a Transfer characteristics of the TFET and its built-in MOSFET at a drain bias of 500 mV. The WSe2/SnSe2 FET threshold voltage is shifted so to match the MOSFET one and favor a more direct comparison. The dotted line is the gate leakage current. Inset: subthreshold slopes of the two devices vs the output current in logarithmic scale. b Difference between TFET and MOSFET output current over the subthreshold region normalized with respect to the MOSFET current. c Corresponding percent gain of the WSe2/SnSe2 TFET over the WSe2 FET transconductances. d Direct comparison of the transconductance efficiencies of the WSe2/SnSe2 heterojunction and the WSe2 MOSFET. The TFET reaches peak transconductance efficiency close to 90 V−1 and outperforms the MOSFET over the entire subthreshold region.