The Study On The Physical Model And Key Process Technologies Of AlGaN/GaN HEMT

The performance of traditional microwave power devices using conventional semiconductors such as Si and GaAs is rapidly approaching the limit set by intrinsic material parameters. The increasing demand for higher power at high frequencies has led to the development of the devices based on emerging wide gap systems such as GaN and SiC. In the past several years, the research of AlGaN/GaN High-Electron-Mobility-Transistors (HEMT) has attracted much attention and remarkable achievements have been made. In this work, the device physics and the fabrication technology of AlGaN/GaN HEMT were investigated. The major results are listed.1. The property of two dimensional electron gas(2DEG) in the triangle quantum well of AlGaN/GaN structure was studied. The energy level and the wave function of the base state in the triangle quantum well can be obtained by solving the coupled Poisson-Schrodinger equations based a modified time independent perturbation method and the technology of variation method. The deduced equations were simple but effective. When the method was applied to the Al_0.2Ga_0.8N/GaN system, the calculated lowest subband level and Fermi-level were 0.19eV and 0.29eV respectively, which agreed with the published numerricl results well.2. With a charge control model for AlGaN/GaN HEMT worked out, an analytical model for the optimization of the field-plate(FP) structure of AlGaN/GaN HEMT at a given drain voltage was suggested. The relationships between the electronic and geometric parameters were calculated and the principles for the design of field-plate structure were obtained. This model indicates an approximately linear relationship between the optimum thickness of the insulator layer and the given drain voltage VDD. The length of the field-plate is not critical to the optimization while the electric field peaks at its two edges do not affect each other effectively. The model is available in both of the gate-field-plate structure. The distribution of electric field in AlGaN/GaN HEMT was investigated. It is found that the acceptor traps effectively affect the electric field peaks in the device's channel. For the relationship between such peaks and the breakdown of the AlGaN/GaN device is known, a critical parameter V_k can be defined to characterize the effects of the acceptor traps on the device's breakdown character.3. In order to understand the mechanism of current collapse in AlGaN/GaN HEMT, a new measurement method was suggested to study the trapping process of electrons which is very difficult to observe. We observed an abnormal trappping time resulting from the transport of the trapped electrons and demonstrated that there was a dynamic equilibrious distribution of the charged surface states. The gate lag measurement was also used to investigate the current collapse effect. With the measurements of current collapse effects, two different kinds of time constant were obtained. The larger kind corresponds to the transportation process of the trapped surface charges. These results indicate that the movements of electrons between the surface states occur in the surface discharging progress. With a specially designed AlGaN/GaN HEMT, the current collapse effect was investigated by the experiments. When periodical square pulse signals were applied on the gate, the sheet resistance of AlGaN/GaN film was investigated. It is demonstrated that the sheet resistance is a function of both signal's amplitude and frequency. A critical signal frequency region is defined. Such a region is related to the surface charging and discharging process, and can be regarded as a basic parameter to characterize current collapse effect.4. A new measurement method for GaN film and its Schottky contacts was developed. It is based on a special back-to-back Schottky diode, instead of the fabrication of Ohmic contacts. The method can be adopted not only for GaN films but also for other semiconductor materials, especially in cases where Ohmic Contacts of high quality are hard to make or their fabricating process affects the film's characteristic. By using the measurement method, the carrier mobility of the epitaxial GaN, the barrier height and the ideality factor of the Schottky contacts can be obtained.5. The fabrication technology of AlGaN/GaN HEMT was studied. The Ohmic Contacts on GaN film was also investigated. The high quality Ti/Al/Ni/Au-GaN Ohmic Contacts were obtained, and the measured specific contact resistivity is at a low level of 6.6×10-6Ω·cm^-2. It is found that the Ti/Al-GaN or Ti/Al/Ni/Au-GaN Ohmic Contact do not form when the annealing temperature is lower than 700℃.The investigation by Auger electron spectroscopic (AES) measurement revealed that the reaction between the Al layer and the Ti layer is one of the two keys for fabrication of Ohmic Contacts on the GaN films, and that the other key is the N's out-diffusion from the GaN's structure into the metal layers. Based on the experiments relative to the fabrication technology, the AlGaN/GaN HEMT was successfully fabricated.In our study of device physics, we aimed to build the pictures of the physical effects relating to the high power microwave applications of AlGaN/GaN HEMT. Such pictures were expected to be used to direct the device design. Efforts for the research of device fabrication technology have also been done. Not only the fabrication processions themselves but also the mechanism of them have been studied. And the later was emphasized in our paper. More works need to be done to strengthen the connections between the researches of the device physics and the device fabrication.