Resonant tunneling via donor<i>X</i>states in the AlAs barrier and binding energies of donors bound to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>X</mml:mi></mml:mrow><mml:mrow><mml:mi>XY</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>X</mml:mi></mml:mrow><mml:mrow><mml:mi>Z</mml:mi></mml:mrow></mml:msub></mml…

Magnetotransport in GaAs/AlAs/GaAs single-barrier heterostructures, incorporating unintentional donors in the barrier, is studied. Resonant tunneling is observed through the quasiconfined states in the AlAs layer which originate from the ${X}_{\mathrm{XY}}$ and ${X}_{Z}$ conduction-band minima and through two distinct states of the donors bound to the ${X}_{\mathrm{XY}}$ and ${X}_{Z}$ valleys. This allowes us to determine directly the binding energies of ${X}_{\mathrm{XY}}$- and ${X}_{Z}$-related donors at the center of a 5-nm AlAs barrier as ${E}_{B}{(X}_{\mathrm{XY}})\ensuremath{\approx}70 \mathrm{meV}$ and ${E}_{B}{(X}_{Z})\ensuremath{\approx}50 \mathrm{meV},$ respectively. Furthermore, we observe an additional oscillatory fine structure of the donor resonances which we attribute to a difference in the binding energies of donors located at different position in the AlAs layer.