NOVELTY - The preparation of poly crystalline silicon using a fluidized bed reactor for manufacturing semiconductor devices or solar cells, comprises introducing a fluidizing gas (10) into a silicon particle bed using a reaction gas (11) inlet unit of the reactor, introducing an inert gas (12) into outer zone of the reactor, maintaining a substantially inert gas atmosphere in the outer zone, heating the silicon particle bed using a heating unit, discharging the polycrystalline silicon particles to the outside of the reactor, and discharging an off-gas (13) having a fluidizing gas.    USE - Useful for manufacturing semiconductor devices or solar cells.    ADVANTAGE - The method ensures effective production of poly crystalline silicon with high purity in economical manner without using additional pulverizing device to control the size and the amount of the seed crystals.    DETAILED DESCRIPTION - The preparation of poly crystalline silicon using a fluidized bed reactor for manufacturing semiconductor devices or solar cells, comprises introducing a fluidizing gas (10) into a silicon particle bed using a reaction gas (11) inlet unit of the reactor, introducing inert gas (12) into outer zone of the reactor, maintaining a substantially inert gas atmosphere in the outer zone, heating the silicon particle bed using a heating unit, discharging the polycrystalline silicon particles to the outside of the reactor, and discharging an off-gas (13) having a fluidizing gas, which passes through the silicon particle bed, a non-reacted reaction gas and a byproduct gas to the outside of the reactor. The fluidized bed reactor has a reactor tube vertically placed within a reactor shell. The reactor tube is encompassed by the reactor shell. The inner space of the reactor shell is divided into an inner zone formed within the reactor tube and an outer zone formed between the reactor shell and the reactor tube. The silicon particle bed is formed and silicon deposition occurs in the inner zone while the silicon particle bed is not formed and the silicon deposition does not occur in the outer zone. The heating zone is installed in the inner and/or the outer zone of the device. The polycrystalline silicon particles are prepared within the inner zone. The inner and outer zones pressure is directly or indirectly measured and/or controlled using an inner- and outer pressure-controlling unit. The difference between the inner zone pressure (Po) and the outer zone pressure (Pi) is maintained within 1-15 bar using a pressure-difference controlling unit to satisfy the condition of 0 bar less than or equal to(Po-Pi)less than or equal to1 bar, when the inner pressure controlling unit is spatially connected to the inner zone by a gas outlet unit, a silicon seed crystals inlet unit or an inner zone connecting unit, which are spatially connected to an upper part of the inner zone instead of the silicon particle bed. The outer zone pressure is controlled between maximum and minimum pressure values measurable in the inner zone. A pressure controlling condition of the inner and/or the outer pressure controlling unit is determined based on gas analysis of the gas that is present within or discharged out of the inner and/or the outer zones using a gas analyzing unit. The packing materials of a packed bed are not fluidized by the flow of the fluidizing gas and are formed in a lower part of the silicon particle bed with the height of the packed bed being positioned below the outlet of the reaction gas inlet unit by the reaction gas is introduced into the silicon particle bed.    DESCRIPTION OF DRAWING(S) - The figure shows a fluidized bed reactor for preparing poly crystalline silicon:    Fluidizing gas; (10)    Reaction gas; (11)    Inert gas; (12)    Off-gas; (13)    Reaction gas inlet unit. (15)