World’s First N-Channel Diamond Field-Effect Transistor: (Left) Atomic force microscope image of diamond epilayer surface morphology. (Middle) Optical microscope image of the diamond MOSFET. (Right) Performance of the MOSFET measured at 300 °C. The drain current increased when the gate voltage (Vg) was increased from -20 V (indicated by a black line) to 10 V (indicated by a yellow line). (Image: National Institute for Materials Science)

A NIMS research team has developed the world’s first n-channel diamond MOSFET. It provides a key step toward CMOS integrated circuits for harsh-environment applications as well as the development of diamond power electronics.

Semiconductor diamond has outstanding physical properties such as ultra-wide bandgap energy of 5.5 eV, high carrier mobilities, and high thermal conductivity, which is promising for applications under extreme environmental conditions with high performance and high reliability, such as high temperatures and high levels of radiation (e.g., in proximity to nuclear reactor cores).

By using diamond electronics, not only can the thermal management demand for conventional semiconductors be alleviated, but these devices are also more energy efficient and can endure much higher breakdown voltages and harsh environments. On the other hand, with the development of diamond growth technologies, power electronics, spintronics, and MEMS sensors operable under high-temperature and strong-radiation conditions, the demand for peripheral circuitry based on diamond CMOS devices has increased for monolithic integration. For the fabrication of CMOS integrated circuits, both p- and n-type channel MOSFETs are required, the same as for conventional silicon electronics. However, n-channel diamond MOSFETs had yet to be developed.

This NIMS research team developed a technique to grow high-quality monocrystalline n-type diamond semiconductors with smooth and flat terraces at the atomic level by doping diamond with a low concentration of phosphorus (diagram on the left in the figure). Using this technique, the team succeeded in fabricating an n-channel diamond MOSFET for the first time in the world. This MOSFET is composed mainly of an n-channel diamond semiconductor layer atop a diamond layer doped with a high concentration of phosphorus (middle diagram in the figure). The use of the latter diamond layer significantly reduced source and drain contact resistance. The team confirmed that the fabricated diamond MOSFET actually functioned as an n-channel transistor. In addition, the team verified the excellent high-temperature performance of the MOSFET as indicated by its field-effect mobility — an important transistor performance indicator — of approximately 150 cm2/V-sec at 300 °C (graph on the right in the figure).

These achievements are expected to facilitate the development of CMOS integrated circuits for the manufacture of energy-efficient power electronics, spintronic devices, and MEMS sensors under harsh environments.