In this paper we present a detailed study of doping and alloying effects in a-Ge:H due to the incorporation of phosphorus, nitrogen, oxygen, boron, carbon and silicon. The film characterization was performed by means of quantitative analysis , optical absorption in the band and subband range and conductivity measurements. N and O were found to form donor states resulting in an increase of the dark conductivity up to five orders of magnitude. The maximum value of 0.3 (Ωcm)-1 has been obtained by N-doping of about 1 at% solid phase concentration, which exceeds the effect of P doping. B doping causes a decrease of the dark conductivity to a minimum of 10-6 (Ωcm)-1 at 150 ppm solid phase concentration. Addition of Si and C up to concentrations of about 5 at % does not influence the subgap absorption, where best values of E0<50 meV and α(0.6 eV)<10 cm-1 were obtained for the Urbach energy and midgap absorption, respectively. A Si content of 1 at % decreases the dark conductivity, whereas the normalized photoconductivity remains at a high level of ŋμτ=10-5 cm2V-1.