This paper reports physically based numerical calculations on the relative importance of hot carrier induced photon emission and impact ionization in generating the substrate current of thin oxide MOS capacitors, In particular, we demonstrate that the generation efficiency of photons with energy above the band gap energy is at least 10(-4) smaller than that of electron-hole pairs by impact ionization. Results provide a direct evidence that photon emission can not explain the substrate current which is measured during tunneling experiments from the gate, and set a lower limit to the probability of hole back-tunneling that could make anode hole injection the dominant substrate current generation mechanism in tunneling experiments from the inverted substrate. (C) 2002 Elsevier Science Ltd. All rights reserved.