Study On The Electrical Reliability Of Gallium-nitride Based High Electron Mobility Transistors

Due to the excellent physical properties of gallium nitride?GaN? material, such as large band gap, high breakdown field, high electron mobility and high electron saturation velocity, GaN-based electrionic devices outperform Si-based ones for operating under extreme environments of high temperature, high voltage and high frequency. In particular, owing to their outstanding high frequency and high power performance, GaN-based high electron mobility transistors?HEMTs?, usually fabricated by AlGaN/GaN heterojunction or lattice-matched In₀.17Al₀.83N/GaN hetreojunction, have emerged as the most promising candidates for power electronics, wireless communication and radar systems. In addition, the stable chemical property of GaN and the high sensitivity of GaN-based HEMT detection devices have recently found more and more applications in the biological and photoelectric al detection areas. At present, impressive progress has been made to improve the performance of GaN-based HEMTs, however, their large-scale commercial applications are still seriously hindered by various electrical reliability issues. To explore the physics nature of these reliability problems and promote the comprehensive development of GaN-based HEMTs have important scientific and practical values.In this thesis, some important electrical reliab ility issues of GaN-based HEMTs are deeply studied, mainly including the Schottky characteristics, capacitance characteristics, transport and degradation mechanisms of gate leakage current. Firstly, devices with different structures are designed and fabricated according to experimental requirements. Secondly, various photoelectrical testing techniques are used to examine the devices, the experimental results are analyzed based on appropriate semiconductor theories, and the physics nature of reliability problems is investigated. Finally, reasonable physical models and feasible methods are proposed to improve the reliability of the devices. The main research content of this thesis can be summarized as follows.1. Two important heterojunction devices, AlGaN/GaN HEMTs and lattice-matched In₀.17Al₀.83N/GaN HEMTs, are fabricated first. In addition, the Schottky diode structures having equivalent Schottky characteristics with HEMTs are also prepared for studing the gate characteristics directly. Numerous studies show that, the fabrication technology of AlGaN/GaN HEMT is relatively mature, but the reliability problems caused by lattice mismatch between the hetero junction interface materials can not be ignored. The lattice- matched In₀.17Al₀.83N/GaN HEMTs have better performance, but the immature material growth technology and various defects in materials can also cause reliability problems. In this thesis, major electrical reliability issues are deeply analyzed by comparing the two different HEMT devices.2. The reverse breakdown mechanism and the forward current transport mechanism of GaN-based HEMTs are investigated, which are useful for improving the Schottky performances. Despite the higher Schottky barrier height, In₀.17Al₀.83N/GaN Schottky diodes have a larger leakage current and a lower reverse breakdown voltage than AlGaN/GaN Schottky diodes, and a premature breakdown behavior can often be observed. The emission microscopy?EMMI? images of the post-breakdown devices suggest that a horizontal premature breakdown behavior, which can be attributed to high density defects in the InAlN barrier layer, happens in the In₀.17Al₀.83N/GaN Schottky diodes, differing from the vertical breakdown in the AlGaN/GaN Schottky diodes. By fitting the forward current with various current models, it is found that the low-bias current is mainly the defect-assisted tunneling current, while the the forward- high-bias current is governed by the conventional thermionic emission mechanism with a significant series resistance effect. In order to improve the Schottky performance of GaN-based HEMTs, it is essential to adopt the appropriate passivation technology and improve the quality of the barrier layer.3. The heterojunction interface defects of GaN-based HEMTs are characterized, and the anomalous phenomenon that the capacitance increases with increasing frequency in the accumulation region is explained, which is helpful for improving their reliability in high frequency applications. The capacitance frequency scattering curves in the depletion region are analyzed by using the capacitance- voltage?C-V? parallel conductance method, and it is found that there exist high-density shallow level interface defects at the heterojunction interface of AlGaN/GaN HEMTs. The defect energy level concentrates mainly from 0.13eV to 0.16eV. O n the other hand, interface defects can hardly be observed at the heterojunction interface of lattice- matched In₀.17Al₀.83N/GaN HEMTs. By testing the frequency-variable C-V curves of GaN-based HEMTs in the accumulation region, an anomalous phenomenon that the capacitance increases with the increas ing frequency at high frequencies is observed, which can not be explained by the traditional model. By considering the non-ideal factors such as the leakage current, series resistance and interface defects, the traditional model is improved. The fitting results using the modified model are in good agreement with experimental results, indicating that this phenomenon is the combined results of the leakage current and capture of surface state electrons.4. The reverse current transport mechanisms of GaN-based HEMTs are clarified, which can provide a theoretical guide for further reducing the gate leakage current. By considering the influence of polarized electric field and based on the measurement of the temperature dependent I-V characteristics, it is found that the reverse current of AlGaN/GaN HEMTs is mainly dominated by the Frenkel-Poole?FP? emission mechanism. When the voltage increases further, the current saturates gradually. The low-bias reverse current of lattice- matched In₀.17Al₀.83N/GaN HEMTs is also mainly carried by the FP emission electrons, while at higher biases the Fowler-Nordheim?FN? tunneling mechanism dominates due to the formation of a triangular potential barrier resulting from the strong polarized electric field. In addition, the zero-bias current balancing problem is explained. By comparing the temperature dependence of the current under the minimum bias conditions, the reverse current is found to be offset by the forward tunneling current.5. A new model is proposed which emphasizes that the high field FN tunneling process causes the gate leakage current degradation of GaN-based HEMTs. Typical leakage current degradation behaviors are observed in the stress- free lattice- matched In₀.17Al₀.83N/GaN HEMTs, suggesting that the inverse piezoelectric effect may not be the major mechanism causing the gate degradation in GaN-based HEMTs. Therefore a new degradation model is tentatively proposed, which emphasizes that the high field FN tunneling process can induce new defects by releasing energy to increase local temperature duo to the thermal effect, and the generation of new defects is considered as the main reason of leakage current degradation. These defects could be mainly N vacancies in the GaN layer. Finally, the gate leakage current degradation phenomenon in lattice- matched In₀.17Al₀.83N/GaN HEMTs caused by the source-drain stress is also investigated. Since the degradation is significantly influenced by the temperature and gate voltage, the channel hot electron should be mainly responsible for the degradation.