Diamond MOSFET, the first in Japan

Latest update time：2024-01-30

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Source : Content compiled and compiled from eetjp by Semiconductor Industry Observation (ID: ic bank ) , thank you.

Japan's National Institute of Materials Science (NIMS) announced the development of the world's first "n-type diamond MOSFET" on January 25, 2024. Its field effect mobility is approximately 150cm²/V·sec at 300℃. It is possible to realize diamond CMOS integrated circuits.

In principle, diamond semiconductors can achieve 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 to realize CMOS structures has not yet been achieved.

In order to form an n-type diamond MOSFET, it is necessary to grow a high-crystal quality diamond n-type channel epitaxial layer (hereinafter referred to as the epitaxial layer) and a highly conductive n+ contact epitaxial layer.

The NIMS research team used NIMS' proprietary microwave plasma chemical vapor deposition (MPCVD) method to precisely control the doping concentration on the [111] crystal plane of a high-temperature and high-pressure synthesized (HPHT) single-crystal diamond substrate. A high-quality n-type diamond epitaxial layer is formed.

Specifically, a lightly phosphorus-doped n-diamond epitaxial layer for the device channel is grown directly on the surface of the HPHT diamond substrate. A heavily 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 homoepitaxial growth of n-type diamond atomically forms steps with an average roughness of about 0.1 nm.

Furthermore, secondary ion mass spectrometry (SIMS) was performed and found that the phosphorus concentration was evenly distributed within the growth surface and that the hydrogen content that deactivated the donor was below the measurement limit. The electron mobility of the diamond epitaxial layer in a high temperature environment of 300°C is 212cm²/V·sec.

The research team verified the operation of the fabricated 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 electronic (n-type) conductivity was confirmed from the polarity. Compared with room temperature, the drain current value at 300°C increases by approximately 4 orders of magnitude, and the field-effect electron mobility at 300°C is approximately 150cm²/V·sec.

In addition, microsecond-level switching speeds are achieved at high temperatures of 300°C. Increasing the gate amplitude increases the conductivity of the channel, allowing for faster switching speeds.

Japanese team releases diamond MOSFET

Waseda University and Power Diamonds Systems (PDS) developed a structure in which the diamond surface is covered with silicon oxide terminals (C-Si-O terminals), which turns off the transistor when the gate voltage is 0V. To this end, they announced the development of a "normally off" diamond MOSFET.

The results were contributed by Professor Hiroshi Kawarada, FU Yu, Norito Narita, Xiahua Zhu, Waseda University Adjunct Professor Atsushi Hiraiwa, PDS's Kosuke Ota, PDS co-founder and CEO Tatsuya Fujishima and others. Details were announced on December 13 at the IEEE International Electronic Devices Meeting (IEDM 2023), an international conference on semiconductor devices/process technologies.

MOSFET is a field effect transistor (FET) with a MOS structure . It has the characteristics of high speed, low on-resistance, and high breakdown voltage. It is especially suitable as a switching element for motor driving. High-speed switching of large currents has been completed.

Regarding diamond semiconductors , which are called the ultimate power semiconductor materials , research and development of diamond MOSFETs using hydrogen termination (CH) structures are being conducted around the world, but due to 2DHG, the transistors are turned on even if the gate voltage is 0V." Normally-on" operation, and it is impossible to achieve the normally-off state of the transistor when the gate voltage is 0V.

Therefore, if a normally-on device is used in a power electronics device , there is no way to safely stop the device when it stops operating normally, so normally-off operation is required. Against this background, the PDS and Waseda University research teams found that due to high-temperature oxidation, the CH bonds of hydrogen atoms covering the diamond surface are converted into CO bonds, and the surface becomes an electron defect, causing its performance to deteriorate. The company has been working on improving this to achieve stable operation of FETs.

In this study, we adopted a device structure in which the diamond surface has silicon oxide (C-Si-O) bonds instead of the traditional CO-Si bonds. As a result, the hole mobility of p-channel MOSFET is 150cm2/V·s, which is higher than the electron channel mobility of SiC n-channel MOSFET, and the signal threshold voltage of normally-off operation is 3~5V, which is the traditional diamond semiconductor Unachievable, a value said to prevent accidental conduction (short circuit).

In addition, the maximum drain current of PDS for horizontal silicon oxide terminated diamond MOSFET exceeds 300 mA/mm, and the maximum drain current of vertical silicon oxide terminated diamond MOSFET exceeds 200 mA/mm. This is said to be the highest value for a normally-off 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 that their usability makes them suitable for mass production. To this end, Professor Kawaharada believes that diamond power semiconductors that are easy to realize in society have been realized, and PDS will continue to strengthen the research and development of diamond MOSFETs in order to popularize and practicalize diamond semiconductors. Their goal is to develop a device process suitable for mass production and achieve higher voltage resistance with a simpler structure.

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