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Increasing passivated AlGaN power

28 d ago

Researchers at the University of Wisconsin-Madison (USA) have developed passivated aluminium gallium nitride metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) on sapphire, achieving a Baliga figure of merit (BFOM) 1.7 times higher than state-of-the-art passivated AlGaN-channel transistors on sapphire. The highest BFOM reached 325 MW/cm², with a breakdown voltage of 2.6 kV. While unpassivated devices can achieve higher breakdowns, their lack of passivation degrades dynamic performance, crucial for power electronics requiring both strong ON-state and OFF-state performance. Aluminium-rich AlGaN on sapphire offers high-field capabilities on a cost-effective, scalable substrate platform compared to other ultra-wide-bandgap (UWBG) materials. The team used metal-organic chemical vapor deposition (MOCVD) on AlN/sapphire templates, with a thin AlN layer between channel and barrier to improve two-dimensional electron gas (2DEG) performance. Hall measurements showed a carrier concentration of 1.1×10¹³/cm² and mobility of 167 cm²/V·s. Fabrication included graded AlGaN for low-resistance ohmic contacts, a 10 nm ALD Al₂O₃ gate dielectric, and 200 nm PECVD SiN passivation. A 1 μm source field plate was added. Devices with varying gate-drain distances (LGD) from 2–20 μm were tested. Longer LGD spread potential, reducing peak fields but increasing ON-resistance. Pulsed operation showed a 10% increase in ON-resistance and reduced drain current due to trapping effects. The 15 μm device had a threshold of −12 V, ON/OFF ratio of 10⁶, and stable operation up to breakdown. Average electric breakdown fields decreased with larger LGD (2.1 MV/cm for 2 μm; 1.35 MV/cm for 15 μm), attributed to field crowding. Compared to prior work, these passivated devices achieved over 30% improvement in average breakdown field (>1.3 MV/cm) at breakdowns above 2 kV. The gate dielectric smoothed field distribution, boosting BFOM. Benchmarking against Ga₂O₃ and diamond lateral transistors showed comparable performance, narrowing the gap in BFOM values. Future improvements may involve systematic field-plate optimization, alternative geometries, and improved mesa termination.