  Backward diodes conduct more efficiently in the reverse bias than in the
forward bias, providing superior high frequency response, temperature
stability, radiation hardness, and 1/f noise performance than a conventional
diode conducting in the forward direction. Here we demonstrate a van der Waals
material based backward diode by exploiting the giant staggered band offsets of
WSe2/SnSe2 vertical heterojunction. The diode exhibits an ultra-high reverse
rectification ratio (R) of ~2.1x10^4 and the same is maintained up to an
unusually large bias of 1.5 V - outperforming existing backward diode reports
using conventional bulk semiconductors as well as one- and two-dimensional
materials by more than an order of magnitude, while maintaining an impressive
curvature coefficient ({\gamma}) of ~37 per V. The transport mechanism in the
diode is shown to be efficiently tunable by external gate and drain bias, as
well as by the thickness of the WSe2 layer and the type of metal contacts used.
These results pave the way for practical electronic circuit applications using
two-dimensional materials and their heterojunctions.
