A buried channel waveguide composed of several core layers (i.e., a expanded-core waveguide) is proved to exhibit single-mode wave characteristics and a mode field compatible with that of a standard single-mode fiber. A sufficient condition for achieving single-mode operation is derived using an effective-index method. The concept of expanded-core waveguides is then applied to the design of MMIC-based beam splitters. Numerical results show that the mode field would exhibit a strong evanescent wave for TM mode in case the core layers are not many enough, even the TE mode field is well confined in the expanded core region. This situation induces substantial polarization dependent loss and a large fiber coupling loss for TM wave. However, this situation can be improved by using more core layers for the expanded-core waveguide.