  We address recent experiments (Science 325, 1367 (2009)) reporting on highly
efficient multiplication of electron-hole pairs in carbon nanotube photodiodes
at photon energies near the carrier multiplication threshold (twice the
quasi-particle band gap). This result is surprising in light of recent
experimental and theoretical work on multiexciton generation in other confined
materials, such as semiconducting nanocrystals. We propose a detailed mechanism
based on carrier dynamics and impact excitation resulting in highly efficient
multiplication of electron-hole pairs. We discuss the important time and energy
scales of the problem and provide analysis of the role of temperature and the
length of the diode.
