A GaN HEMT with a polarization-graded AlGaN buffer is performed by two-dimensional analysis of drift-diffusion simulations. The bulk trap-induced current collapse of the proposed structure is effectively restrained in contrast to that of conventional HEMTs with a GaN or AlGaN buffer, resulting from the fact that the high and flat back-barrier altitude in the proposed structure prevents the two-dimensional electron gas (2DEG) from spilling over from the channel, with the reduction of hot carriers injecting into the buffer followed by trapping in deep acceptorlike levels. Simultaneously, the off-state breakdown voltage is remarkably enhanced, due to the strong electric breakdown field of the polarization-graded AlGaN buffer and the restraint of the buffer leakage current. In addition, the relationship between the off-state breakdown voltage and the thickness of the polarization-graded AlGaN buffer is analyzed.
A metal-source flow-rate modulation epitaxy method is reported to enhance the hole concentration of Mg-doped AlGaN grown by metal organic chemical vapor deposition. The hole concentration of p-type AlGaN (Al content 0.43) is increased to 2.3 × 1017 cm−3 at room temperature by this method, which is about ten times higher than that of the conventional growth. The resistivity was found to be as low as 12.7 Ω·cm. Furthermore, the effective acceptor activation energy (EA) in the AlGaN films (Al content 0.32–0.43) was determined to be 20–22 meV, several times smaller than EA in p-GaN. Secondary ion mass spectroscopy measurements demonstrated that uniformly Mg-doped AlGaN structures with spatially modulated Al compositions were formed using this technique. It is suggested that the enhancement of hole concentration benefits from the modulation of the valence band edge.
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