Re-engineering transition layers in AlGaN/GaN HEMT on Si for high voltage applications

We report on the study of step-graded AlGaN transition layers (TLs) in metalorganic chemical vapor deposition-grown GaN HEMT-on-silicon toward improving the breakdown field while minimizing buffer-induced current dispersion. The transition layers include three AlGaN epi-layers of 75%, 50%, and 25% Al-content, downgraded from bottom to top. The growth temperature and carbon doping are varied independently to assess the transition layer's role in reducing current collapse and leakage current. We observe that the introduction of High Temperature (HT) AlGaN increases the lateral but decreases the vertical leakage, the latter being attributed to the reduction of V-pit density. Temperature-dependent data indicate that the increased lateral (mesa) leakage current in HT AlGaN layers is due to space charge limited current, the activation energy of which yields the positions of the defect states within the bandgap. The increase in mesa leakage current in HT AlGaN layers is attributed to the formation of point defects such as oxygen in nitrogen site (ON) and VGa–ON complexes. The introduction of C-doping in the top AlGaN transition layer with 25% Al-content helps reduce lateral leakage in both mesa and 3-terminal configurations. The combination of HT AlGaN (75% Al-content) with C-doped AlGaN (25% Al-content) is found to be the optimal TL design that yielded a minimum buffer-induced current dispersion with a 65% channel recovery when the substrate was swept to −300 V and back; moreover, it also enabled a vertical breakdown field of 2.05 MV/cm defined at 1 A/cm2 for a buffer thickness of 1.65 μm.