Abstract-In this work the advantages of GaN HEMTs grown on native GaN substrates over GaN/Si or GaN/Sapphire substrates are investigated, and correlated with epitaxial quality. TEM plane view and cross section analysis of GaN/GaN revealed dislocation density lower than 1 × 10 6 cm −2 , which is at least 3 orders of magnitude lower than the case of GaN/Si or GaN/Sapphire. In the case of GaN/Si, the dislocations did not necessarily originate from the substrate/nucleation layer interface, but the strain relief and isolation buffer stacks were main contributors to the dislocation density. GaN/GaN HEMTs demonstrated superior electrical and thermal performance. GaN/GaN demonstrated 3 orders of magnitude lower off-state leakage, current collapse (Ron increase) after stress bias less than 15% compared to 50% in the case of GaN/Si, and 2% drop of the onstate current due to self-heating in DC operation as compared to 13% and 16% for GaN/Si and GaN/Sapphire respectively. The GaN/Si thermal performance approached GaN/GaN only by substrate removal. Therefore GaN/GaN can allow high on-state current, low off-state leakage current, minimal current collapse, and enhanced thermal dissipation capability at the same time, which can be directly correlated to the absence of high dislocation densities.

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In this article, the gate current in AlGaN/GaN high-electron mobility transistors is modeled in a surface potential-based compact model. The thermionic emission, the Poole-Frenkel emission, and the Fowler-Nordheim tunneling are the dominant mechanisms for the gate current in the forward-and reverse-bias regions. These conduction mechanisms are modeled within the framework of the ASM-GaN compact model, which is a physics-based industry-standard model for GaN HEMTs, hence yielding a consistent model for the drain and gate currents. The proposed model captures the gate voltage, drain voltage, temperature, and gate-length dependencies of the gate current. The results of dc gate-leakage measurements of two GaN HEMT, differing only in terms of gate length, over a wide range of temperature, showing these current-conduction mechanisms, are presented, and the proposed model is validated accordingly. The developed gate current model, implemented in Verilog-A, is in excellent agreement with the experimental data.

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.