Processing approaches and gate dielectrics for AlGaN/GaN-based MISHFET

摘要

The excellent electrical properties of group-III nitride semiconductors enabled the development of galliumnitride- (GaN-) based heterostructure field effect transistors (HFET) with high breakdown voltage and high switching frequencies at the same time. Power densities of such high-frequency power amplifiers significantly exceed those of comparable Si-, GaAs-, or SiC-based devices. One inherent weakness, however, is the insufficient electric strength of the gate, which is typically realized as a Schottky contact and thus goes along with high leakage currents as well. The concept of the „Metal Insulator Semiconductor HFET” (MISHFET) is one approach to overcome this weakness. Moreover, it is considered as key technology for future device generations. In this work, AlGaN/GaN MISHFET with gate dielectrics silicon nitride (Si3N4), silicon oxide (SiO2), aluminum oxide (Al2O3), hafnium oxide (HfO2), and lanthanum lutetium oxide (LaLuO3) are investigated. Different deposition techniques such as Plasma Enhanced Chemical Vapor Deposition (PECVD), Metal Organic Chemical Vapor Deposition (MOCVD), Atomic Layer Deposition (ALD), Pulsed Laser Deposition (PLD) and thermal oxidation of the barrier were utilized. The established baseline process of GaNBET institute was extended by the additional processing steps necessary for MISHFET processing. Relevant electrical device parameters were set into relation to material properties of the investigated dielectrics, both theoretically and experimentally: Lower leakage currents are being correlated with conduction band offsets between dielectrics and semiconductor, while sheet carrier density and threshold voltage shift, both as a function of the corresponding dielectric and its thickness, are not only predicted on the basis of a fundamental theoretical model, but are also proven by experiment. The experimental results allow for an evaluation of the general feasibility of a certain dielectric or a certain deposition technique in conjunction with the surface-sensitive semiconductor. It is being demonstrated both theoretically as well as experimentally that electrical properties of MISHFET such as channel current density, carrier mobility, transconductance and RF switching frequencies exceed those of conventional HFET.