The authors have studied the electrical conductivity of n⁺-n^--n⁺ GaAs structures in which the thickness of the n^- layer is comparable with the mean donor separation. At low temperatures, electrical conduction is dominated by a process in which electrons tunnel across the sample via shallow donor impurities close to the centre of the n^- layer. By studying the magnetoresistance of such samples they have investigated the effect of a magnetic field on the donor wavefunction in GaAs. Measurements are compared with the approximate analytical expression for the donor wavefunction and with numerical solutions of the ground-state hydrogenic wavefunctions in megatesla fields developed by astrophysicists.