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In this study rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with 22-nm and 12-nm AlGaN barrier layers are fabricated, respectively. Using the measured capacitance–voltage and current–voltage characteristics of the prepared devices with different Schottky areas, it is found that after processing the device, the polarization Coulomb field (PCF) scattering is induced and has an important influence on the two-dimensional electron gas electron mobility. Moreover, the influence of PCF scattering on the electron mobility is enhanced by reducing the AlGaN barrier thickness. This leads to the quite different variation of the electron mobility with gate bias when compared with the AlGaN barrier thickness. This mainly happens because the thinner AlGaN barrier layer suffers from a much stronger electrical field when applying a gate bias, which gives rise to a stronger converse piezoelectric effect.

Based on the measured capacitance—voltage (C—V) curves and current—voltage (I–V) curves for the prepared differently-sized AlN/GaN heterostructure field-effect transistors (HFETs), the I–V characteristics of the AlN/GaN HFETs were simulated using the quasi-two-dimensional (quasi-2D) model. By analyzing the variation in the electron mobility for the two-dimensional electron gas (2DEG) with the channel electric field, it is found that the different polarization charge distribution generated by the different channel electric field distribution can result in different polarization Coulomb field (PCF) scattering. The 2DEG electron mobility difference is mostly caused by the PCF scattering which can reach up to 899.6 cm2/(V·s) (sample a), 1307.4 cm2/(V·s) (sample b), 1561.7 cm2/(V·s) (sample c) and 678.1 cm2/(V·s) (sample d), respectively. When the 2DEG sheet density is modulated by the drain—source bias, the electron mobility for samples a, b and c appear to peak with the variation of the 2DEG sheet density, but for sample d, no peak appears and the electron mobility rises with the increase in the 2DEG sheet density.

Using the measured capacitance—voltage and current—voltage characteristics of the rectangular AlN/GaN heterostructure field-effect transistors (HFETs) with the side-Ohmic contacts, it was found that the polarization Coulomb field scattering in the AlN/GaN HFETs was greatly weakened after the side-Ohmic contact processing, however, it still could not be ignored. It was also found that, with side-Ohmic contacts, the polarization Coulomb field scattering was much stronger in AlN/GaN HFETs than in AlGaN/AlN/GaN and In0.17Al0.83N/AlN/GaN HFETs, which was attributed to the extremely thinner barrier layer and the stronger polarization of the AlN/GaN heterostructure.

Using the measured capacitance-voltage curves of Ni/Au Schottky contacts with different areas and the current-voltage characteristics for the AlGaAs/GaAs, AlGaN/AlN/GaN and In0.18 Al0.82N/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, the two-dimensional electron gas (2DEG) electron mobility for the prepared HFETs was calculated and analyzed. It was found that there is an obvious difference for the variation trend of the mobility curves between the III–V nitride HFETs and the AlGaAs/GaAs HFETs. In the III–V nitride HFETs, the variation trend for the curves of the 2DEG electron mobility with the gate bias is closely related to the ratio of the gate length to the drain-to-source distance. While the ratio of the gate length to the drain-to-source distance has no effect on the variation trend for the curves of the 2DEG electron mobility with the gate bias in the AlGaAs/GaAs HFETs. The reason is attributed to the polarization Coulomb field scattering in the III–V nitride HFETs.