We have studied the electrical characteristics of p‐GaAs/n‐Si (100) heterojunction diodes grown by molecular beam epitaxy in an effort to determine the interface polarity. A Ga or As pre‐exposure was used prior to the growth of p‐GaAs on n‐Si substrates to prevent antiphase domains. The Ga prelayer induces a shift in the built‐in voltage of −0.2 V, while the As prelayer shifts it as much as +2.0 V depending upon the As coverage. From the shift, the electrical charge and its polarity can be determined which is not possible by structural analysis. These electrical measurements are very sensitive to the interface charge properties and show very clearly that even with a submonolayer pre‐exposure, antiphase domain‐free material can be obtained.