Illumination-induced degradation of minority carrier lifetime was studied in n-type Czochralski silicon co-doped with phosphorus and boron. The recombination centre that emerges is found to be identical to the fast-stage centre (FRC) known for p-Si where it is produced at a rate proportional to the squared hole concentration, p2. Since holes in n-Si are excess carriers of a relatively low concentration, the time scale of FRC generation in n-Si is increased by several orders of magnitude. The generation kinetics is non-linear, due to the dependence of p on the concentration of FRC and this non-linearity is well reproduced by simulations. The injection level dependence of the lifetime shows that FRC exists in 3 charge states (-1, 0, +1) possessing 2 energy levels. The recombination is controlled by both levels. The proper identification of FRC is a BsO2 complex of a substitutional boron and an oxygen dimer. The nature of the major lifetime-degrading centre in n-Si is thus different from that in p-Si - where the dominant one (a slow-stage centre, SRC) was found to be BiO2 – a complex involving an interstitial boron.