Effect of surface passivation on two-dimensional electron gas carrier density in AlGaN/GaN structures

The effect of surface passivation on two-dimensional electron gas (2DEG) carrier density in undoped AlGaN/GaN heterostructures is investigated by capacitance-voltage (C-V) measurements. All oxides used (SiO2, Al2O3, and Ta2O5) experienced a decrease in 2DEG carrier concentration with increasing thickness of the respective oxide layers between the gate and the AlGaN layer. In contrast, the 2DEG carrier concentration increased markedly with increasing Si3N4 layer thickness. An elementary polarization model Was used to fit the behavior for all materials and thicknesses leading to quantitative results. In combination with bowing measurements, the fitting suggests that the effect of the Si3N4 on the 2DEG carrier concentration can be traced back mainly to an increase in piezoelectric polarization charge due to strain, whereby changes in the 2DEG carrier concentration caused by oxides are explained by 2DEG charge accumulation due to fixed interface charges at the AlGaN/oxide interface and in the bulk of the oxides.