Carbon Nanotube Length Variation in Correlated CNFETs

Correlation of carbon nanotube FETs (CNFETs) due to aligned CNT growth was shown to improve functional yield of CNFET-based circuits under diameter and the number of CNTs variations. Circuits with correlated CNFETs were investigated based on the assumption that all transistors in correlated sets were identical, due to sharing the same sets of tubes. Therefore, all transistors in the correlated sets have the same failure probability and the same drive current. Though CNTs reach long lengths, not all grow to be equally long. CNTs grown on the substrate will reach various lengths due to early catalyst precipitation and varying CNT diameter. Thus, assuming 100% correlation in correlated-CNFETs is too optimistic. In this work we analyze the impact of variation in CNT length on functional yield and performance of CNFETs. Our results show that variation in tube length can increase failure probability of rows of correlated transistors by 85%. and increase variation in total delay by 1.51X when compared to correlated CNFETs without variation in CNT length. The impact of CNT length variation on gate delay with and without tube diameter and tube count variations is analyzed.