A new technique to measure the electron drift mobility, µD, in n-type doped a-Si:H is described. µD is defined from the d. c. conductivity, σ, and the density of electrons in the band tail, nBT, by the expression σ = nBT e µD. The temperature dependence of the mobility was measured from 200K to 450K for both the thermal equilibrium and the frozen in states at different doping levels, using sweep out measurements to obtain nBT for the same thermal conditions. Doping up to 10-2 PH3/SiH4 suppresses µD by about an order of magnitude at room temperature, and increases its activation energy. Numerical modelling shows that a shift of the mobility edge into the conduction band by about 100 meV gives the best fit to the data, and is attributed to potential fluctuations induced by charged dopant and defect states.