By forming a highly stable Al2O3 gate oxide on a C-H bonded channel of diamond, high-temperature, and high-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) has been realized. From room temperature to 400 °C (673 K), the variation of maximum drain-current is within 30% at a given gate bias. The maximum breakdown voltage (VB) of the MOSFET without a field plate is 600 V at a gate-drain distance (LGD) of 7 μm. We fabricated some MOSFETs for which VB/LGD > 100 V/μm. These values are comparable to those of lateral SiC or GaN FETs. The Al2O3 was deposited on the C-H surface by atomic layer deposition (ALD) at 450 °C using H2O as an oxidant. The ALD at relatively high temperature results in stable p-type conduction and FET operation at 400 °C in vacuum. The drain current density and transconductance normalized by the gate width are almost constant from room temperature to 400 °C in vacuum and are about 10 times higher than those of boron-doped diamond FETs.

Although the two-dimensional hole gas (2DHG) of a hydrogen-terminated diamond surface provides a unique p-type conducting layer for high-performance transistors, the conductivity is highly sensitive to its environment. Therefore, the surface must be passivated to preserve the 2DHG, especially at high temperature. We passivated the surface at high temperature (450 °C) without the loss of C-H surface bonds by atomic layer deposition (ALD) and investigated the thermal reliability of the Al2O3 film. As a result, C-H bonds were preserved, and the hole accumulation effect appeared after the Al2O3 deposition by ALD with H2O as an oxidant. The sheet resistivity and hole density were almost constant between room temperature and 500 °C by the passivation with thick Al2O3 film thicker than 38 nm deposited by ALD at 450 °C. After the annealing at 550 °C in air The sheet resistivity and hole density were preserved. These results indicate the possibility of high-temperature application of the C-H surface diamond device in air. In the case of lower deposition temperatures, the sheet resistivity increased after air annealing, suggesting an insufficient protection capability of these films. Given the result of sheet resistivity after annealing, the increase in the sheet resistivity of these samples was not greatly significant. However, bubble like patterns were observed in the Al2O3 films formed from 200 to 400 °C by air annealing at 550 °C for 1 h. On the other hand, the patterns were no longer observed at 450 °C deposition. Thus, this 450 °C deposition is the sole solution to enabling power device application, which requires high reliability at high temperatures.

Use of two-dimensional hole gas (2DHG), induced on a hydrogenated diamond surface, is a solution to overcoming one of demerits of diamond, i.e., deep energy levels of impurities. This 2DHG is affected by its environment and accordingly needs a passivation film to get a stable device operation especially at high temperature. In response to this requirement, we achieved the high-reliability passivation forming an Al2O3 film on the diamond surface using an atomic-layer-deposition (ALD) method with an H2O oxidant at 450 °C. The 2DHG thus protected survived air annealing at 550 °C for an hour, establishing a stable high-temperature operation of 2DHG devices in air. In part, this achievement is based on high stability of C-H bonds up to 870 °C in vacuum and above 450 °C in an H2O-containing environment as in the ALD. Chemically, this stability is supported by the fact that both the thermal decomposition of C-H bonds and reaction between C-H bonds and H2O are endothermic processes. It makes a stark contrast to the instability of Si-H bonds, which decompose even at room temperature being exposed to atomic hydrogen. In this respect, the diamond 2DHG devices are also promising as power devices expectedly being free from many instability phenomena, such as hot carrier effect and negative-bias temperature instability, associated with Si devices. As to adsorbate, which is the other prerequisite for 2DHG, it desorbed in vacuum below 250 °C, and accordingly some new adsorbates should have adsorbed during the ALD at 450 °C. As a clue to this question, we certainly confirmed that some adsorbates, other than those at room temperature, adsorbed in air above 100 °C and remained at least up to 290 °C. The identification of these adsorbates is open for further investigation.

Various insulators are used as gate dielectrics and passivation layers in wide-bandgap (WBG) semiconductor devices as well as in advanced Si devices, and the understanding of their current conduction mechanism is essential to achieve their high performance and high reliability. Because these insulators are more or less charged, the conduction current is mostly caused by the Fowler-Nordheim (FN) tunneling into charged insulators, ruling out the conventional analytic FN formula. In order to facilitate the analysis of these currents, we focused on the method, named sheet-charge approximation (SCA), of approximating the charge distribution in the insulators by a charge sheet that has the same areal density and centroid as those of the original. Using, as references, the results obtained exactly calculating the tunneling current in the framework of the Wentzel-Kramers-Brillouin approximation, we confirmed the advantage of SCA over the previous method using a tunneling-endpoint field, the error of SCA-estimated areal charge densities being at most 30% for rectangular charge distributions of which charge centroids are known as in stacked films. In a more general case where the centroid is unknown, the SCA usually provides only a charge moment with reference to the insulator/anode interface, being unable to decompose the moment into the areal charge density and centroid. However, this demerit of SCA can be overcome through a reverse-biased current-voltage measurement using a capacitor formed on a heavily doped substrate or a capacitor with a diffusion layer attached, which measurement provides a charge moment with reference to the original cathode/insulator interface. Using these two kinds of charge moments, we can separately extract the areal charge density and centroid. Hence, the SCA has practical significance as a tool for analyzing conduction currents through charged insulators, especially through stacked films, and accordingly will play an important role in improving the performance and reliability of gate dielectrics and passivation layers for various WBG semiconductor devices as well as of high-k gate stacks for advanced Si devices.