NOVELTY - Producing polycrystalline silicon by the thermal decomposition of silane in a fluidized bed reactor having a reaction chamber, comprises: introducing a first feed gas comprising silane into a core region of the reaction chamber, where the reaction chamber contains silicon particles and the first feed gas contains less than 80 vol.% of silane, and silane thermally decomposes in the reaction chamber to deposit an amount of silicon on the silicon particles; and introducing a second feed gas into a peripheral region of the reaction chamber.    USE - The polycrystalline silicon is useful for producing integrated circuits and photovoltaic (i.e. solar) cells.    ADVANTAGE - The process is capable of efficiently producing polycrystalline silicon with high yield while limiting or reducing the amount of silicon deposits on the reactor by controlling pressure of the fluidized bed reactor within the specified range, rapidly heating the first and second fluidized gases in the reaction chamber to promote deposition of polycrystalline silicon and/or controlling the diameter of the withdrawn polycrystalline silicon particulate within the specified range.    DETAILED DESCRIPTION - Producing polycrystalline silicon by the thermal decomposition of silane in a fluidized bed reactor having a reaction chamber, where the reaction chamber has a core region, a peripheral region and a cross-section through which feed gases pass, and the fluidized bed reactor produces at least 100 kg/hour of polycrystalline silicon per square meter of reaction chamber cross-section, comprises: introducing a first feed gas comprising silane into the core region of the reaction chamber, where the reaction chamber contains silicon particles and the first feed gas contains less than 80 vol.% of silane, and silane thermally decomposes in the reaction chamber to deposit an amount of silicon on the silicon particles; and introducing a second feed gas into the peripheral region of the reaction chamber, where the concentration of silane in the first feed gas exceeds the concentration in the second feed gas, the overall concentration of silane in the feed gases fed into the reaction chamber is less than 15 vol.%, and the pressure in the reaction chamber is at least 3 bar.