On the basis of the SCHOTTKY space charge theory of semiconductor rectifiers a theory for pnheterojunctions is given. The presence of a thin layer with an extremely small carrier lifetime at the interface between the two semiconductors is considered to be typical for pn-heterojunctions. The pn-heterojunction is found to correspond to two metal—semiconductor contacts in series, the boundary concentrations of current carriers, however, not being independent upon the voltage applied. The d. c. properties are mainly determined by the ratio of the densities of imperfections in the two semiconductors. The straight-line portion of the logarithmic forward characteristic has a slope between e/k T and e/2 k T, depending on the value of this ratio. The reverse current rises exponentially with voltage; the reverse resistance has a maximum.