  We have systematically studied the transport properties of the
La$_{2-x}$Y$_{x}$CuO$_{4}$(LYCO) films of T'-phase ($0.05\leq x \leq 0.30$). In
this nominally "undoped" system, superconductivity was acquired in certain Y
doping range ($0.10\leq x \leq 0.20$). Measurements of resistivity, Hall
coefficients in normal states and resistive critical field ($H^\rho_{c2}$)in
superconducting states of the T'-LYCO films show the similar behavior as the
known Ce-doped n-type cuprate superconductors, indicating the intrinsic
electron-doping nature. The charge carriers are induced by oxygen deficiency.
Non-superconducting Y-doped Pr- or Nd-based T'-phase cuprate films were also
investigated for comparison, suggesting the crucial role of the radii of A-site
cations in the origin of superconductivity in the nominally "undoped" cuptates.
Based on a reasonable scenario in the microscopic reduction process, we put
forward a self-consistent interpretation of these experimental observations.
