The potential barrier heights, thickness, and capacitance of abrupt heterojunction space‐charge regions under forward voltages are treated analytically, and a model for this is developed. Physical effects such as free‐carrier charges and interface states in the heterojunction space‐charge region are considered. Comparison of the present model and other models for a Gaas/Si heterojunction diode is included. Experimental aspects regarding the heterojunction capacitance are also briefly discussed.