Revealing the Positive Bias Temperature Instability in Normally-OFF AlGaN/GaN MIS-HFETs by Constant-Capacitance DLTS

Effects of oxide/III-nitride interface and oxide states on the threshold voltage instability ( $\boldsymbol{V}_{\mathbf{TH}}$ -Instability) of normally-OFF Al 2 O 3 /AlGaN/GaN MIS-HFETs, i.e., metal-insulator-semiconductor heterojunction field-effect transistors, were revealed by constant-capacitance deep level transient spectroscopy (CC-DLTS). It is confirmed that a technique of in-situ remote plasma pretreatments could effectively suppress the $\boldsymbol{D}_{\mathbf{it}}$ with level depth ( $\boldsymbol{E}_{\mathbf{C}}-\boldsymbol{E}_{\mathbf{T}}$ ) larger than 0.4 eV down to below $1.3\times 10^{12}\ \mathbf{cm}^{-2}\mathbf{eV}^{-1}$ , in spite of the presence of a discrete level with $\boldsymbol{E}_{\mathbf{C}}-\boldsymbol{E}_{\mathbf{T}}$ and capture cross section ( $\boldsymbol{\sigma}_{\mathbf{n}}$ ) being 0.33 eV and $4.0\times 10^{-15}\ \mathbf{cm}^{2}$ respectively. However, electron charging of oxide states occurs when the MIS-HFETs are pulsed by a high positive gate bias (e.g., > 8 V), as confirmed by a reduced tunnel constant $\boldsymbol{d}_{0}$ of 0.79 nm. High electric field induced tunnel filling of gate oxide states could be an assignable cause for the positive bias temperature instability in normally-OFF III-nitride MIS-HFETs.