Group III-nitride semiconductors such as (Al, Ga, In) N and their alloys have increasingly attracted the attention of worldwide researchers due to their unique material properties. AlGaN/GaN is the most widely proven material, allowing it to be employed in a number of nanoelectronics applications due to its significant high-power and high-frequency characteristics. High power density is achieved by AlGaN/GaN high electron mobility transistor (HEMT) with a significant critical breakdown field and the capacity to sustain a large channel current. However, one important consideration is that the AIGaN/GaN HEMT's breakdown voltage (BV) is significantly lower than the theoretical value. In this research work, the theoretical relationship between the BV of III-Nitride HEMT on β-Ga2O3 substrate, field-plate length, and AIN layer thickness has been investigated. The results show that the breakdown voltage of the proposed HEMT increases accordingly with AIN thickness, but it saturates gradually. Then, as the length of the field-plate increases, the breakdown voltage rises. As a result, the breakdown voltage of a device improves with the field-plate length and is limited by the AIN back-barrier layer thickness.