A sine-gated single-photon detector (SPD) intended for use in a quantum key
distribution (QKD) system is considered in this paper. An "avalanche delay"
effect in the sine-gated SPD is revealed. This effect consists in the
appearance of an avalanche triggered at the next gate after the photon arrival
gate. It has been determined experimentally that the nature of this effect is
not related to the known effects of afterpulsing or charge persistence. This
effect negatively affects the overall error rate in the QKD system. The
influence of the main detector control parameters, such as temperature, gate
amplitude and comparator's threshold voltage, on the avalanche delay effect was
experimentally established