Developing power transistors for high-voltage applications requires materials with a wide bandgap and high critical electric field strength. Monoclinic β-Ga2O3 stands out with a bandgap of about 4.8 eV and a calculated critical field of up to 8 MV/cm, making it a promising platform for power electronics. However, uniformly doped channels in such transistors suffer from a trade-off between mobility and thickness: high mobility requires light doping, but then the channel must be thick, which increases the gate length and exacerbates short-channel effects. Modulation doping in β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures solves this problem by forming a two-dimensional electron gas with high mobility without ionized impurities in the channel. Previously, such structures were studied mainly for radio-frequency applications, while their potential for high-voltage operation remained largely unexplored. This work fills that gap by demonstrating a record breakdown voltage for modulation-doped gallium oxide transistors.
The heterostructures were grown by metalorganic chemical vapor deposition (MOCVD) in an Agnitron Agilis 100 reactor on Fe-doped (010) β-Ga2O3 substrates. First, an unintentionally doped β-Ga2O3 buffer layer about 100 nm thick was deposited, followed by a 5 nm β-(Al0.19Ga0.81)2O3 spacer, after which silicon δ-doping was introduced with a dose of 1 nmol, and a 20 nm β-(Al0.19Ga0.81)2O3 barrier layer was grown. To form source and drain contacts, etching was performed to a depth of 80 nm through a SiO2 mask to expose the channel, and then a heavily doped n++ β-Ga2O3 layer 120 nm thick was regrown. Ti/Ni contacts were deposited by electron-beam evaporation and annealed at 470 °C. Active regions were isolated by plasma etching, and 9 nm of Al2O3 was deposited by plasma-enhanced atomic layer deposition to suppress gate leakage. A 120 nm Ni gate was formed by evaporation, and the device was passivated with a 250 nm SiNx layer deposited at 350 °C. Breakdown voltage measurements were performed in Fluorinert liquid to prevent surface breakdown in air.
Hall measurements of the channel showed a carrier density of 3.2×10¹² cm⁻², mobility of 132 cm²/V·s, and sheet resistance of 14.8 kΩ/sq. A transistor with dimensions LGS/LG/LGD=1.5/2/3 μm exhibited a threshold voltage of −0.7 V, subthreshold swing of 94 mV/dec, ION/IOFF ratio greater than 10⁸, and off-state leakage current below 1.5×10⁻¹⁰ A/mm. The maximum drain current reached 54 mA/mm, and the on-resistance was 149 Ω·mm. The breakdown voltage increased monotonically with gate-drain distance: from 545 V at LGD=3 μm to 6.6 kV at LGD=28 μm. The average lateral electric field saturated at about 2.4 MV/cm for LGD>13 μm. The specific on-resistance for the device with LGD=28 μm was 196 mΩ·cm², yielding a power figure of merit of 222 MW/cm², a record for β-(AlxGa1-x)2O3/β-Ga2O3 MODFETs. Two-dimensional electrostatic modeling for LGD=13 μm at 3 kV showed that the peak field is localized at the gate edge and reaches 12 MV/cm in the barrier layer, indicating the potential for further improvement using field management techniques.
This work demonstrates that modulation-doped β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures can withstand voltages of several kilovolts without field plates or edge termination. This distinguishes it from related studies that focused either on the radio-frequency properties of such MODFETs or on high-voltage transistors with uniformly doped channels, where complex field management methods were required. Here, the record breakdown voltage is achieved thanks to the material itself and the two-dimensional electron gas architecture, simplifying the design and opening the way to scaling. The result places β-Ga2O3 MODFETs on par with the best lateral gallium oxide transistors and confirms their potential for next-generation power electronics.
Further development may proceed along the integration of field plates and optimization of edge termination to bring the average breakdown field closer to the theoretical limit of β-Ga2O3 (8 MV/cm). Improving heterostructure quality, for example, reducing parallel conduction in the δ-doped layer, would increase the two-dimensional electron gas mobility and reduce channel resistance. Scaling the gate length is also promising to lower the specific on-resistance while maintaining a high breakdown voltage. Reliability studies under prolonged high-voltage operation will be an important step toward practical application.
The results will influence the development of power converters, inverters for electric vehicles, energy transmission systems, and switched-mode power supplies, where compact devices with breakdown voltages of several kilovolts are required.
Immediate next steps include implementing field plates to suppress the peak field at the gate edge, optimizing the δ-doping profile to enhance mobility, and conducting reliability tests at high voltages and temperatures.
In the field of gallium oxide power electronics, the unresolved problem remains achieving high breakdown voltage simultaneously with low specific on-resistance. This work shows that modulation doping can mitigate this trade-off, but the peak field at the gate edge still limits the average breakdown field, requiring further research on electric field management.
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What the work does
This work created a modulation-doped field-effect transistor on a β-(Al0.19Ga0.81)2O3/β-Ga2O3 heterostructure with a two-dimensional electron gas, grown by MOCVD. The device showed a record breakdown voltage of 6.6 kV at a gate-to-drain distance of 28 µm, an average lateral field of 2.4 MV/cm, and a power figure of merit of 222 MW/cm².
Where its strength is
The achieved combination of high breakdown voltage and power performance demonstrates the potential of modulation doping in gallium oxide for high-voltage applications, overcoming the trade-off between mobility and channel thickness.
Where it could go next
Worth trying
Why this may matter
The development of high-voltage Ga2O3 transistors could lead to compact power converters for electric transport and power grids, reducing losses and improving efficiency.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
A breakdown voltage of 6.6 kV is sufficient for direct switching in medium-voltage distribution networks, which typically operate at 4–35 kV, and the device itself is only 100 μm wide.