Ultrawide‑Bandgap β‑Ga₂O₃ Schottky Barrier Diodes: Material Fundamentals, Device Design, and Power Electronics Outlook

Abstract

Gallium oxide (β‑Ga₂O₃) offers an ultrawide bandgap (~4.8 eV), a high breakdown field (~8 MV cm⁻¹), and a superior Baliga figure of merit, positioning it as a leading candidate for next‑generation high‑power devices such as Schottky barrier diodes (SBDs). This review synthesizes the fundamental physical properties of β‑Ga₂O₃, evaluates recent progress in Ga₂O₃‑based SBDs, and compares strategies that enhance breakdown voltage and on‑resistance. Finally, it outlines the prospects for Ga₂O₃ SBDs in power electronics.

Background

The demand for high‑performance power semiconductors is escalating across electric power, industrial control, automotive, and consumer electronics. Ultrawide bandgap materials meet this need by enabling high voltage, high‑temperature operation. β‑Ga₂O₃ stands out with its 4.8‑eV bandgap, ~8 MV cm⁻¹ critical field, and low electron mobility (~200 cm² V⁻¹ s⁻¹), which together yield a Baliga figure of merit that surpasses Si, SiC, and GaN. Unlike SiC and GaN, β‑Ga₂O₃ substrates can be grown cost‑effectively via floating‑zone (FZ) or edge‑defined film‑fed growth (EFG) at atmospheric pressure, enabling large‑area, high‑quality single crystals. These attributes make β‑Ga₂O₃ an attractive platform for unipolar devices such as SBDs and MOSFETs.

Physical Properties of β‑Ga₂O₃

β‑Ga₂O₃ is the thermally stable monoclinic phase among five known Ga₂O₃ polymorphs. Its lattice constants (a = 1.22 nm, b = 0.30 nm, c = 0.58 nm) and ultrawide bandgap (4.7–4.9 eV) confer a critical field nearly double that of SiC and GaN. The material’s conductivity originates from oxygen‑vacancy–induced donor levels, which provide free electrons and enable n‑type doping with Si or Sn donors (10¹⁵–10¹⁹ cm⁻³). The resulting resistivity spans 10⁻³–10¹² Ω cm, while the bandgap can be tuned by dopants. Key challenges remain: (1) p‑type doping is ineffective due to deep acceptor levels; (2) thermal conductivity is low (0.1–0.3 W cm⁻¹ K⁻¹), limiting high‑power operation; (3) electron mobility is modest, restricting high‑frequency performance. Nonetheless, the ultrawide bandgap affords excellent UV‑sensing capabilities in the 250–280 nm range.

Basic Concept of Schottky Barrier Diodes

In an SBD, the Schottky barrier height (Φ_B) depends on the metal work function and the semiconductor electron affinity, but interface states can perturb this relationship. For β‑Ga₂O₃, Ni and Pt are common Schottky metals, while Ti and In serve as low‑resistance ohmic contacts after high‑temperature annealing. Optimizing doping (10¹⁶–10¹⁷ cm⁻³) ensures a depletion width that suppresses tunneling. Edge‑termination techniques—field plates (FP) and trench structures—redistribute the electric field, raising breakdown voltage (V_br) and reducing leakage. Surface passivation and high‑quality substrates further enhance V_br stability.

Development of β‑Ga₂O₃‑Based Schottky Barrier Diodes

Early devices on (010) β‑Ga₂O₃ single crystals achieved V_br ≈ 150 V, with Φ_B ≈ 1.3–1.5 eV. Subsequent work on (100)-oriented substrates (EFG‑grown) demonstrated V_br > 200 V and on‑resistance (R_on) as low as 2.9 mΩ cm², with forward current densities exceeding 400 A cm⁻². Al‑doped β‑Ga₂O₃ films grown by pulsed‑laser deposition (PLD) yielded SBDs with R_on = 2.1 mΩ cm² and 10¹¹ current on/off ratios. Halide vapor‑phase epitaxy (HVPE) produced 7 µm thick n⁻ layers on n⁺ substrates, maintaining a nearly constant Φ_B from 21 °C to 200 °C and enabling high‑temperature operation.

Advanced edge‑termination engineering—field plates and trenching—has pushed V_br into the kilovolt range while keeping R_on below 5 mΩ cm². For example, a Pt‑capped HVPE‑n⁻/(001) n⁺ device reached V_br = 1076 V, R_on = 5.1 mΩ cm². A trench‑terminated device achieved V_br > 400 V and R_on = 2.4 mΩ cm², outperforming commercial 600 V SiC SBDs in switching loss.

Despite the lack of reliable p‑type doping, heterojunction p‑Cu₂O/n‑Ga₂O₃ diodes have shown V_br up to 1.49 kV and R_on = 8.2 mΩ cm², suggesting a viable pathway for bipolar devices. However, the higher threshold voltage and minority‑carrier storage effect remain concerns.

Practical applications demonstrate that β‑Ga₂O₃ SBDs can rectify AC signals up to 100 kHz—comparable to SiC—making them suitable for power converters and motor drives.

Conclusions

β‑Ga₂O₃ Schottky barrier diodes are in a rapid development phase. Ongoing advances in substrate quality, epitaxial growth, and device engineering continue to lower R_on and raise V_br. While challenges such as low thermal conductivity and the absence of p‑type doping persist, the material’s ultrawide bandgap and high critical field promise superior performance in high‑voltage, high‑temperature power electronics.

Abbreviations

• AC – Alternating current
• BFOM – Baliga’s figure of merit
• CVD – Chemical vapor deposition
• EFG – Edge‑defined film‑fed growth
• FFT – Fast Fourier transform
• FP – Field plate
• FZ – Floating zone
• HRTEM – High‑resolution transmission electron microscopy
• HVPE – Halide vapor‑phase epitaxy
• IMECAS – Institute of Microelectronics of the Chinese Academy of Sciences
• MOCVD – Metal‑organic chemical vapor deposition
• MOSFET – Metal‑oxide‑semiconductor field‑effect transistor
• NICT – National Institute of Information and Communications Technology
• PLD – Pulsed laser deposition
• SBD – Schottky barrier diode
• TE – Thermionic emission
• TFE – Thermionic field emission
• WBG – Wide bandgap
• XRD – X‑ray diffraction