The fabrication of heterostructure that has large quantities of p-n, n-n junction with highly dispersed small nanoparticle dopants for metal oxide semiconductor (MOS) gas sensor is one of the important issue in gas sensor research for enhancing the gas sensing performance. Despite many intensive tries in this area, the ideal heterostructure could not be fabricated by all of the conventional bottom-up fabrication methods. Herein, the almost ideal p-n heterostructure nanowire arrays as a gas sensing channel was fabricated by our top-down lithographic technique. The fabricated p-n heterostructure have high resolution of ultrathin (~20 nm thickness) and high aspect ratio (~10) composed of polycrystalline MOS with ultra-small grains of highly dispersed p-type dopants (~ 5 nm) and n-type (~ 10 nm) channel material by $Ar^+$ ion bombardment. Due to its almost ideal structure as a p-n heterostructure and unique morphological characteristics, fabricated p-n nanowire arrays ($WO_3$/CuO) exhibited ultra-high sensitivity ($R_a$/$R_g$ = 224 at 100 ppb) toward ethanol with distinguishable gas response characteristics on wide range of ppb level to % level as well as rapid response (< 21 s) and fast recovery (< 140 s) properties in all concentration range. In addition, our unique top-down lithographic technique can make various combination of heterostructure by simple change of sputtering target materials of p-type and n-type.