Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

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Perspective: Ga2O3 for ultra-high power rectifiers and MOSFETS

We fabricated inversion channel diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with normally off characteristics. At present, Si MOSFETs and insulated gate bipolar transistors (IGBTs) with inversion channels are widely used because of their high controllability of electric power and high tolerance. Although a diamond semiconductor is considered to be a material with a strong potential for application in next-generation power devices, diamond MOSFETs with an inversion channel have not yet been reported. We precisely controlled the MOS interface for diamond by wet annealing and fabricated p-channel and planar-type MOSFETs with phosphorus-doped n-type body on diamond (111) substrate. The gate oxide of Al2O3 was deposited onto the n-type diamond body by atomic layer deposition at 300 °C. The drain current was controlled by the negative gate voltage, indicating that an inversion channel with a p-type character was formed at a high-quality n-type diamond body/Al2O3 interface. The maximum drain current density and the field-effect mobility of a diamond MOSFET with a gate electrode length of 5 μm were 1.6 mA/mm and 8.0 cm2/Vs, respectively, at room temperature.

Inversion channel diamond metal-oxide-semiconductor field-effect transistor with normally off characteristics

Diamond has unique physical properties, which show great promise for applications in the next generation power devices. Hydrogen-terminated (C–H) diamond metal–oxide–semiconductor field-effect transistors (MOSFETs) often have normally-on operation in devices, because the C–H channel features a p-type inversion layer; however, normally-off devices are preferable in power MOSFETs from the viewpoint of fail safety. We fabricated hydrogen-terminated (C–H) diamond MOSFETs using a partially oxidized (partial C–O) channel. The fabricated MOSFETs showed a high breakdown voltage of over 2 kV at room temperature and normally-off characteristics with a gate threshold voltage $\text{V}_{\mathrm{th}}$ of −2.5–−4 V.

Normally-Off C–H Diamond MOSFETs With Partial C–O Channel Achieving 2-kV Breakdown Voltage

Recent advances in diamond power semiconductor devices

Diamond power devices: state of the art, modelling, figures of merit and future perspective

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.

C-H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation

Complementary power field effect transistors (FETs) based on wide bandgap materials not only provide high-voltage switching capability with the reduction of on-resistance and switching losses, but also enable a smart inverter system by the dramatic simplification of external circuits. However, p-channel power FETs with equivalent performance to those of n-channel FETs are not obtained in any wide bandgap material other than diamond. Here we show that a breakdown voltage of more than 1600 V has been obtained in a diamond metal-oxide-semiconductor (MOS) FET with a p-channel based on a two-dimensional hole gas (2DHG). Atomic layer deposited (ALD) Al2O3 induces the 2DHG ubiquitously on a hydrogen-terminated (C-H) diamond surface and also acts as both gate insulator and passivation layer. The high voltage performance is equivalent to that of state-of-the-art SiC planar n-channel FETs and AlGaN/GaN FETs. The drain current density in the on-state is also comparable to that of these two FETs with similar device size and VB.

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Durability-enhanced two-dimensional hole gas of C-H diamond surface for complementary power inverter applications

More than 1600V breakdown voltages have been obtained in hydrogen terminated (C-H) diamond planar p-channel MOSFETs with gate-drain distance of 16–22 μm. The drain current density exceeds 100mA/mm in the FETs. The blocking voltage and drain current characteristics are comparable to those of n-channel AlGaN/GaN FETs and planar SiC MOSFETs in a similar device size. Atomic layer deposited Al2O3 works as gate insulator and passivation layer. It also induces the 2 dimensional hole gas ubiquitously on C-H diamond surface not only in planar, but in a trench gate structure. The first diamond vertical MOSFET has also operated using the trench structure.

Diamond MOSFETs using 2D hole gas with 1700V breakdown voltage

By forming a highly stable Al 2 O 3 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 −263°C (10K) to 400°C (673K), the variation of maximum drain-current is within 50% at a given gate bias. The maximum breakdown voltage (V B,max ) of the MOSFET without a field plate is 996V at a gate-drain distance (L GD ) of 9µm. We fabricated some MOSFETs satisfying V B,max /L GD > 200V/µm (2MV/cm). This value is superior to those of lateral SiC or GaN FETs.

Tetsuya Yamada

Papers

C-H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation

Normally-Off C–H Diamond MOSFETs With Partial C–O Channel Achieving 2-kV Breakdown Voltage

Durability-enhanced two-dimensional hole gas of C-H diamond surface for complementary power inverter applications

Diamond MOSFETs using 2D hole gas with 1700V breakdown voltage

Wide temperature (10K–700K) and high voltage (∼1000V) operation of C-H diamond MOSFETs for power electronics application