As a potential candidate for solid-state switches in low-power electronic circuits, the Tunnel Field Effect Transistor (TFET) has attracted the attention of device designers in the past few years. Although simulations have shown that ideal hetero TFETs can achieve sub-thermal sub-threshold swing (SS), the fabrication of a TFET with sufficient on-current and sub-thermal SS over a few decades of drain current remains to be done. Non-idealities in a TFET such as interface traps, band tails, or surface roughness exhibit stronger influence on TFET characteristics in the sub-threshold region. In Ref.  we observed that among all of these non-idealities the strongest effect is due to interface traps. On the other hand, simulations have shown that channel quantization severely degrades the on-current . In this work, we analyse experimental transfer characteristics of InAs/Si nanowire TFETs (diameter 100 nm) and find reasons for the degradation of SS and on-current. We give an estimate for the Dit that still would allow a sub-thermal SS.