Abstract
This thesis focuses on high performance AlGaN/GaN heterojunction devices on Si substrate for high power and high frequency applications. For high voltage applications, the contact engineering was investigated to improve device off-state characteristics in GaN on Si devices by using two different approaches, including a selective Si diffusion structure for AlGaN/GaN SBDs and a hybrid Schottky-Ohmic drain electrode structure for AlGaN/GaN HEMTs. Both two approaches are proposed to alleviate the E-field peaks at the alloy spikes and further enhance the breakdown voltage by manipulating the E-field around Ohmic contact. First, a selective Si diffusion approach is proposed to reduce Schottky onset voltage from 1.3 V to 1.0 V and enhance the reverse blocking capability up to 20%simultaneously. With Si diffused layer underneath cathode, a low and stable contact resistance and relatively smooth ohmic metal morphology can be obtained. Second, a hybrid drain device shows nearly zero drain onset voltage and reduces drain leakage current by one order of magnitude, comparing with that of traditional ohmic drain devices. Both the gate-drain and buffer breakdown can be improved. We also investigated the impact of Schottky extension length Lext on device characteristics. After optimizing Lext, the breakdown voltage can be enhanced up to 60% without clear on-resistance RON degradation. For high frequency applications, we proposed a method to directly extract the more convergent results for parasitic substrate resistance and capacitance in small signal model of GaN-on-Si HEMTs by using derivative method. The model is verified by in-house 200-nm T-gate AlGaN/GaN HEMTs. Also, the simulation is performed to analyze the impact of substrate effect on device high frequency characteristics.