On the redox origin of surface trapping in AlGaN/GaN high electron mobility transistors

Abstract

Water-related redox couples in ambient air are identified as an important source of the surface trapping states, dynamic on-resistance, and drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species—hydroxyl groups (OH) was found at the AlGaN surface at room temperature. It was also found that these species, as well as the current collapse, can be thermally removed above 200 °C in vacuum conditions. An electron trapping mechanism based on the H2O/H2 and H2O/O2 redox couples is proposed to explain the 0.5 eV energy level commonly attributed to the surface trapping states. Finally, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface.

Impact of 2D-Graphene on SiN Passivated AlGaN/GaN MIS-HEMTs Under Mist Exposure

“…Hence, these results confirm that the moisture plays a critical role in the degradation of the characteristics of conventional GaN-based MIS-HEMT devices, in agreement with recent reports indicating that the moisture on the AlGaN surface can modify significantly the 2DEG [11], [20]. Moreover, Gao et al showed that this adverse effects are also present even with a 20-nm-thick SiN passivation layer, but it can be mitigated with a thicker SiN layer (200 nm) [11].…”

“…Hence, these results confirm that the moisture plays a critical role in the degradation of the characteristics of conventional GaN-based MIS-HEMT devices, in agreement with recent reports indicating that the moisture on the AlGaN surface can modify significantly the 2DEG [11], [20]. Moreover, Gao et al showed that this adverse effects are also present even with a 20-nm-thick SiN passivation layer, but it can be mitigated with a thicker SiN layer (200 nm) [11]. In the present case, the passivation layer is 100 nm thick, and according to the results it seems not to be thick enough to impede either the water permeation through the SiN layer and reaching the AlGaN surface [17], or the electrons from the gate in reverse bias being injected onto the passivation surface, as Gao et al suggested [11].…”

“…On the other hand, SiN passivation is widely used to prevent current collapse, even though the exact mechanism is still not clear. Gao et al recently proposed that water-related redox reactions play a significant role in the physical origin of surface trapping states both in unpassivated and passivated AlGaN/GaN HEMTs [10], [11]. Although they showed that a thick SiN passivation and a fluorocarbon top layer can mitigate this effect [12], a robust solution to avoid water adsorption and diffusion is still under research.…”

Negative differential resistance: Another banana?

“…For AlGaN / GaN high-electron-mobility transistors (HEMTs) under ambient moisture, Gao et al found current collapse by the ionization and deionization of water molecules at the device surface [21,22].…”

Avoidance of instable photoluminescence intensity from AlGaN bulk layers

“…The generated holes drift in opposite direction toward the crystal surface where the negative surface charge is lowered. The accumulation of positive charges at the surface changes the configuration of adsorbates , predominantly consisting of chemically bound oxide or hydroxide and physically bound water . The ratios between chemically bound species as well as the pH value of the adsorbed water layer will adjust to the more positive charge at the semiconductor surface .…”