The physical mechanism of IG Random Telegraph Noise (IG-RTN) has been studied in deeply scaled pMOSFETs subject to Negative Bias Temperature Stress (NBTS). Using carrier separation technique, we identify the majority carriers in IG-RTN are channel holes. By investigating the electric field and temperature dependence of the capture time τc and emission time τe in IG-RTN, it is found that the physical origin of IG-RTN are NBTS-induced switching traps; Further quantitative analysis of IG-VG reveal that IG-RTN is related to a tunneling process. Based on these results, we propose a tunneling model through NBTS-induced switching traps to explain the discrete gate leakage. The model provides a good agreement between the predicted and experimental data.