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The vertical leakage current mechanisms of high electron mobility transistors (HEMT) grown by metalorganic chemical vapor deposition (MOCVD) on Si and GaN substrate under forward and reverse bias are analyzed at ambient temperatures from 25 C to 200 C. For the GaN/Si case, a thermally activated vertical conduction with two temperature regimes and activation energies of 0.06 eV and 0.43 eV is found. In contrast to that, the GaN/GaN case shows a single activation energy of 0.67 eV for the ratelimited vertical conduction. For forward vertical bias, Poole-Frenkel (PF) conduction is identified as the dominant conduction mechanism at higher fields for both substrates. In reverse bias, space charge limited and PF conduction are identified as dominant conduction mechanism for GaN/Si and GaN/GaN, respectively. The deviation in vertical conduction mechanism is related to a significant reduction in the dislocation density by three orders of magnitude and homoepitaxially lattice matched growth for the GaN/GaN HEMT.

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