We investigate electronic properties of heterojunctions between metallic and semiconducting single-wall carbon nanotubes. Ineffective screening of the long range Coulomb interaction in one-dimensional nanotube systems drastically modifies the charge transfer phenomena compared to conventional semiconductor heterostructures. The length of depletion region varies over a wide range (from the nanotube radius to the nanotube length) sensitively depending on the doping strength. The Schottky barrier gives rise to the asymmetry of the I-V characteristics of heterojunctions, in agreement with recent experimental results by Yao {\it et al.} and Fuhrer {\it et al.} Dynamic charge build-up near the junction results in a step-like growth of the current at reverse bias.