The main goal of the work presented in this dissertation is to achieve a good performance of metal-semiconductor-metal photodetectors (MSM-PD's) for both Si and ZnO materials. This dissertation is mainly divided into two categories. First, Si-based MSM-PD's with conventional linear interdigitated fingers were designed, fabricated, measured, and simulated. Growth, characterization, and application, such as MSM-PD's and solar cells of ZnO thin films were including in the second part of the work. The dark leakage current and sensitivity of Si-based MSM-PD's were improved by adding a-Si:H and/or SiO 2 layers on top of c-Si, choosing metals with higher work function or using cryogenic processing, and choosing geometry design with smaller size of finger width and spacing. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) showed a good crystalline quality of the detectors formed with cryogenic processing. The size of metal grains became smaller and more crystals were formed as temperature decreased. In the second category, ZnO thin films were deposited either by RF magnetron sputtering or laser assisted molecular beam deposition (LAMBD). Some LAMBD samples were post-deposition excimer laser annealed with pulse energy levels of 103 mJ/cm 2 or 206 mJ/cm 2 , and some RF sputtered samples were annealed at 300 °C in air for 30 minutes. X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) show both RF sputtered and LAMBD ZnO films were close to a 1:1 Zn:O stoichiometry, with energy bandgap of ~3.3 eV at room temperature. The surface morphology was observed from field emission scanning electron microscopy (FESEM) and appears to be very rough, implying 3-D growth; crystals formed more tightly and uniformly with heat treatment under air ambient. The performance of both ZnO MSM-PD's and solar cells was evaluated by means of room temperature dark and photo I-V measurements. Both dark current and responsivities were improved after heat treatment for RF sputtered ZnO MSM-PD's. The value of responsivity for heat-treated devices was increased from 41.9 mA/W to 59.3 mA/W, and the dark current of the as-deposited RF ZnO MSM-PD was found to be three orders lower in magnitude compared to the heat-treated film.