It is revealed that the reflectance in the whole stop band of a heterostructure, including the defect mode, approaches 1; while the phase-shift depends on the sub-photonic crystal (PC) that faces the incident direction. For every sub-peak in the defect mode or every pass band of the sub-PC facing the incident direction, the phase shift is 2π. These properties are useful for designing optical phase devices several orders more sensitive than those using amplitude-signal. In this paper, a sensitive optical phase logic gate (NAND) is described as an example of optical phase devices. Moreover, these properties are also useful for the study of related physical phenomena.