The properties of deep donor states (DX centers) in III-V alloys are discussed in relation to their influence on device characteristics and performance. The techniques to avoid or minimize such deleterious effects in AlGaAs-based devices are discussed, along with their physical basis, and some guidelines for improved III-V device design are established. New results about the benefits of proper donor selection, the role of In alloying, the advantage of δ doping in layers and in modulation-doped devices, and the use of AlInAs and InGaP as alternative wide band-gap III-V alloys are presented.