Ion implantation of dopants in semiconductors at low temperature generally reduces dynamic annealing and may enhance the substitutional fraction upon subsequent annealing. Si+ implantation at multiple ion energies in the range 30-360 keV into GaN for n-type doping was carried out at either 27 or - 180 degrees C substrate temperature, followed by annealing at 1150-1400 degrees C for 5 min. At total doses of 1.0 x 10(14) cm(-2), the use of cryogenic implants does not produce improved electrical characteristics. The sheet carrier concentration saturates with increasing annealing temperature and reaches similar to 1 x 10(14) cm(-2) at 1350- 1400 degrees C, corresponding to a net electrical activation efficiency of,50%. These values are independent of the implant temperature. The associated room temperature electron mobility in the implanted layers is similar to 100 cm(2)/Vs after annealing at 1350- 1400 degrees C, corresponding to a minimum sheet resistance of similar to 650 Omega/square. (C) 2005 The Electrochemical Society.