Germanium‐silicon heterojunctions have been prepared by an alloy process. By controlling the impurity concentrations, abrupt junctions having widths from 6000Aå to as low as 80Aå have been realized. Forward and reverse bias characteristics of several typical heteroj unctions are presented. No Esaki effect was observed for thin heterojunctions (<100Aå). This is attributed to masking by an abnormally high excess current. This excess current is a tunneling current via defect states caused by the large dislocation densities present in the junction.