The doping of semiconductor nanocrystals (NCs) is crucial for the optimization of the performance of devices based on them. In contrast to recent progress on the doping of compound semiconductor NCs and silicon NCs, the doping of germanium (Ge) NCs has lagged behind. Here it is shown that Ge NCs can be doped with phosphorus (P) during synthesis by a nonthermal plasma. It is found that there are more P atoms in the NC near-surface region than in the NC core. P doping modifies the surface state of Ge NCs. Compressive strain can be incuced in Ge NCs by P which can explain the P-doping-enhanced oxidation resistance of Ge NCs. Stable dispersions of P-doped Ge NCs in acetonitrile can be cast to produce films for field-effect transistors (FETs). FET analysis shows that the electrical conductivity and electron mobility of a Ge-NC film increase with the increase of the P doping level, although the electrical activation efficiency of P in the Ge-NC film is low. Finally, atomic layer deposition of aluminum oxide at the surface of P-doped Ge NCs is shown to improve the performance of the FETs.