Triboelectricity harvesting techniques are capable of converting mechanical energy into electricity, which show tremendous potential for powering miniaturized sensors, portable electronics, etc. Specifically, converting mechanical energy into direct-current (DC) power with sufficient current density is one of the key challenges. In conventional triboelectricity nanogenerators (TENGs), polymer-based materials are widely used. Due to the large impedance however, they can only generate electrostatic charges via contact electrification and obtain dielectric displacement alternating current (AC) with low current density. Moreover, the AC power output has to be rectified for practical use. Therefore, exploring new physical mechanisms and new material systems for harvesting triboelectric power is highly desirable. By using C-AFM techniques, we found that a high DC output can be generated from two-dimensional semiconducting materials without applying any external bias. The nanoscale electrical measurement results suggest that the triboelectric DC generation has different physical mechanism compared with the dielectric displacement current generation in conventional TENGs. From the analysis of electronic properties at the probe-sample interface, we found that the phenomenon is related to the surface potential difference between the metal-semiconductor materials under a thermodynamic non-equilibrium condition. By a precise control of the interface structure, we found strong evidences showing the tunneling behavior of the triboelectric charges. Accordingly, a new physical model has been proposed to describe the phenomenon. And also, we have successfully demonstrated the concept at a macro-scale level. This new concept shows a promising future as a green energy harvesting technique with a broad range of material candidates and device configurations.