  The continuous down-scaling of transistors has been the key to the successful
development of current information technology. However, with Moore's law
reaching its limits, the development of alternative transistor architectures is
urgently needed. Transistors require a switching voltage of at least 60 mV for
each tenfold increase in current, that is, a subthreshold swing (SS) of 60 mV
per decade (dec). Alternative tunnel field-effect transistors (TFETs) are
widely studied to achieve a sub-thermionic SS and high I60 (the current where
SS becomes 60 mV/dec). Heterojunction (HJ) TFETs show promise for delivering a
high I60, but experimental results do not meet theoretical expectations due to
interface problems in the HJs constructed from different materials. Here, we
report a natural HJ-TFET with spatially varying layer thickness in black
phosphorus without interface problems. We achieved record-low average SS values
over 4-5 dec of current (SSave_4dec = ~22.9 mV/dec and SSave_5dec = ~26.0
mV/dec) with record-high I60 (I60 = 0.65-1 uA/um), paving the way for
application in low-power switches.
