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A highly sensitive photodiode was fabricated with a metal-porous silicon-silicon-metal structure. The porous silicon film was found to be an excellent light trap and antireflection coating for the photodiode. It was demonstrated that close to unity quantum efficiency could be obtained in the wavelength range of 630 to 900 nm. The detector response time is about 2 ns for a 9 volts reverse bias. It was also demonstrated that a relative sensitivity of higher than 90 percent could be obtained with incident angles of 40 degree(s), 80 degree(s)2, and 58 degree(s) for s-,p- and randomly-polarized light. The uniformity and stability of the photodiode were also studied. Porous silicon films can also be used as solar cells to improve the acceptance angle of the sunlight. Possible machanisms to this photodiode and roles of the porous silicon film are discussed.