Power electronics is the technology that is being developed to achieve high-efficiency power conversion of electric power by static means required for energy savings which are directly related to the performance of power devices. Wide Bandgap (WBG) materials such as SiC and GaN have been systematically studied to achieve high operation voltage and energy conversion efficiency. It has improved the power density and efficiency of power electronics systems. Several advanced improvements are required to be considered for further development such as high breakdown voltage, low energy loss, low cost, and ease of integration of cutting-edge semiconductor technology.

Even though an increasing number of publications on Ga₂O₃ have been reported, the progress & performance of beta-Ga₂O₃ lag behind that of SiC & GaN. Behind every single power application, the metal-oxide-semiconductor-field-effect-transistor (MOSFET) is the most significant component of power application.

Fundamental properties of gallium oxide:

1) Crystalline structure of Ga₂O₃:

Corundum (α), and monoclinic (β) are the most commonly used polymorphs of Ga2O3 single crystals. There are other polymorphs such as defective spinel (γ), and orthorhombic (ε), with the δ phase broadly acknowledged as a form of orthorhombic phase.

Fig 1.(a) Fig 1.(b)

In Fig. 1 (a), all phases of Ga2O3 could stay steady under normal conditions and would translate into other phases under high temperatures or pressure. Among others, the most stable one is the monoclinic phase of Ga2O3 (β-Ga2O3).

Gallium (Ga) atoms exist only in two forms of four and six ligands as shown in Fig. 1 (b). [GaO6] octahedrons and distorted [GaO4] tetrahedrons are separately stacked in the crystal structure of β-Ga2O3. Physical and optical properties, for illustration, diverse warm conductivity at (-201), (100), (010) due to diverse positions of Ga and O atoms

2) Physical properties of β-Ga₂O₃:

The physical properties of some significant semiconductors are summarised in Table I. Here, the highest thermal conductivity of β-Ga₂O₃ is 0.27 W cm^-1 K^-1 and the lowest thermal conductivity is 0.1-0.3 W cm^-1 K^-1. By different figures-of-merits (FOMs), the comparison between various semiconductors could be carried out.

In addition to p-type doping, thermal conductivity is another issue for β-Ga₂O₃ power applications as shown in Table I. Therefore, it is believed that β-Ga₂O₃ will best exploit its potential in unipolar power devices, provided that the disadvantage of thermal conductivity is poor.

3) Single crystal Ga₂O₃:

Large-area crystals with low defect density are generally required to fabricate electronic devices suitable for high-voltage and high-power applications. Melt-grown methods have a good chance of achieving low-cost commercial substrates, and emerging competitive economic advantage over other WBG SiC, AIN, and diamond are examples of semiconductors.

Edge-defined film fed (EFG) is a widely used method to produce Ba2O3 crystals. There are also other methods such as floating zone (FZ), and vertical Bridgman (VB) growth methods.

Fabrication process for Ga₂O₃ FETs:

i) Gate dielectrics and surface passivation on Ga₂O₃ devices:

An effective grid-controlled ability is critical for the device performance of Ga2O3-based MOS devices, which must meet the following premise. First and foremost, the gate dielectric must be unreactive during processing and have thermodynamic stability with the semiconductor. High channel mobility, interface quality, and defect density are also in demand. Finally, the band offsets of gate dielectrics/Ga2O3 are discussed as one of the critical parameters, suitable gate dielectrics must possess sufficient band offsets (typically greater than 1 eV for conduction band offset and valence band offset) to act as electron or hole barriers, respectively.

The reported band offsets and band alignment types extracted from various dielectrics deposited on Ga2O3 or AlxGa1-xO is summarised in Table II.

ii) Contacts and Etching on Ga₂O₃:

To reduce the devices' power losses and specific on-resistance (Ron), it is important to have reliable ohmic contacts with low specific contact resistance and a suitable high temperature. Doping Si ions in Ga₂O₃/Ti/Au helps achieve ohmic contacts with low specific resistance and resistivity of 4.6×10-6 Ω.cm2 and 1.4 mΩ.cm.  Donor ions like Sn and Ge are also used in ion implantation to form drain/source electrodes of Ga2O3 MOSFETs.

Wet etching and dry etching processes are used in patterning structures and isolating devices of Ga₂O_3. The quality of the crystal decides the rate of wet etching. Liquid solutions of HF and HCl are effective to etch Ga2O3 at room temperature, meanwhile, H3PO4, KOH, and H₂SO₄ are used to wet etch Ga2O3 at higher temperatures. A higher resolution pattern process can be achieved with dry etching of Ga₂O₃. High-density plasma etches techniques like electron cyclotron resonance (ECR, which operates at 2.45 GHz), inductively coupled plasma (ICP, which operates at 2–13.56 MHz) and reactive ion etching (RIE) are used in dry etching. For Chlorine-based dry etching of β-Ga2O3, ICP was preferred over RIE considering etch rate and surface roughness due to higher plasma densities.

