Characterization of charge trapping mechanisms in GaN vertical Fin FETs under positive gate bias

In this paper, we present a comprehensive analysis of the charge trapping mechanisms that affect the GaN based vertical Fin FETs when the devices are submitted to positive gate bias. Devices with higher channel width show lower threshold voltage: with 2D simulations of the electron density we are able to explain the phenomenon and propose a trade-off to improve the technology. By using double pulse measurements and threshold voltage transients, two trapping/detrapping mechanisms under positive gate bias can be identified according to two voltage ranges. At low positive gate bias, electrons (previously trapped inside the oxide during the fabrication process) are detrapped towards the gate metal (mechanism 1). At higher gate bias, electrons are trapped at the GaN/oxide interface, moving the threshold towards positive values (mechanism 2). The second mechanism is observable at higher time of stress and it is predominant for higher voltages. Moreover, mechanism 2 is found to be recoverable only when the device is exposed to UV-light and electrons trapped in a specific level in the oxide acquire the energy necessary to escape and reach the n-type GaN and/or the UV-generated holes accumulate at the interface may reduce the trapped electron density. We demonstrate our hypothesis by calculating the interface state density in trapping/detrapping conditions by using photo-assisted Capacitance-Voltage measurements.