Avalanche leakage current mechanism in vertical GaN devices on foreign substrates

Avalanche breakdown, the most desirable breakdown mode in power devices, is typically characterized by a positive temperature coefficient (PTC) of breakdown voltage (BV) extracted from leakage current–voltage (I–V) characteristics. Here, we report a suppression and reversal of this PTC in avalanche-capable gallium nitride (GaN) p–n diodes with high dislocation densities grown on foreign substrates and elucidate the underlying mechanism. Vertical GaN p–n diodes with optimized edge termination are fabricated on GaN, patterned sapphire, and sapphire substrates, providing a controlled increase in dislocation density. Although all devices exhibit robust avalanche behavior in circuit-based tests, their breakdown I–V characteristics show distinct temperature dependences, evolving from a clear PTC to a negative temperature coefficient (NTC) with increasing dislocation density. This behavior arises from the interplay between avalanche carrier generation and dislocation-assisted carrier transport. Avalanche-generated carriers traverse the depleted drift region via dislocation-assisted variable-range hopping (VRH), which intrinsically exhibits an NTC. The superposition of avalanche generation and VRH transport flattens the breakdown I–V characteristics and yields a weakened PTC or NTC of the extracted BV. These results indicate that an NTC of BV in high-dislocation-density power devices does not preclude avalanche capability, suggesting the limitation of leakage I–V-based avalanche identification.