Metal organic chemical vapor deposited GaN thin films on n-Si (111) substrates with cubic β-SiC (c-SiC) and porous β-SiC (PSC) buffer layers were characterized and compared with each other. We used rapid thermal chemical vapor deposition (RTCVD) to grow the c-SiC layers, and some of them were electrochemically anodized to obtain the PSC layers. Then the deposited GaN films were characterized with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy. To study the GaN thin films for ultraviolet-detecting applications, we measured the photo and dark currents of a metal-semiconductor-metal (MSM) photodiode with and without the irradiance of an ultraviolet (UV) light source (366 nm, 6 mW/cm2). The photo and dark current ratios (PDCRs) of both structures are high at room temperature, especially for the devices on the PSC buffer layer; a value of as high as 6.75 × 105 under −1 V bias has been achieved. We attribute the high PDCR to the suppression of the induced stress in the GaN film by the PSC buffer layer.