The Investigation Of The Stress Effect On The Electrical Properties Of AlGaN/GaN HEMTs

GaN has a large band gap, good physical and chemical stability, large saturation velocity, and high thermal stability. The GaN-based devices, especially the AlGaN/GaN HEMT, have attracted much attention in the use for the high voltage, high-power, and high frequency conditions. However, the reliability of the device has always been the problem for the wide applications. The device is unavoidably subjected to the stresses, which can cause the degradation and failure of the device. Therefore, the thesis conducts a comprehensive and systematic study of the stress effect on the electrical properties of the AlGaN/GaN HEMT.(1)The residual stress was measured by the Raman scattering and the x-ray diffraction technique. The results demonstrated that a tensile residual stress around 305 MPa existed in the device due to the lattice mismatch and the different thermal expansion coefficient between different materials.(2)The four-point bending test rig was designed to apply the uniform in-plane stress on the AlGaN/GaN HEMT. The results demonstrated that the current changed linearly with the external stress which resulted from the piezoelectric effect of the GaN. Moreover, the kink effect was also affected by the stress which could be attributed to the change of energy band structure under the external stress.(3) The dynamic test rig was designed to apply the alternating stress on the AlGaN/GaN HEMT. The results demonstrated that the output current and the transconductance decreased with the increase of the stress cycles. The reason was associated with the defects in the AlGaN layer, which increased in size under the stress, resulting in the degradation of the device.(4) The stress effect was also studied for the AlGa As/GaAs single quantum lasers. It was found that the stress had an influence on the band structure. The experimental results were consistent the theoretical results.