Study On GaN-Based Double Heterostructure

Due to its advantages such as high breakdown field strength, high electron drift saturated velocity and high electron density, AlGaN/GaN HEMT will play an increasingly important role in the high-frequency, high-power applications. Since the first AlGaN/GaN HEMT fabricated in1993, great improvements has been achieved in its performance. However, it is still a far way from maturation to date due to some inevitable problems. Up to date, many researches dedicated to improve the performance of AlGaN/GaN HEMT have been done to satisfy the increasingly demands in the future mobile communication technology. Among them, the research on the GaN-based double-heterojunction (DH) HEMT is one of the hotspots. It is found that the performance of the AlGaN/GaN HEMT is greatly influenced by the density and distribution of the two-dimensional electron gas (2DEG). By introducing a back-barrier layer in the DH AlGaN/GaN heterostructure, better2DEG confinement is obtained, and the device characteristics will improved.In this paper, the properties of GaN-based DH materials and HEMTs are researched. The main achievements are listed as follows:1. The energy band diagram and carrier distribution of GaN-based single and double heterostructures are studied by the one-dimensional self-consistent simulation. The effect of different kinds of back-barrier layers on the carrier distribution and confinement are mainly studied in our simulation. With the increase of the Al-components and the thickness of the back-barrier layer, the2DEG concentration of the main channel becomes lower and the2DEG carrier confinement becomer stronger. Finally, we found the right parameters of the double heterostructure2. Different kinds of AlGaN/GaN double heterostructure materials are grown by MOCVD. These materials show bigger interface mismatch on the condition of high Al-content back-barrier layer, and the relaxation of crystal lattice caucused by the large thickness of back-barrier layer. The results of the Hall test show that the carrier mobility of the DH materials was a little smaller than that of the SH materials. Because the interface roughness scattering and alloy disorder scattering become stronger when we insert an AlGaN back barrier. The carrier distributions are studied by mercury probe CV measurement, the results are consist with the results of theoretical simulation3. The AlGaN/GaN single and double-heterojunction HEMTs were fabricated. The output characteristics, transfer characteristics, the shift of threshold voltage and breakdown voltage of the HEMTs were researched. The results show that our AlGaN/GaN double-heterojunction HEMT have good DC characterization. Attributed to the improved carrier confinement, in a very wide range of gate voltage bias, the device remains relatively high transconductance; the threshold voltage shift of the DH device is smaller.