TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The reaction chamber further comprises a core region and a peripheral region, and contains silicon particles, where the first feed gas contains less than 20 vol.% of silane, and the concentration of silane in the first feed gas exceeds the concentration in the second feed gas. The fluidized bed reactor comprises a distributor (2) for distributing gases into the reaction chamber, where the fluidized bed reactor produces at least 100 kg/hour of polycrystalline silicon per square meter of reaction chamber cross-section. The temperature of all feed gases with a concentration of silane of at least 1% is less than 400 degrees C, preferably less than 100 degrees C prior to entry into the reaction chamber. The pressure in the reaction chamber is at least 4 bar, preferably 4-20 bar. The average residence time of gas introduced into the reaction chamber is less than 20 seconds, preferably less than 0.1-20 seconds. In the process, at least 150 kg/hour of silicon deposits on the silicon particles per square meter of reaction chamber cross-section or at least 250 kg/hour, preferably 100-1000 kg/hour of silicon deposits on the silicon particles per square meter of reaction chamber cross-section. The process further comprises maintaining a pressure in the inner chamber greater than 1 bar below the pressure within the reaction chamber or at least 5 bar, preferably 5-20 bar below the pressure within the reaction chamber. The overall concentration of silane in feed gases introduced into the reaction chamber is less than 12 vol.%, preferably 10-15 vol.%. The fluidized bed reactor further comprises an annular inner chamber formed between a reaction chamber wall and an outer shell. The reaction chamber is heated to at least 500 degrees C, preferably 700-800 degrees C. The particulate polycrystalline silicon is withdrawn from the fluidized bed reactor, where the Sauter mean diameter of the participate polycrystalline silicon is 600-2000 mu m, preferably 800-1300 mu m. The reaction chamber is not partitioned into separate portions. The fluidized bed reactor further comprises an annular wall, where the fluidized bed reactor has a generally circular cross-section having a center and a radius R, where the core region extends from the center to less than 0.975R, preferably at least 0.9R, and the peripheral region extends from the center region to the annular wall. The second feed gas comprises one or more compounds comprising silicon tetrachloride, hydrogen, argon and helium. Preferred Process: The process further comprises: either introducing a first feed gas (5) comprising silane into the core region of the reaction chamber and introducing a second feed (7) gas into the peripheral region of the reaction chamber; or introducing one or more feed gases into a distributor to distribute the gases into the reaction chamber, where the temperature of each feed gas is less than 400 degrees C prior to introduction into the distributor. In the process, a spent gas is withdrawn from the fluidized bed reactor, where the pressure of the spent gas is at least 3 bar, preferably 4-20 bar. EXAMPLE - A bed of silicon particles with Sauter mean diameter of 655 mu m were fluidized using a mixture of silane and hydrogen gas. The silane volume concentration in the feed gas was maintained at 10.1% with the gas mixture, which was preheated to 193 degrees C before entering into the fluidized bed reaction chamber. The reactor was maintained at an absolute pressure of 7.5 bar and a temperature of 698 degrees C. Silane concentration in the gas discharge was measured to assess the conversion inside the reactor. The silane conversion was found to be 99.5%. The productivity of the reactor under this operation was calculated to be 560 kg/hours per square meter of the reactor.