We have investigated the conduction in heavily doped mu c-Si:H samples
grown by glow discharge by measuring the conductivity and the Hall co
efficient in a wide temperature range. We show that the conductivity o
f these materials cannot be satisfactorily interpreted using the model
s already present in the literature. We put forward a model in which t
he material is considered composed of small crystalline grains embedde
d in an amorphous tissue and in which potential barriers exist between
adjacent grains due to the band discontinuities. A comparison between
detailed calculations and transport data shows that the tunneling (pr
obably aided by the presence of localized states) is the fundamental c
onduction mechanism. (C) 1995 American Institute of Physics.