The field dependence of the hole generation rate, also known as the impact ionization coefficient a, in thin SiO2 (< 20 am) was characterized by measuring the negative flat-band shift due to hole trapping. In thicker oxides, a = aoe -H/E where H = 78 MV/cm for electric fields ranging from 7 to 14 MV/cm, which covers the field range from the onset of significant Fowler-Nordheim current to instant breakdown. The similar field dependcences of a and charge-to-breakdown supports the model that hole generation and trapping leads to oxide wearout. Because of the fact that positive charge generation is observed for oxide voltage well below the SiO2 bandgap, we propose that the generated holes arise from transition between band tails in the amorphous SiO2. It is also observed that a decreases rapidly when the applied oxide voltage is very low; thus a is a function of both oxide field and voltage in general. This suggests that ultra-thin oxide with low operating voltages might be a good candidate for high endurance E2PROM devices at very low oxide field.