The characteristics of polarizations, including spontaneous polarization (PSP) and piezoelectric polarization (PPE) in Al0.25Ga0.75N/AlN/GaN were obtained by temperature-dependent x-ray diffraction measurements of AlN and GaN layers, and the sheet carrier density (ns) in 20-nm-thick barrier Al0.25Ga0.75N/AlN/GaN heterostructure was studied by Hall measurement at different temperatures from 300 K to 600 K. It is found that the PSP/e (∼1 × 1013 cm−2, e is the electron charge) is larger than PPE/e (∼7 × 1012 cm−2). Excluding the influence of background carrier density, the change of PSP/e with temperature is similar to that of ns, while the tendency of PPE/e is opposite. The contributions to ns change by PSP and PPE are ∼−7.3 × 1011 cm−2 and ∼1.9 × 1011 cm−2, respectively. Therefore, the total variation of PSP and PPE is ∼−5.4 × 1011 cm−2, which is close to the value ∼−5.5 × 1011 cm−2 obtained by Hall data. All these results demonstrate that the PSP is dominant in temperature-dependent properties of two-dimensional electron gas. Moreover, the reduction of PSP with increasing temperature might aggravate current collapse in high electron mobility transistors devices.

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