Ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics were attained with 25 nm thick Pb(Zr<sub>0.52</sub>Ti<sub>0.48</sub>)O<sub>3</sub> (PZT)/Si<sub>3</sub>N<sub>4</sub> gated AlGaN/GaN heterostructure. The PZT films used in the gate of the device was deposited by magnetron rf-sputtering at the substrate temperature of 700 <sup>o</sup>C. Increasing the PZT deposition temperature from that in previous device structures from 600 <sup>o</sup>C to 700 <sup>o</sup>C we obtained much improved device performance in terms of the IV characteristics inclusive of hysteretic behavior. The pinch-off voltage was about 7 V in FFET device compared to 6 V in a the control (conventional) AlGaN/GaN device. Counterclockwise hysteresis appeared in the transfer characteristic curve of a FFET with a maximal drain current shift of about 10 mA at the gate-to-source voltage of -6 V.