The electrical properties of nanometer-sized Schottky contacts which were successfully formed on n-GaAs and n-InP substrates by a combination of an electrochemical process and an electron-beam (EB) lithography. were characterized both experimentally and theoretically. The detailed I-ri measurements using a conductive AFM system showed nonlinear log I-ii characteristics with large n value in range of 1.2-2.0 which cannot be explained by a standard 1D thermionic emission model. A computer simulation showed that this nonlinear characteristics can be explained by a new 3D thermionic emission model where Fermi-level pinning on the surrounding free surface modifies the potential distribution underneath the nano-contact. Calculation of C-V characteristics showed an extremely small change of the depletion layer width with bias due to the environmental Fermi-level pinning. On the other hand, it was also found that Fermi-level pinning at the metal-semiconductor interface itself is greatly reduced, resulting in a strong dependence of barrier height on the metal workfunction.