The piezoelectric polarization of the AlGaN/GaN high-electron-mobility transistors (HEMTs) is strongly related to the strain state in the active area. Therefore, understanding the strain behavior inside the channel is crucial to the device electrical performance improvement of devices. This paper, for the first time, reveals the potential of optimizing flip-chip structures with active-region bumps to modulate the strain state of the AlGaN/GaN HEMT for enhancing the piezoelectric effect. The thermo-mechanical strain is observed to be affected by the physical dimensions and the material properties of the package. Thus, incorporating device strain engineering into packaging design will be very important for GaN devices due to their strong piezoelectric effects.