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This paper reports the theoretical and experimental study of the inverse piezoelectric effect in AlGaN/GaN heterostructure field-effect transistors (HFETs) with field plate (FP) electrodes. The theoretical analysis based on the 2-D Monte Carlo simulation predicted that introducing the FP structure drastically decreases the elastic energy due to the inverse piezoelectric effect. The extension of gate-connected or source-connected FP electrode and thinning a SiN film under FP were found effective to improve the critical voltage (Vcrit) for degradation under reverse bias stress. Also, calculated trends of Vcrit as a function of FP lengths and a SiN film thickness were qualitatively verified by step-stress measurements of field-plated HFETs fabricated on Si substrates. These results clearly indicate that the optimization of the FP structure minimizes degradation associated with the inverse piezoelectric effect in AlGaN/GaN HFETs.