Charge trapping and interface state generation in rapid thermal processed (RTP) oxide and NH3-nitrided oxide metal oxide semiconductor capacitors have been investigated by internal photoemission (IPE) electron injection from both the Al gate and the Si substrate. It is found that the generation rate of the interface state (D-it) by IPE electron photoinjection from the Si substrate is one order of magnitude higher than that from the Al gate for both RTP oxide and nitrided oxide samples at low electron injection fluence, N-inj < 1 x 10(16) electron/cm(2). The polarity dependence of interface state generation is discussed by means of the hydrogen species induced D-it generation model. Photocurrent voltage (photo-IV) technique has been used to determine the centroid and density of trapped charge after IPE electron photoinjection into the dielectrics from both electrodes.