The electrical surface passivation of mesa-shaped pin diodes was investigated by three different processes : (1) wet thermal oxidation at 700 degrees C, (2) plasma enhanced oxidation at 360 degrees C, (3) plasma enhanced hydrogenation at room temperature. With the plasma-assisted low-temperature processes (2) and (3) low values of leakage currents were achieved compared with the reference process (1), whereas after removal of the oxide passivation layer by HF and subsequent water rinsing the leakage current was significantly higher. According to our model, current transport is supported by mobile charges at the Si surface which are mainly present after process (2) and after removal of the thermal oxide passivation layer by HF. The plasma hydrogen termination of the Si surface lasts for several days since buried hydrogen serves as a reservoir against the decay of Si-H-x surface sites by desorption.