Silicon Carbide (SiC)-based devices have been proposed to replace conventional Silicon-based devices, which are already approaching the material limit in power electronics. SiC has shown great promise for use in high temperature, high power, and high frequency applications. In addition, SiC is the only wide-bandgap semiconductor that can grow silicon dioxide (SiO2) by thermal oxidation. Among power devices based on SiC, MOSFETs are representative discrete device for power conversion applications in high voltage due to low conduction loss and high input impedance.
However, carrier mobility degradation and poor gate oxide reliability in SiC-based FETs are serious issues, which may result from residual impurities and other related defects in SiC MOS structures. Negative bias temperature instability (NBTI) is a critical issue that has to be solved. This is because a negative bias causes a negative shift of threshold voltage (Vth), which results in an increase in off-current, which degrades the breakdown voltage.
In this work, (i) SiC MOSFET was fabricated to analyze the effect of different gate dielectric oxides, such as thermal oxide (Device TO) and deposited-oxides (Device DO), on the BTI of 1200 V 4H-SiC MOSFETs. (ii) The Vth shift of SiC MOSFETs under negative bias stress was investigated. The stress time dependent ΔVth (10000s) - ΔVth (1s) of Device TO and DO, which have different values of 0.51 V and 0.09 V, respectively. From the time dependences, we applied a two component model that considers nitrogen(N)-related traps. The relatively small value of ΔVth is indicative of the decrease in the oxide traps and activation of nitrogen-related traps. (iii) Interface properties of MOS capacitors with N2O-O2-N2O oxide film (Device TO) and TEOS oxide film (Device DO) have been investigated. The Nit of the Device DO was decreased about 17 times compared to Device TO. These results indicate to the reduction of the N-related defects into the oxide network. Controlling of nitrogen in SiO2 is important for the long-term Vth stability of SiC-MOSFETs under NBT stress.