wafer is used). The inhibition effect was greater when the temperature increases from 25℃ to 125℃.

                                                                                         (a) Cross-sectional structure of the JBS structure                        (b) Top view of the microscope

Figure 2. Iridium gallium oxide applied to the JBS structure

      (A) Electrical characteristics of 25°C                          (B) Electrical characteristics of 125°C

Figure 3. Inverse direction characteristics

Future work

    The JBS structure is planned to be applied to the second generation diode of FLOSFIA's corundum gallium oxide (α-Ga₂O₃) power device "GaO" series. Low starting voltage is expected to cause the decrease of forward voltage drop Vf. The next goal is to apply the new P-type semiconductor "iridium gallium oxide" to transistors such as MOSFET and IGBT. For example, converters include commercial power supplies such as AC adapters, drive circuits for robots, white appliances such as electric vehicles, air conditioners or refrigerators, and power regulators for solar cells, aiming to use Ga₂O₃ power supplies to make breakthrough on the "miniaturization of converters and limits of low cost"^※8. Depending on the type of device, the converter can be miniaturized to a small percentage, and the cost reduction is expected to reach 50% of the entire power converter (FLOSFIA estimate).

【Glossary】

※1 Semiconductor ecology

The efforts made by minimizing the impact of the rapid development of semiconductors on the global ecological environment, in order to achieve comprehensive ecology, reduce energy loss [low energy loss], reduce manufacturing process loss [low process loss], and reduce limited earth resource loss in the manufacturing process [low physical loss] in the peripheral circuit system, and contribute to a sustainable new future. FLOSFIA is working with a number of companies to achieve the complete semiconductor ecology.

※2 Power Semiconductor

A semiconductor used for power conversion, which is capable of carrying a higher voltage and a larger current than a normal semiconductor, such as transistors, diodes, and thyristors.

※3 Gallium Oxide

As a new material of power semiconductor, it attracts much attention, with a variety of crystal structures, among which corundum structure (α structure), and β-gallium structure (β structure) are known. Compared with  β-gallium structure, a special crystal structure using only gallium oxide, gallidum structure also has sapphire and indium oxide, which is expected to be used in heterogeneous stacked crystal growth and devices.

※4 JBS structure

Closing potential barrier of the Schottky structure. The depletion layer diffused out of the embedded new P-type semiconductor layer reduces the electric field strength of the Schottky interface and reduces the leakage current.

※5 Iridium oxide gallium

Terternary mixed crystal, with a corundum structure, consisting of gallium, iridium and oxygen.

※6 Ultra-wide band gap semiconductor

The band gap of SiC and GaN is about 3 eV, which is called wide-band gap semiconductors, while the band gap of gallium oxide, diamond and AlN are even larger than these (generally 4 eV or greater).

※7 Mist Dry Method

The Mist CVD method, developed by Professor Shigeo Fujida and the research team at Kyoto University, is an exclusive improved deposition method improved by FLOSFIA; FLOSFIA uses the Mist Dry method to synthesize gallium oxide and control the concentration of impurities.

※8 miniaturization of converters and limits of low cost

As for the miniaturization and low cost of the converter, the high frequency of the working frequency is considered necessary. When silicon (Si) is used, it is difficult to miniaturize power converters. For example, because conversion loss increases at high frequency, additional heat dissipation measures are required. As a new semiconductor material, silicon carbide (SiC) can realize the miniaturization of the converter in high frequency operation, but limit its cost effectiveness due to the particularity of its synthesis method and process technology.