Trapping processes and band discontinuities in Ga2O3FinFETs investigated by dynamic characterization and optically-assisted measurements

In this paper, we analyze the threshold voltage stability of beta-Ga2O3FinFETs for power applications using Al2O3 as gate insulator. In dynamic characterization measurements, when the filling bias condition is moved from off-state to on-state a positive threshold voltage shift is induced, caused by the trapping of electrons in the insulator or at the insulator interface with the semiconductor. The threshold voltage variation was found to be stable in rest condition, but illumination by 280 nm UV light was able to slowly recover the threshold voltage even below its value before the filling condition was applied, suggesting the presence of natively trapped charge into the oxide even in the as-grown device. In order to obtain more information on the role of the external illumination, monochromatic excitation in the range from 1.5 eV to 5 eV was applied to the device before a transfer characteristic measurement. Results show that photon energies lower than 2.2 eV cause a positive threshold voltage shift, caused by charge trapping during the measurement phase and not related to illumination. Photon energies between 2.2 eV and 3.5 eV promote electron detrapping, leading to a partial recovery in the threshold voltage. Finally, energies above 3.5 eV cause an additional charge trapping process. The physical origin of the photon energy difference was investigated by monochromatic light-induced current transients, and a suitable model considering the conduction band discontinuities between the gate metal and the oxide and between the oxide and the semiconductor was developed to explain the experimental data.