Based on the disorder induced gap state (DIGS) model, an attempt is made to control the insulator–semiconductor (I–S) interfaces of GaAs and In0.53Ga0.47As by an ultrathin surface‐oxidized silicon (Si) interface control layer (ICL). A Si ICL is grown on GaAs or InGaAs by molecular beam epitaxy (MBE), and is partially oxidized. Then, a thick SiO2 or Si3N4 layer is deposited by insitu photo‐CVD processes. An insitu XPS analysis confirms formation of the intended structures. In the GaAs structure, the state density in the midgap region is remarkably reduced. However, the interface Fermi level is blocked by a high density of interface states near the conduction band minimum, contrary to the recent reports of ‘‘complete unpinning.’’ These states are Si‐derived intrinsic states, judging from the band line‐up. On the other hand, no such states exist in the InGaAs structure and completely ‘‘unpinned’’ behavior with a very small hysteresis is realized after annealing. The result is interpreted in terms of successful pseudomorphic matching of Si ICL to InGaAs combined with excellent I–S matching between Si ICL and SiO2.