MOS device interfaces are investigated using carrier injection and monoenergetic positron beam experiments. Carrier injection reveals that the holes injected into gate S1O2 film seem to be the main cause of the interface state generation and the dielectric breakdown of thin-gate SiO2. Positron annihilation experiments show that the positron diffuse along the electric field in the Si and the gate SiO2 and are trapped in the interface region before annihilation. The obtained value of 5 at the SiO2/Si interface was 0.500 ±0.003. The behavior of holes in the SiO2 and SiO2/Si interface are simulated using the monoenergetic positron annihilation technique.