We have combined current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurements onn− GaAs–Al0.4Ga0.6As–n+ GaAs capacitors with different thicknesses of Al0.4Ga0.6As to determine the conduction‐band discontinuity at the Al0.4Ga0.6As–n+ GaAs interface. Undoped AlxGa1−xAs deposited by molecular‐beam epitaxy contains negative charge. We calculate the effect of band bending in undoped AlxGa1−xAs due to negative charge on measured barrier heights and onC‐Vcurves. The temperature dependence ofI‐Vcurves is analyzed in terms of thermionic emission to derive barrier heights at then+ GaAs–Al0.4Ga0.6As interface &fgr;G. Measured values of &fgr;Gare proportional to the square of the insulator thickness,w, showing that negative charge is uniformly distributed through undoped Al0.4Ga0.6As. Combining values of Fermi energy and band bending at zero bias with &fgr;Gat zero Al0.4Ga0.6As thickness, we find that the value of the barrier discontinuity forn+ GaAs–Al0.4Ga0.6As is ∼0.30 V. This corresponds to a ratio of conduction‐band discontinuity &Dgr;ECto band‐gap difference &Dgr;EGof 0.63±0.03, which is appreciably less than the value 0.85±0.03 that is widely accepted. We find that the dielectric constant of Al0.4Ga0.6As is more temperature dependent than that of GaAs between 77 and 250 K.