Reliability Of Enhancement-mode AlGaN/GaN HEMTs Treated By Fluorine Plasma Ions Under Electrical Stress

With the development of wide band-gap semiconductor devices, enhancement-mode Al Ga N/Ga N HEMTs have recently attracted a lot of attention. For RF and monolithic integrated microwave circuit, enhancement-mode HEMTs can reduce the circuit size and cost. For digital circuits, they can realize direct-coupled logic, make the circuit simply. Moreover, enhancement-mode HEMTs are promising candidates for microwave power amplifiers and high-voltage switches. The methods to realize enhancement-mode HEMTs contains the structure of groove gate, thinning the Al Ga N barrier layer, p-type gate electrode, etc. Among these technologies, the fluoride-base plasma treatment is a very important approach to realize enhancement-mode HEMTs. It has the advantages of simple process, easy change in the threshold voltage. Despite the early work has achieved good performance, the reliability under electrical stress need to be investigated in depth. Research on characteristics of fluorine plasma treatment enhancement-mode Al Ga N/Ga N HEMTs under electrical stress are carried out in this paper. For the purpose that the fluorine plasma treatment devices can have long-term stability in the wide range of applications.Firstly, the fabrication of enhancement-mode HEMTs treated by fluorine plasma ions on sapphire wafer has been introduced. After annealed at 350℃ for 5 minutes, the devices show slightly degradation in the threshold voltage, but the characteristics such as transconductance have improved significantly. The threshold voltage of enhancement-mode HEMTs we fabricated in this paper is 0.5V and a peak transconductance of 180 m S/mm.Secondly, the degradation under typical gate over-drive stress has been studied. It is observed in this paper that the threshold voltage of enhancement mode devices show a significant negative shift during the long-term on-state gate overdrive stress. But no degradation observed under the identical electric stress applied on depletion-mode HEMTs with the same size and structure except the fluorine ions implantation process. On the other hand, a trap filling procedure is applied on the fresh enhancement-mode HEMTs, no significant shift of the threshold voltage is observed. The degradation does not originate from the presence of as-grown traps in the Al Ga N barrier layer or the generated traps during fluorine ions implantation process. By comparing the relationships between the shift of threshold voltage and the cumulative injected electrons under different stress conditions, a good agreement is observed. The degradation of enhancement mode HEMTs under gate overdrive stress can be explained by the ionization of fluorine ions in Al Ga N barrier layer by electrons injected from 2DEG channel. Furthermore, our results show that there are little new traps generated in the Al Ga N barrier layer during the gate overdrive stress, and the ionized fluorine ions cannot recapture the electrons.Finally, the influence of both OFF-state and ON-state high-field stress on enhancement-mode Al Ga N/Ga N HEMTs are studied. Under OFF-state stress, the threshold voltage of the device shows a negative shift, and the higher the electric field, the more obvious the degradation. Our research shows that the negatively charged fluorine ions may experienced a strong Coulomb force, which could lead to a physical migration of several fluorine ions away from the gate edge. Under ON-state stress,the threshold voltage has a slight positive shift at first,then shows an obvious negative shift with the increasing stress time. The effect of hot electron is in significant at the beginning of the stress. The defects and traps in the buffer layer or in the passivation layer can capture electrons, may induce slight depletion effects on the 2DEG, then the threshold voltage shows a positive shift. With the stress time increases, hot electron effect weakened gradually, the movement of fluorine ions induced by high electric field becoming more and more obvious, and ultimately leads to a negative shift of the threshold voltage of the device. Moreover, different degradation model of the fluorine plasma treatment devices under different typical stress is proposed. respectively.