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• The mechanism of current modulation in the open region of split-gate AlGaN/GaN HFETs can be explained by the fringe electric field (FEF) effect and polarization Coulomb field (PCF) scattering effect for the applied biased split-gate. • Experimental results and TCAD simulation reveal the role of the PCF scattering in split-gate AlGaN/GaN HFETs. • The PCF scattering modulates the current by affecting the mobility of the two-dimensional electron gas (2DEG) and is closely related to the additional polarization charges generated by the inverse piezoelectric effect. • This study further promotes the research and application of split-gate AlGaN/GaN HFETs. In this study, we fabricated two similar split-gate AlGaN/GaN heterostructure field-effect transistors (SG AlGaN/GaN HFETs). However, minor variations in structural design resulted in substantial differences in the current–voltage characteristics. The mechanism of current modulation in the open region of the HFET can be explained by the fringe electric field (FEF) effect and the polarization Coulomb field (PCF) scattering for the applied biased split-gate. In particular, the effects of FEF and PCF scattering on the two-dimensional electron gas in the open region were analyzed separately when only the open region was on. Experimental results and TCAD simulation reveal the role of PCF scattering in SG AlGaN/GaN HFETs and provide evidence for the universality of PCF scattering in electronic devices with the AlGaN/GaN heterostructure. This study is meaningful for the structural optimization and electrical performance enhancement of SG AlGaN/GaN HFETs. [ABSTRACT FROM AUTHOR]