This study presents the reliable operation of Cr₂O₃:Mg/β-Ga₂O₃ p-n heterojunction diodes at 600°C. The diodes demonstrate a room-temperature on/off ratio of >10⁴ at ±5 V and a breakdown voltage of -390 V. The leakage current increases with temperature up to 600°C, attributed to Poole-Frenkel emission with a trap barrier height of 0.19 eV. After a 140-hour thermal soak at 600°C, the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm² to 1.59 V and 7.1 mΩ cm², respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface. The study also shows that the diodes exhibit stable reverse bias leakage current during prolonged high-temperature testing, but experience forward-bias changes in the form of increased on-state resistance, turn-on voltage, and forward current drop. The observed evolution in forward bias electrical properties is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface after prolonged operation at 600°C due to the temperature-induced diffusion of Mg dopants in Cr₂O₃. The study provides insights and design considerations for oxide-based ultra-wideband gap (UWBG) semiconductors for p-n heterojunctions applicable in harsh operating environments. The results highlight the importance of further reliability assessments for β-Ga₂O₃-based devices. The study also demonstrates the potential of Cr₂O₃:Mg/β-Ga₂O₃ p-n heterojunction diodes for high-temperature applications, with the calculated pO₂-temperature phase stability diagram predicting that Ga₂O₃ and Cr₂O₃ should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The study also shows that the diodes exhibit stable reverse bias leakage current during prolonged high-temperature testing, but experience forward-bias changes in the form of increased on-state resistance, turn-on voltage, and forward current drop. The observed evolution in forward bias electrical properties is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface after prolonged operation at 600°C due to the temperature-induced diffusion of Mg dopants in Cr₂O₃. The study provides insights and design considerations for oxide-based ultra-wideband gap (UWBG) semiconductors for p-n heterojunctions applicable in harsh operating environments. The results highlight the importance of further reliability assessments for β-Ga₂O₃-based devices.This study presents the reliable operation of Cr₂O₃:Mg/β-Ga₂O₃ p-n heterojunction diodes at 600°C. The diodes demonstrate a room-temperature on/off ratio of >10⁴ at ±5 V and a breakdown voltage of -390 V. The leakage current increases with temperature up to 600°C, attributed to Poole-Frenkel emission with a trap barrier height of 0.19 eV. After a 140-hour thermal soak at 600°C, the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm² to 1.59 V and 7.1 mΩ cm², respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface. The study also shows that the diodes exhibit stable reverse bias leakage current during prolonged high-temperature testing, but experience forward-bias changes in the form of increased on-state resistance, turn-on voltage, and forward current drop. The observed evolution in forward bias electrical properties is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface after prolonged operation at 600°C due to the temperature-induced diffusion of Mg dopants in Cr₂O₃. The study provides insights and design considerations for oxide-based ultra-wideband gap (UWBG) semiconductors for p-n heterojunctions applicable in harsh operating environments. The results highlight the importance of further reliability assessments for β-Ga₂O₃-based devices. The study also demonstrates the potential of Cr₂O₃:Mg/β-Ga₂O₃ p-n heterojunction diodes for high-temperature applications, with the calculated pO₂-temperature phase stability diagram predicting that Ga₂O₃ and Cr₂O₃ should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The study also shows that the diodes exhibit stable reverse bias leakage current during prolonged high-temperature testing, but experience forward-bias changes in the form of increased on-state resistance, turn-on voltage, and forward current drop. The observed evolution in forward bias electrical properties is attributed to the accumulation of Mg and MgO at the Cr₂O₃/Ga₂O₃ interface after prolonged operation at 600°C due to the temperature-induced diffusion of Mg dopants in Cr₂O₃. The study provides insights and design considerations for oxide-based ultra-wideband gap (UWBG) semiconductors for p-n heterojunctions applicable in harsh operating environments. The results highlight the importance of further reliability assessments for β-Ga₂O₃-based devices.