The recent emergence of two dimensional (2D) materials has enabled the realization of atomically thin heterostructure devices with vertically stacks of disparate 2D materials. The monolayer thick structure of these materials allows electrostatic doping modulation of the overlying layers in a vertically stacked heterostructure.1 While a majority of work is focused on stacking varying combinations of 2D materials only, an all 2D structure is not necessary to achieve gate-tunable devices. Here, we demonstrate a gate-tunable p-n heterojunction diode using one dimensional semiconducting single-walled carbon nanotubes (s-SWCNTs) and 2D single-layer molybdenum disulfide (SL-MoS2) as p-type and n-type semiconductors, respectively.2 The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that can be tuned with an applied gate bias over a wide range of charge transport behavior ranging from insulating to rectifying with forward-to-reverse bias current ratios exceeding 104. The transfer characteristics of this p-n heterojunction has a unique 'anti-ambipolar' characteristic with two off-states at either extremes of gate voltage range with a current maxima in between them.2 The continuous transition from a positive to negative transconductance in an anti-ambipolar characteristic enables operation of analog communication circuits with a reduced number of circuit components compared to unipolar transistors. This anti-ambipolar characteristic can be widely generalized to heterojunctions of other materials such as s-SWCNTs and n-type amorphous indium gallium zinc oxide (a-IGZO), ultimately leading to all solution processed heterojunctions on a wafer scale.3 References: 1. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. ACS Nano 2014, 8, 1102ï¿½1120. 2. Jariwala, D.; Sangwan, V. K.; Wu, C.-C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 18076ï¿½18080. 3. Jariwala, D.; Sangwan, V. K.; Seo, J. T.; Xu, W.; Smith, J.; Kim, C. H.; Lauhon, L. J.; Marks, T.; Hersam, M. C. 2014, (submitted).