A conceptual consideration is given to a zero-energy state (ZES) at thesurface of unconventional superconductors. The reflection coefficients innormal-metal / superconductor (NS) junctions are calculated based on aphenomenological description of the reflection processes of a quasiparticle.The phenomenological theory reveals the importance of the sign change in thepair potential for the formation of the ZES. The ZES is observed as thezero-bias conductance peak (ZBCP) in the differential conductance of NSjunctions. The split of the ZBCP due to broken time-reversal symmetry states isnaturally understood in the present theory. We also discuss effects of externalmagnetic fields on the ZBCP.
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