It is known that when an n-channel metal-oxide-semiconductor field-effect transistor is biased with a high positive gate voltage, a hole current appears in the substrate cathode. Recent experiments indicate that the holes are generated within the oxide. We show that this hole generation mechanism is linked to oxide time-dependent breakdown. When the hole fluence reaches a certain critical value, breakdown occurs. This is in agreement with a hole-trapping-induced breakdown model. For very thin oxides the hole generation rate can become so low that the substrate hole current is dominated by the tunneling of valence-band electrons which is not expected to contribute to oxide breakdown. A different mechanism of hole generation such as hot-hole tunneling from the anode may be responsible for oxide breakdown in the important case of low gate voltage (<6 V).