The authors have investigated the electrical properties for one typical set of Al-doped ZnSe layers: one is conductive (type I) and the other is resistive (type II). The two types of ZnSe: Al layers are grown on different substrates by molecular-beam epitaxy under the same conditions: type I is on a 15 tilted (001) GaAs substrate and type II is on an exact (001) substrate. In capacitance-voltage curves, it is found that the measured capacitance in type II is 20 times smaller than that of type I. Moreover, it is estimated that the net-doping density of type II (similar to 10(15) cm(-3)) is two orders of magnitude lower than that of type I (similar to 10(17) cm(-3)). Frequency-dependence measurements of capacitance indicate that type II suffers from deep levels more seriously than type I. In photoluminescence spectra, it is shown that strong deep-level emission peaks exist at the energies of 2.03 and 2.24 eV, but their emission features are almost the same in types I and II. In photocapacitance spectra, it is found that electron-trap centers exist in the midgap region, located at 1.1, 1.2, and 1.3 eV below the conduction-band minimum, and the photocapacitance generated in the midgap of type II is seven times larger than that of type I. Moreover, it is estimated that the total density of the three trap centers in type II (similar to 10(17) cm(-3)) is similar to the net-doping density of type I. Consequently, it is suggested that the midgap trap centers contribute to the carrier compensation in ZnSe: Al layers and induce the different electrical properties in types I and II. c 2008 American Vacuum Society.