Phosphorus‐doped silicon samples grown by various methods after irradiation by fast neutrons of a WWR‐M reactor and subsequent annealing at room temperature were investigated. The calculation of the temperature dependence of effective carrier concentration was carried out in the framework of Gossick's model, taking into account the recharges of defects both in the conducting matrix of n‐Si and in the space charge region of defect clusters. The distribution function of electrons in the acceptor level of bistable defect (CiCs)0 was determined in the case when the concentration of this defect is a function of the Fermi level in the conducting matrix of n‐Si. The concentration of bistable CiCs defect and its energy level at (Ec – 0.123 eV) in the forbidden band of silicon were calculated. The absence of reconciliation between the introduction rate of A‐centers in n‐Si, irradiated by fast neutrons of the reactor, and the concentration of oxygen in 1016–1018 cm–3 limits was determined. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)