Impact of n-GaN cap layer doping on the gate leakage behavior in AlGaN/GaN HEMTs grown on Si and GaN substrates

The gate leakage characteristics of n-GaN- and i-GaN-capped AlGaN/GaN high-electron-mobility transistor (HEMT) heterostructures grown on various substrates of Si, SiC, and GaN were investigated. HEMT heterostructures were grown by metal-organic vapor phase epitaxy, and the effect of n-GaN cap layer doping on the gate leakage characteristics was investigated depending on the dislocation densities. For i-GaN capped HEMT heterostructures grown on GaN substrates, the current–voltage characteristics were well explained by the theoretical calculation based on thermionic emission, thermionic-field emission, and field emission. Alternatively, for the AlGaN/GaN HEMT heterostructures grown on Si substrates that contain a high threading dislocation density of 8.2 × 109 cm−2, a drastic increase in the reverse leakage current of approximately five orders of magnitude was observed compared to the reverse leakage current observed for the i-GaN-capped HEMT heterostructures. Conductive atomic force microscope analysis revealed that the dislocation-induced surface pits acted as leakage paths only for the n-GaN-capped HEMT heterostructures. Furthermore, the temperature dependence of the leakage current through the surface pit was investigated using a conductive AFM system, and the temperature-dependent behavior associated with one-dimensional variable-range-hopping (1D-VRH) was confirmed. These results indicate that the surface pits have a large impact on the leakage characteristics of the n-GaN capped structures with high dislocation density, and the electron transport through dislocations based on 1D-VRH plays an important role. Furthermore, we demonstrated a significant reduction in the reverse leakage current for n-GaN-capped AlGaN/GaN HEMT heterostructures using low-dislocation-density GaN substrates, i.e., by reducing dislocation-induced leakage paths. Therefore, we believe that the HEMT heterostructures grown on GaN substrates have an advantage in suppressing frequency dispersion and current collapse with minimized impact on the gate leakage behavior of the Schottky-gated HEMTs with an n-GaN cap layer.