The properties and modifications of surface, sub-surface and bulk regions of Cz-Si after plasma hydrogenation at moderate temperatures (RT -450 °C) and annealing up to 600 °C were studied. Experimental investigations were done by IR-Absorption and Raman spectroscopy, sprea-ding resistance probe analysis, C(V) measurements and scanning electron microscopy. The incorporation of hydrogen from a plasma in-to p-type Cz-Si results in an enhanced thermal donor (TD) formation, if (i) a one-step-process is applied, where the plasma hydrogenation is done at 400 -450 °C (t > 30 min), or if (ii) a two-step-process is applied, where the plasma treatment is done at 250 °C (1h) and then a subsequent annealing is required (t > 10 min). By these processes counter doping of the initial p-type material and therefore a p-n junction formation occurs [1,2]. Prototypes of devices (diodes) were fabricated by such processes [3]. C(V) measurements on diode structures created by TD generation show the performance of linearly graded junctions. By IR-absorption spectroscopy (at 10 K), we can show that the TDD species (thermal double donors) are responsible for counter doping of the p-type Cz-Si. TDD1 up to the TDD6 species could be identified for the neutral and positively charged state (Fig. 1). By depth resolved spreading resistance probe (SRP) analysis the evolution of the n-type regions due to TDD counter doping were investigated in dependence on the treatment/ annealing time. We could verify that neutral atomic hydrogen H 0 is the species which controls the hydrogen enhanced TD formation. From the location of the p-n in dependence on the annealing time a diffusion constant D can be estimated, which comes close to the extrapolated 'Van-Wieringen-Warmholz' value for neutral H 0 [1].