Subband gap impact ionization and excitation in carbon nanotube transistors

Impact excitation IE and impact ionization II play important roles in carbon nanotube CNT optoelectronics and device reliability. The Boltzmann transport equation BTE in both the real and k spaces is solved to study subband gap II and IE in a CNT metal-oxide-semiconductor field-effect transistor MOSFET . We show that even when the band bending is smaller than the CNT band gap, considerable II or IE can occur. The subband gap II rate varies exponentially with the applied drain voltage, but the current varies linearly due to a small amount of excess carriers. In contrast, solving the BTE by assuming a constant electric field indicates that both the II rate and current varies exponentially. Subband gap II or IE explains why considerable light emission was observed even when the potential drop is smaller in a recent experiment on bright CNT light emitters. The observed exponential variation of light intensity versus the linear variation of current alone, however, is not sufficient to distinguish between subband gap II and subband gap IE. The results also indicate that the bias condition for the maximum hot carrier degradation in CNT MOSFETs is the same as in Si MOSFETs. © 2007 American Institute of Physics. DOI: 10.1063/1.2435821