Unlike conventional planar-doped-barrier (PDB) diodes which have relative low built-in accelerating fields, tunneling of electrons through the energy band gap becomes an important. and at times dominant, current transport mechanism under the high accelerating field which is essential for high performance PDB electron emitters. Thermionic emission across the triangular barrier is also found to be important. A systematic study on the temperature dependent I-V characteristics of PDB diodes with high accelerating field has been carried out, and different carrier transport mechanisms across the PDB diodes will be discussed in this article.