NMOS surface-channel transistors using shallow trench isolation (STI) is known to show reverse narrow width effect (RNWE) such that the threshold voltage becomes smaller when the channel width decreases. We found that by using a phosphorus deep S/D implant in addition to an arsenic deep S/D implant, the threshold voltage first becomes larger when the channel width decreases and then later becomes smaller when the channel width further decreases for NMOS transistors with very small gate lengths. We attribute such an anomalous narrow width effect to an enhancement of TED due to Si interstitials generated by the phosphorus implant. PMOS transistors show up a much stronger anomalous narrow width effect compared to NMOS transistors. We attribute such an anomalous narrow width effect to an enhancement of phosphorus and arsenic TED due to Si interstitials generated by the deep boron S/D implant.