Flexible functional devices are extremely suitable for malleable, sustainable, and portable applications such as smart clothing, flexible electronics and medical applications. Here, we present a scalable, transparent, and flexible piezoelectric nanogenerator (STF PNG) fabricated by forming a p-n NiO/GaN heterojunction using an electrochemical lift-off process to transfer GaN onto a flexible substrate. Several actuation sources such as airflow, finger forces for bending, vibrations with a frequency of 20 Hz, and cyclic stretching-releasing agitation by a linear motor were applied to generate piezoelectric bias. Peak piezoelectric output voltage and current of 30 V and 1.43 mu A, respectively, were measured. Such high piezoelectric bias was generated by suppressing free carrier screening and junction screening; the former was achieved due to removal of compressive stresses from GaN after the lift-off process, while the latter was achieved by the deposition of a highly resistive p-type NiO layer on transferred GaN and a sandwiched Polydimethylsiloxane resulting in a very high junction resistivity of the p-n NiO/GaN heterojunction STF PNG. As a result, our approach provides a new strategy for novel and highly efficient design of semiconductor-based flexible PNGs for a wide variety of applications.