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On January 25, 2024, Japan's National Institute of Materials Science (NIMS) announced the development of the world's first N-type diamond MOSFET. Its field effect mobility at 300 is about 150 cm v·sec. It is possible to realize diamond CMOS integrated circuits.
In principle, diamond semiconductors are capable of achieving high dielectric strength and high-speed switching even in high-temperature and high-radiation environments. However, due to the difficulty of controlling doping, the formation of N-channel MOSFETs necessary for the realization of CMOS structures has not yet been realized.
In order to form N-type diamond MOSFETs, it is necessary to grow a diamond N-channel epitaxial layer with high crystal quality (hereinafter referred to as the epitaxial layer) and an N+ contact epitaxial layer with high conductivity.
The NIMS research team used NIMS' proprietary microwave plasma chemical vapor deposition (MPCVD) method to precisely control the doping concentration on the [111] crystal planes of high-temperature and high-pressure synthesis (HPHT) single crystal diamond substrates. A high-quality n-type diamond epitaxial layer is formed.
Specifically, the light phosphorus-doped N-diamond epitaxial layer used for the device channel grows directly on the surface of the HPHT diamond substrate. The heavy phosphorus-doped n+ layer is then deposited on the n-layer to form an ohmic contact. When confirmed using atomic force microscopy (AFM), it was found that the homogeneous epitaxial growth of n-type diamond was formed atomically with an average roughness of about 01 nm steps.
In addition, secondary ion mass spectrometry (SIMS) was performed to reveal that the phosphorus concentration was uniformly distributed within the growth surface and that the hydrogen content that inactivated the donor was below the measurement limit. The electron mobility of the diamond epitaxial layer at 300 °C is 212 cm v·sec.
The research team verified the operation of the manufactured MOSFETs. As a result, the drain current flowing between the source and drain (n+ layer) can be controlled by the voltage applied to the gate, and the electronic (n-type) conductivity is confirmed from the polarity. The drain current at 300 increases by about 4 orders of magnitude compared to room temperature, and the field-effect electron mobility at 300 is about 150 cm V·sec.
In addition, microsecond switching speeds are achieved at high temperatures of 300°C. Increasing the gate amplitude increases the conductivity of the channel, resulting in faster switching speeds.
The Japanese team released a diamond MOSFET
Waseda University and Power Diamonds Systems (PDS) have developed a structure in which the diamond surface is covered with silicon oxide terminations (C-Si-O termimates) that turn off the transistor when the gate voltage is 0V. To this end, they announced the development of a "constant-off" diamond MOSFET.
The results were contributed by Prof. Hiroshi Kawarada, Fu Yu, Norito Narita, Xiahua Zhu, Adjunct Professor Atsushi Hiraiwa of Waseda University, Kosuke OTA of PDS, and Tatsuya Fujishima, Co-Founder and CEO of PDS, among others. Details were announced on December 13 at the IEEE International Electron Devices Conference (IEDM 2023), the international conference on semiconductor device process technology.
MOSFET is a MOS structure field effect transistor (FET), which has the characteristics of high speed, low on-resistance, and high breakdown voltage, and is especially suitable as a switching element for motor drive, and high-speed switching and high current have been completed.
With regard to diamond semiconductors, which are said to be the ultimate power semiconductor materials, research and development of diamond MOSFETs using hydrogen termination (CH) structures are being carried out all over the world, but due to 2DHG, it is impossible to achieve a normally off state where the transistor cut-off is possible even when the gate voltage is 0V.
Therefore, if a normally open type device is applied to a power electronic device, it will not be possible to safely stop the device when the device stops operating normally, so normally off operation is required. Against this background, the PDS and Waseda University research team found that the C-H bond of the hydrogen atom covering the surface of the diamond was transformed into a CO bond due to high-temperature oxidation, and the surface became an electronic defect, resulting in a deterioration of its performance. The company has been working to improve this to achieve stable operation of FETs.
In this study, we used a device structure in which the diamond surface has silicon oxide (C-SI-O) bonds instead of traditional Co-Si bonds. As a result, the hole mobility of the P-channel MOSFET is 150 cm 2 V·s, which is higher than that of the SiC N-channel MOSFET, and the signal threshold voltage for normally-off operation is 3 5 V, which is not possible with conventional diamond semiconductors, and is said to be a value that can prevent accidental conduction (short circuit).
In addition, PDS has a maximum drain current of more than 300 mA for horizontal SiOxide-terminated diamond MOSFETs and more than 200 mA for vertical SiOxide-terminated diamond MOSFETs. This is said to be the highest value of a Changguan diamond MOSFET in the series.
Both companies claim that by covering their surfaces with C-Si-O bonds, they have become stable devices that are more resistant to high temperatures and oxidation than traditional CH surfaces, and the company believes their availability makes them suitable for mass production. To this end, Professor Kawawara believes that diamond power semiconductors that are easy to realize in society have already been realized, and PDS will continue to strengthen the research and development of diamond MOSFETs with a view to popularizing and commercializing diamond semiconductors. Their goal was to develop a device process that was suitable for mass production and achieved higher voltage resistance with a simpler structure.
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