Abstract Boron-doped and phosphorus-doped polycrystalline silicon thin films were deposited on glass and SiO 2 substrates at a rather low temperature of 250°C by electron cyclotron resonance chemical vapor deposition (ECR-CVD) using SiH 4/Ar/H 2/B 2H 6 and SiH 4/Ar/H 2/PH 3 downstream plasma technique. The effects of in situ doping concentration and hydrogen dilution on the structural and electrical properties of heavily doped polycrystalline silicon thin films have been systematically investigated. The largest grain size of the heavily doped poly-Si films with ∼700 nm thickness (growth rate ∼20 nm/min) is approximately 500 nm, and the surface roughness is about 30 nm. With increasing the doping gas flow rate, the resistivity rapidly decreased and the minimum values of 0.025 Ω cm at [B 2H 6]/[SiH 4] ∼9×10 −2 for p-type poly-Si films and 0.036 Ω cm at [PH 3]/[SiH 4] ∼7×10 −2 for n-type poly-Si films were achieved. The Hall mobility decrease as the carrier concentration increase can be explained by the impurity scattering increase with increasing the doping gas flow rate. X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated a change from <110> to <111> with slight decrease in the grain size. Furthermore, the grain shape of the deposited films was changed from elliptical to round. Similar to the undoped poly-Si thin films, larger hydrogen dilution ratio would increase the grain size, change the grain shape from round to elliptical. Otherwise, the Hall mobility simply decreased with increasing the hydrogen dilution ratio until 88% but it rapidly increased with further hydrogen dilution. On the other hand, the carrier concentration exhibited an entirely converse behavior with the hydrogen dilution ratio. The relationships between electrical properties and structure properties with regarding to the hydrogen dilution ratio and doping gas flow rate were attributed to the high surface coverage of atomic hydrogen species, doping precursors disturbance, solid solubility limitation and impurity scattering effect.