Based on the band diagram analysis and systematic measurements, comprehensive description of the output characteristics of tunnel FETs (TFETs) operation is proposed. We show that both tunneling junctions have to be considered simultaneously to explain TFET behavior correctly. For the first time, we present and investigate in detail untruncated I D (V D ) measurements of TFETs. We prove that competition between the two tunneling junctions explains these experiments. Insights on the links between I D (V G ) and I D (V D ) curves are provided, which reveal the origin of the tunneling current in the device. Our theory also enables to clarify previously reported I D (V D ) results.