AlGaN/GaN heterostructure field-effect transistors (HFETs) with three kinds of gate lengths were fabricated, and the theory of polarization Coulomb field (PCF) scattering with the electron systems in the AlGaN/GaN HFETs was studied. There are two methods of analysis and calculation of the PCF scattering in AlGaN/GaN HFETs: one is by considering the 2-dimensional electronic gas (2DEG) of the gate-source, gate-drain, and gate regions as three independent electron systems and the other is by considering the 2DEG of the drain–source channel as a unified electron system. The calculation and analysis of the additional polarization charges underneath the gate region for the prepared AlGaN/GaN HFETs indicate that the theory of PCF scattering in AlGaN/GaN HFETs with three independent electron systems is more accurate.