We study recently reported drain current (I@sub d@)-drain voltage (V@sub d@) characteristics of a carbon nanotube metal-semiconductor diode device with the gate voltage (V@sub g@) applied to modulate the carrier density in the nanotube.@footnote 1@ The diode was kink-shaped at the metal-semiconductor interface. It was shown that (1) larger negative V@sub g@ blocked I@sub d@ more effectively in the negative V@sub d@ region, resulting in the rectifying I@sub d@-V@sub d@ characteristics, and that (2) positive V@sub g@ allowed I@sub d@ in the both V@sub d@ polarities, resulting in the non-rectifying characteristics. The negative V@sub d@ was the Schottky reverse direction, judging from the negligible I@sub d@ behavior for a wide region of -4 V < V@sub d@ < 0 V, with V@sub g@ = -4 V. Such negative V@sub g@ would attract positive charges from the metallic electrodes (charge reservoir) to the nanotube and lower the nanotube Fermi energy (E@sub F@). With larger negative V@sub g@, the experiment showed that the Schottky forward direction (V@sub d@ > 0) had a smaller turn-on voltage and the Schottky reverse direction (V@sub d@ < 0) was more resistant to the thermionic breakdown. Therefore, the majority carriers in the transport would be electrons since they can see a lower tunneling barrier (shallower built-in potential) in the forward direction when E@sub F@ is lowered, and a thicker tunneling barrier (Schottky barrier) in the reverse direction due to the reduction in the electron density when E@sub F@ is lowered. @FootnoteText@ @footnote 1@Z. Yao, H. W. Ch. Posma, L. Balents, and C. Dekker, Nature 402, 273 (1999).