We introduce quite a novel method, photo-excited charge-collection spectroscopy (PECCS) which employs monochromatic photon probes on the gate of working transistors to measure the interfacial trap density-of-state (DOS) of Organic-, Oxide-, Nanowire-, and Nanosheet-based FETs. As a model FET for the present photon-probe measurement, we have used Organic-p channel and ZnO-n channel FETs due to their wide bandgap and easiness for ohmic contact. With the photo-induced threshold voltage Vth shift observed from respective photo-transfer curves, the interfacial trap DOS located below the valence/conduction band maximum/minimum of Organic heptazole/ZnO was quantitatively measured. Using the same technique, PECCS, we also probe the DOS and band gap of ZnO nanowire and molybdenum disulfide (MoS2) layers. In particular, MoS2 nanosheets recently appeared as an alternative nanosheet material that may overcome the drawbacks of graphene, even if prepared by the same exfoliation technique. Bulk MoS2 is known to have an indirect bandgap of ~1.2 eV, but the few angstrom-thin single-layered MoS2 has recently been reported to exhibit a direct bandgap of 1.8 eV. In this presentation, we demonstrate a transparent top-gate photo-transistor with single-, double-, and triple-layer MoS2 nanosheets, which was respectively analyzed by PECCS.