We disclose a method to continuously track the movement of eyeballs in all the possible directions, including lateral, vertical, diagonal, and rotational movements, using a compact arrayed sensor network that can be attached on the temple area of the face. Disclosure are principles of sensing, fabrication methods, and unique applications of highly sensitive and biocompatible flexible piezoelectric eye-movement sensors using group III-nitride (III-N) thin films with single-crystalline structure. We claim several details in this invention disclosure. First, the III-N TFs, including aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and their alloys, are used for sensing an element of the eye movement sensors. Second, the sensors can be comfortably attached on the temple area of a face to detect the various movements of eyeballs. Third, detailed eye-related motions are monitored by the sensor network. Fourth, measurement results of the sensor can be used (1) to evaluate the eye and brain fatigue, (2) as an indicator for the diagnosis of several brain-related diseases, and (3) compact eye tracking in personal electronic systems including virtual reality and augmented reality.