The Characteristic Research Of AlGaN/AlN/GaN Heterostructure Schottky Contacts

As one of the most important members of the3rd generation semiconductors, GaN-based materials are promising candidates in high temperature, high voltage, microwave, and high power applications due to their wide band gap, high breakdown electric field, high electron saturated velocity and excellent chemical stability. Among them AlGaN/GaN heterostructure field effect transistors (HFET) is a new microwave power device, and it will play the most important part in the next generation wireless communication power amplifiers because of it’s high transconductance, high breakdown voltage and high cutoff frequency compared with traditional microwave devices.We have made progress in the research of the AlGaN/GaN HFET since the first AlGaN/GaN HFET has been made in1993. Nowadays, state-of-the-art AlGaN/GaN HFETs have the structures with a thin AlN interlayer added to the AlGaN/AIN interface. The insertion of a thin AlN barrier interlayer can prevent the diffusion of the two-dimensional gas (2DEG) in the triangular quantum well towards AlGaN barrier layer and reduce the alloy scattering that influenced the2DEG transport performance. However, there are still some unsolved problems for AlGaN/GaN HFETs such as the current collapse effect at high frequency, the degeneration of reliability at high temperature etc. These problems have stopped AlGaN/GaN HFET devices from a large-scale application. Taking into consideration these problems, we did research on some aspects as follows.1. Research on the2DEG electron mobility of AlGaN/AlN/GaN SBDs. We raised the power consumption method to calculate the series resistance under the Schottky contact of AlGaN/AlN/GaN SBDs, and demonstrated the validity of this meathod. Using the measured I-V curves and C-V curves, the series resistance under the Schottky contact and the2DEG electron mobility was obtained with the power consumption method. The results showed the series resistance under the Schottky contact is an important part of the whole SBD resistance, and can not be neglected.2. Research on the strain of AlGaN layer of Schottky drain AlGaN/AlN/GaN HFETs. Schottky drain AlGaN/AlN/GaN HFETs with different gate contact areas and conventional AlGaN/AlN/GaN HFETs with the same areas as control group have been formed on the AlGaN/AIN/GaN heterostructure. The drain series resistance and the2DEG electron mobility in the channel between the gate and drain were calculated with the power consumption method and compared with the control group. The results showed that Schottky drain AlGaN/AIN/GaN HFETs have smaller drain series resistance, higher2DEG electron mobility in the channel between gate and drain. The reason is that when Ohmic contacts were formed, the transverse diffusion of considerable metal atoms towards the gate contacts happened. This resulted in strong polarization Coulomb field scattering between the gate and drian related to the strain variation in the AlGaN barrier layer. However, no transverse diffusion of metal atoms happened from the drain contacts of Schottky drain AlGaN/AIN/GaN HFETs without thermal annealing. Therefore the drain contacts of Schottky drain AlGaN/AlN/GaN HFETs have less effect on the strain variation in the AlGaN layer between the gate and drain, and the polarization Coulomb field scattering between the gate and drain decreases.3. Research on the interface traps in AlGaN/AlN/GaN heterostructures. Based on the measured I-V curves, C-V curves, G-V curves and the equivalent models of AlGaN/AIN/GaN heterostructures, the density of interface traps, the time constants and the trap state energy of AlGaN/AIN/GaN heterostructure devices with different Ohmic contact specific resistivity were calculated. The results demonstrated that the Ohmic contact annealing is one of the most important factors that have an influence on the density of interface traps, the time constants and the trap state energy of AlGaN/AlN/GaN heterostructures.