MgO has been suggested as a possible high-k dielectric for future complementary metal-oxide semiconductor processes. In this work, the time dependent dielectric breakdown (TDDB) characteristics of 20 nm MgO films are discussed. Stress induced leakage current measurements indicate that the low measured Weibull slopes of the TDDB distributions for both n-type and p-type devices cannot be attributed to a lower trap generation rate than for SiO2. This suggests that much fewer defects are required to trigger breakdown in MgO under voltage stress than is the case for SiO2 or other metal-oxide dielectrics. This in turn explains the progressive nature of the breakdown in these films which is observed both in this work and elsewhere. The reason fewer defects are required is attributed to the morphology of the films.