Reliability Investigation Of GaN-based High Electron Mobility Transistors Under Inverse Piezoelectric Effect

Ga N based high electron mobility transistors(HEMTs) are of tremendous interest for next generation high power and high frequency devices, mainly due to their high breakdown voltage, high electron density, and high electron saturation velocity. However, the greatest impediments today preventing the further development of Ga N HEMT technology is its limited electrical reliability. Since the Ga N based HEMTs are always working on the high voltage and high power condition, the inverse piezoelectric effect becomes one of the most important factors affecting the device reliability. In this dissertation, the physics model of the inverse piezoelectric effect in Ga N was established based on the theoretical analysis and software simulation. Besides, the inverse piezoelectric effect in Ga N based HEMT was characterized by designing the experiments and device layout properly, and the mechanisms of the device degradation under the inverse piezoelectric effect was analyzed. Morover, the impacts of Si N passivation on the inverse piezoelectric effect of Ga N based HEMTs was investigated, and the device degradation modes under forward gate bias stress were also discussed. Finally, the effect of mechanical stress on the characteristics of the device was investigated.In this dissertation, Ga N based HEMT device was introduced, including the advantage of Ga N material over other semiconductors, the reason why the Ga N based HEMTs are suitable for the high frequency and high power application, and the history of the Ga N material growth and the Ga N based HEMT fabrication. The main reliability problems limiting the Ga N based HEMTs, including hot carrier effect, current collapse effect, and the inverse piezoelectric effect were analyzed, and research background of the Ga N based HEMT reliability was also discussed.The theory of the inverse piezoelectric effect in Ga N based HEMT was deeply discussed, and physical model was established. Based on the knowledge of polarization properties of Ga N material, such as spontaneous polarization, piezoelectric polarization, and inverse piezoelectric polarization, the theory of the inverse piezoelectric effect for Al Ga N material was introduced. Besides, the relationship between the elastic energy density and the electric field was deduced. The electric field distribution in Ga N based HEMT was simulated using the Silvaco-ATLAS semiconductor devices simulation software. Based on the simulation, the electric field at any point of the Al Ga N barrier can be obtained, when the device is working at a certain condition. Then the elastic energy density distribution in the Al Ga N barrier can be calculated based on the relation between the elastic energy and the electric field. By comparing the elastic energy density in the Al Ga N barrier with the maximum elastic energy density that Al Ga N material can sustained, the critical voltage beyond which the devices start to degrade can be deduced.Ga N based HEMTs with excellent properties were fabricated, and the inverse piezoelectric effect in Ga N based HEMTs were characterized by properly designing the electrical stress experiments. In the step-stress experiments, the change in the stress gate current was monitored, the voltage at which the sudden increase of the stress gate current happens was defined as the critical voltage. The experimental results reveal that, the reverse gate leakage current also increases sharply beyond the critical voltage, which was explained based on the gate leakage mechanisms in Ga N based HETMs. By designing the forward and reverse testing schemes, the device characteristics before and after the step-stress experiments were compared. It is concluded that, the trap states that induced by the inverse piezoelectric effect located at the source side edge of the gate contact.Effects of the inverse piezoelectric effect on the device characteristics were systematically investigated by designing the device structure properly, and the device degradation mechanisms were deeply analyzed. By performing the step-stress experiments on the symmetrical structure Ga N based HEMTs, we prove that the electric field peak under the gate edge is of vital importance to the inverse piezoelectric effect. By removing the Si N passivation and the metal contacts using the wet etching process, the material morphology under gate contact were investigated under the scanning electron microscopy(SEM). It is concluded that the inverse piezoelectric effect would not necessarily induce the physical damage in the Al Ga N barrier. The effect of the bias condition on the critical voltage of the device degradation was also investigated, and the phenomena were explained based on the electric field simulation. Combined the energy band analysis with the trap generation theory, device degradation mechanisms under the inverse piezoelectric effect were discussed.Impacts of Si N passivation on the inverse piezoelectric effect was investigated. The transient properties of the Ga N based HETMs and the surface leakage mechanisms were also discussed. By performing the step-stress experiments we found that the critical voltage for the sudden degradation of the devices could not be observed after the Si N passivation removal. The comparison of transient properties in the Ga N based HEMTs with and without Si N passivation reveals that, Si N dielectric passivates the slow trap states at the Al Ga N surface, while introduces a great density of fast trap states at the same time. The surface leakage mechanisms related to the surface traps were discussed aided by the energy band diagram along the lateral direction. It was supposed that slow trap states at the Al Ga N surface would capture the electrons tunneled from the gate electrode, inducing the extension of the surface depletion region, thus the inverse piezoelectric effect was shield.Device degradation under the forward gate bias stress was investigated in this dissertation, and the consumption of the oxide interfacial layer under electrical stress was also discussed. A forward shift of the threshold voltage and a significant increase of the gate leakage current were observed after the forward gate bias stress. By performing the frequency dependent capacitance and conductance experiments we found that, the density of the oxygen donors at the Al Ga N surface decreases apparently after stress. The material composition under gate contact was analyzed by X-ray photoelectron spectroscopy(XPS) after the gate contact removal, and a weaker Ga-O peak on the stressed semiconductor surface was found. It was supposed that the interface oxygen was stripped by Ni under gate contact during the electrical stress, inducing the consumption of the interface oxide interfacial layer and the device degradation.A setup for applying mechanical stress on the chip was invented, and the effect of tensile stress on the electrical characteristics of Ga N based HEMTs was investigated. It was found that, the output current and the transconductance of the device both increased under tensile stress. Based on the polarization model of Ga N material, it is proposed that tensile stress could bend the enrgy band of the material, inducing the ionization of deeper surface donors. The ionization of the surfac donors would induce the increase of the channel carrier density, as well as the carrier mobility.