PbZr_0.2Ti_0.8O₃ (PZT) gate insulator with the thickness of 30 nm is grown by pulsed laser deposition (PLD) in AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs). The ferroelectric effect of PZT AlGaN/GaN MIS-HEMT is demonstrated. The polarization charge in PZT varies with different gate voltages. The equivalent polarization charge model (EPCM) is proposed for calculating the polarization charge and the concentration of two-dimensional electron gas (2DEG). The threshold voltage (V _th) and output current density (I _DS) can also be obtained by the EPCM. The theoretical values are in good agreement with the experimental results and the model can provide a guide for the design of the PZT MIS-HEMT. The polarization charges of PZT can be modulated by different gate-voltage stresses and the V _th has a regulation range of 4.0 V. The polarization charge changes after the stress of gate voltage for several seconds. When the gate voltage is stable or changes at high frequency, the output characteristics and the current collapse of the device remain stable.