The properties of planar metal-semiconductor-metal (MSM) optical sensor structures on dc photocurrent gain have been investigated an experimentally. The structure has two co-planar Mo/n-Si Schottky-barrier junctions on silicon of resistivity 9-12 Ω-cm and the electrode separation is 2000 µm. The current-voltage (I-V) characteristics measurements under illumination in visible range showed a rapid increase in photocurrent at higher biases examined. The dependence of I-V characteristics and noise measurements, such photocurrent increased was ascribed to avalanche multiplication of carriers photogenerated in the Schottsky junction as applied reverse-biased. From low-frequency (10-50 kHz) signal measurements, it was found that multiplication factor larger than 100 at 10 kHz and 30 at 50 kHz was achieved.