A normally off hydrogen-terminated diamond (H-terminated diamond) field-effect transistor (FET) has been successfully achieved with Ti/TiOₓ gate materials. A 5-nm Ti film was deposited on H-terminated diamond by electron beam evaporation technique and then it was thermally oxidized in air at 120 °C for 10 h to form Ti/TiOₓ, which was confirmed by X-ray photoelectron spectroscopy. The threshold voltage of H-terminated diamond FET is -0.14 V at VDS of -8 V, indicating a normally off operation. The normally off property could be attributed to the difference of work difference between Ti and H-terminated diamond, which may deplete the hole carriers in H-terminated diamond 2-D hole gas (2DHG) conduction channel. The maximum mobility of H-terminated diamond FET is 313 cm²/Vs at VGS of -0.2 V. And, the fixed negative charge density is 3.37 x 10₁₁ cm⁻². The results demonstrate that Ti/TiOₓ H-terminated diamond FET is a good solution to fabricate normally off device with a simple fabrication process, undamaged 2DHG channel, and uncontaminated interface between Ti and TiOₓ gate materials, which may promote the development of normally off H-terminated diamond FET.

Fabrication of single crystal hydrogen-terminated diamond MOSFET with dielectrics of LiF/Al2O3 has been successfully carried out. After patterning source and drain electrodes, 60nm LiF/ 20nm Al2O3 were deposited as dielectrics. The output and transfer characteristics were investigated, indicating the typical p-type channel MOSFET. The on/off ratio was ~109, which was high enough for practical applications. The fixed and trapped charge in LiF/Al2O3 were also examined. Based on the capacitance-voltage curves, the dielectric constant of LiF/Al2O3 was calculated to be 36.1, which was ascribed to the LiF supercapacitor structure. To the best of our knowledge, this LiF supercapacitor structure was first used in hydrogen-terminated diamond MOSFET to improve the dielectric constant.

The dynamic on-resistance (R_ON) behavior of one commercial GaN HEMT device with p-GaN gate is investigated under hard-switching conditions. The non-monotonic performance of dynamic R_ON with off-state voltage ranging from 50 to 400 V is ascribed to the “leaky dielectric” model. The highest normalized R_ON value of 1.22 appears at 150 and 200 V. The gradual increase and following maximum of dynamic R_ON are found when the device is exposed to a stress voltage for an extended stress time under 100 and 200 V, which is due to a much longer trapping time compared to detrapping time related to deep acceptors and donors. No obvious R_ON degradation, thanks to the suppressed trapping effect, is observed at higher V_DS. From the multi-pulse test, the dynamic R_ON is seen to be insensitive to the frequency. It is demonstrated that the leakage, especially under source and drain contact, is a key issue in the dynamic resistance degradation.