Diamond is a promising material for power applications owing to its excellent physical properties. Two-dimensional hole gas (2DHG) diamond metal–oxide–semiconductor field-effect transistors (MOSFETs) with hydrogen-terminated (C-H) channel have high current densities and high breakdown fields but often show normally- ON operation. From the viewpoint of safety, normally- OFF operation is required for power applications. In this letter, we used ion implantation to form a shallow and thin nitrogen-doped layer below the C-H channel region, which realized normally- OFF operation. Nitrogen-ion implanted length is fixed at 5 or 10 $\mu \text{m}$ . Nitrogen is a deep donor (1.7 eV) and the nitrogen-doped layer prevents hole accumulation near the surface. The threshold voltage was as high as −2.5 V and no obvious dependence on the threshold voltage of nitrogen-ion implanted length is observed. The breakdown field was 2.7 MV/cm at room temperature. Of 64 devices with a common gate length, 75% showed normally- OFF operation. We confirmed the threshold voltage shift by a thin and shallow nitrogen-doped layer formed by ion implantation.

Normally-OFF Two-Dimensional Hole Gas Diamond MOSFETs Through Nitrogen-Ion Implantation

High temperature characteristics of bipolar-mode operation of normally-off diamond junction field-effect transistors were investigated up to 573 K. As an important factor, the current gain depending on the gate current was analyzed with a theoretical model. We found that the experimental current gain decreased with the rise in the gate current, in agreement with the theoretical estimation considering the recombination at the end regions. We achieved 4–9 times higher drain currents in the bipolar-mode compared with the unipolar-mode operation at a DC current gain of 10. Furthermore, the bipolar-mode currents at the high temperatures of 473 and 573 K became two orders of magnitude larger than the unipolar-mode current at room temperature with a large DC current gain of $10^{2}$ .

High Temperature Bipolar-Mode Operation of Normally-Off Diamond JFET

https://tsukuba.repo.nii.ac.jp/record/2001445/files/CARBON_168-659.pdf

Energy distribution of Al2O3/diamond interface states characterized by high temperature capacitance-voltage method

Recent developments in the use of diamond materials as metal-oxide-semiconductor field-effect transistors (MOSFETs) are introduced in this article, including an analysis of the advantages of the device owing to the unique physical properties of diamond materials, such as their high-temperature and negative electron affinity characteristics. Recent research progress by domestic and international research groups on performance improvement of hydrogen-terminated and oxygen-terminated diamond-based MOSFETs is also summarized. Currently, preparation of large-scale diamond epitaxial layers is still relatively difficult, and improvements and innovations in the device structure are still ongoing. However, the key to improving the performance of diamond-based MOSFET devices lies in improving the mobility of channel carriers. This mainly includes improvements in doping technologies and reductions in interface state density or carrier traps. These will be vital research goals for the future of diamond-based MOSFETs.

Recent progress in diamond-based MOSFETs

This article reviews the state of the art in inversion-type p-channel diamond MOSFETs. We successfully developed the world’s first inversion-channel homoepitaxial and heteroepitaxial diamond MOSFETs. We investigated the dependence of phosphorus concentration (NP) of the n-type body on field-effect mobility (μFE) and interface state density (Dit) for the inversion channel homoepitaxial diamond MOSFETs. With regard to the electrical properties of both the homoepitaxial and heteroepitaxial diamond MOSFETs, they suffer from low μFE and one main reason is high Dit. To improve the interface quality, we proposed a novel technique to form OH-termination by using H-diamond followed by wet annealing, instead of the previous OH-termination formed on O-diamond. We made precise interface characterization for diamond MOS capacitors by using the high-low C–V method and the conductance method, providing further insights into the trap properties at Al2O3/diamond interface, which would be beneficial for performance enhancement of the inversion-type p-channel diamond MOSFETs.

/pdf/inversion-type-p-channel-diamond-mosfet-issues-3quq3irooz.pdf

Inversion-type p-channel diamond MOSFET issues

The electrical properties of Al2O3/p-type diamond (111) MOS capacitors were studied with the goal of furthering diamond-based semiconductor research. To confirm the formation of an inversion layer in the p-type diamond body, an n-type layer for use as a minority carrier injection layer was selectively deposited onto p-type diamond. To form the diamond MOS capacitors, Al2O3 was deposited onto OH-terminated diamond using atomic layer deposition. The MOS capacitor showed clear inversion capacitance at 10 Hz. The minority carrier injection from the n-type layer reached the inversion n-channel diamond MOS field-effect transistor (MOSFET). Using the high-low frequency capacitance method, the interface state density, D it, within an energy range of 0.1–0.5 eV from the valence band edge energy, E v, was estimated at (4–9) × 1012 cm−2 eV−1. However, the high D it near E v remains an obstacle to improving the field effect mobility for the inversion p-channel diamond MOSFET.

https://iopscience.iop.org/article/10.7567/JJAP.57.04FR01/pdf

Direct observation of inversion capacitance in p-type diamond MOS capacitors with an electron injection layer

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Direct observation of inversion capacitance in p-type diamond MOS capacitors with an electron injection layer