Semiconductor‐insulator‐semiconductorn+‐SI‐n+InP structures have been used effectively for current confinement in channeled substrate buried heterostructure lasers. The semi‐insulating (SI) layer is produced by the implantation of Fe into an+‐InP substrate. The properties of this implanted layer and the role of the deep acceptor levels of Fe in controlling the transport properties of SI‐InP are studied in this and the following paper (Part II). The principle of compensation is discussed and applied to the implanted layer, and current injection phenomena are described, emphasizing the important role of space‐charge‐limited (SCL) current. The electrical characteristics of then+‐SI‐n+structures reveal the classic trap‐controlled SCL current behavior at low temperatures with a trap‐filled limit achieved at 1.2 V, and an approximately trap‐free SCL current at room temperature.