Bipolar charge detrapping induced current instability in HfSiON gate dielectric pMOSFETs after negative bias and temperature stress is studied by using a fast transient measurement technique. Both single electron and single hole emissions are observed, leading to post-stress current degradation and recovery, respectively. The NBT stress voltage and temperature effect on post-stress current evolution is explored. Clear evidence of electron and hole trapping in NBT stress is demonstrated. A bipolar charge trapping/detrapping model and charge detrapping paths based on measured charge emission times are proposed.