We calculate electron transmission in carbon nanotube field-effect-transistors, based on a tight-binding model. For positive gate voltages, a quantum dot is formed in the nanotube between two regions doped p type by metal contacts, and the resulting quantum dot states give rise to n-type conduction. While p-type currents for negative gate voltages exhibit robustness, n currents sensitively depend on the depletion layer between regions doped p type by the contacts and n type by the gate. This feature explains the polarity of electron transport observed in carbon nanotube transistors. (C) 2002 American Institute of Physics.