The electrical characteristics of magnesium (Mg)-doped GaAs epitaxial layers grown with different substrate temperatures have been investigated by Hall effect and capacitance-voltage (C-V) measurements at room temperature. The carrier concentration obtained by Hall measurements was decreased from 1.4 x 10(19) to 3.4 x 10(16) cm(-3) when increasing the substrate temperature in the range 460-540 degrees C, and thus the maximum attainable doping density was N-A-N-D = 1.4 x 10(19) cm(-3). The depth profiles of doping density, obtained by C-V measurements, show that carriers largely out-diffused toward the surface. This could be attributed to the surface segregation effect, which was predominant in the samples with a high doping density than those with a low doping density. Mg-doped GaAs layers have higher mobility than Be-doped GaAs layers around N-A-N-D = 10(18) cm(-3). Thus, it is expected that Mg could be effectively used as p-type dopants for highly doped nanostructured semiconductors. (C) 2008 Elsevier B.V. All rights reserved.