Fig. 2. (a) Etch rate of β-Ga2O3 vs. the Cl2-content in the Cl2/BCl3 ICP plasma [161], (b) etch rates vs. substrate temperature for BCl3/Ar and Cl2/Ar ICP etch [161]. (c) Etch rate of UID (-201) β-Ga2O3 vs. RIE power at various ICP powers [160], and (d) Etch rate vs. ICP power at 60W RIE power [160

Overview of Ga₂O₃ MOSFETs:

Using the Si+ ion implantation doping method, M. H. Wong et al. fabricated a Ga2O3 Field-Plated MOSFET (FP-MOSFET) consisting of a gate-connected field plate and chemical vapor deposited (CVD) SiO2 passivation layer with an off-state breakdown voltage of 755 V, current on/off ratio above 109, and stable high-temperature operation against 300°C thermal stress.

Fig. 3. Schematic cross-section of (a) generic Ga2O3 FP-MOSFET [22], (b) source-connected FP-MOSFET [23]

One way is to reduce the channel thickness to ensure that the channel is completely depleted below zero Vgs, including β-Ga2O3 on insulator field effect transistors (GOOI FETs) [17], [136], [137]. An alternative method is to adopt a thin active region to allow full channel depletion at zero gate bias (Vgs), which also includes the modulation of the doping concentration of channel (Nd), gate metal work function (Фm), and interfacial charge (Qit) of the oxide/Ga2O3 interface.

Fig. 4. (a) Schematic view of GOOI FET (b) Thickness-dependent ID-Vgs plots of various GOOI FETs from D-mode to E-mode [17]. (c) Temperature dependence characteristics [183] of ION and peak Gm, int and (d) Temperature dependence of Vth for β-Ga2O3 GOOI FETs on AlN/Si substrate

Developing the effective shallow acceptor of Ga2O3 and effective Ga2O3 p-type doping technology and severe self-heating effects from ultra-low thermal conductivity (κ) of Ga2O3 are the major challenges for the development of Ga2O3. These can be solved by considering the following points:

● Developing high-quality single crystals with large diameter sizes.

●  epitaxial growth.

● E-mode operation

● Thermally stable Schottky contacts.

● Process optimization.

● Gate modulation of transistors.

● Reduction of RON.

● Modelling and device simulation in Ga2O3.

● Device reliability and future analysis (FA).

● Thermal management.

                                                                    TABLE III

Power electronics are operated and implemented in electric motor controls, photovoltaic inverters, electric vehicle drives, rail transport, ships, windmills, and smart power grid for converting the energy in power systems. Today, devices based on Si and SiC dominate the high-voltage/high-power market, and ultra-wide bandgap (UWBG) materials (like Ga2O3, and diamond) are considered likely for the high-power market (>1 kW). Gas sensors and solar blind photodetectors are applications for Gallium (III) oxide [Ga2O3] semiconductors.

Conclusion:

Significant progress has been seen in the development of β-Ga2O3 single crystals and films in the past few years. Researchers demonstrated several effective solutions for the absence of p-type doping, which include a low doping channel, high work function gate metal, vertical moderately-doped fins, thin channel geometry using gate recess process, GOOI FET structure, and a thin depletion layer by gate region interface states. More investigation is in demand for p-type doping and device fabrications. It is necessary to develop β-Ga2O3 FETs for power applications and to reduce the gate length for the high-frequency application. Having addressed the above issues, it is anticipated that Ga₂O₃ devices will compete with medium to high-power Si and SiC devices. 

References:

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[4] S. J. Pearton, J. Yang, P. H. Cary IV, F. Ren, J. Kim, M. J. Tadjer, and M. A. Mastro, “A review of Ga2O3 materials, processing, and devices”, Applied Physics Reviews, vol. 5, no. 1, pp. 011301, Jan. 2018, 10.1063/1.5006941.

[5] M. A. Mastro, A. Kuramata, J. Calkins, J. Kim, F. Ren, and S. J. Pearton, “Perspective—opportunities and future directions for Ga2O3”, ECS Journal of Solid-State Science and Technology, vol. 6, no. 5, pp. 356-359, Apr. 2017, 10.1149/2.0031707jss.