applies to all the embodiments of the present invention the second embodiment of fabricating p i n diodes is described in fig4 a to fig4 g in contrast to the first embodiment where the p pillar regions are formed before the pillars in the second embodiment the p region in the top portion of the pillars are formed following the construction of the pillars referring to fig4 a a semiconductor substrate 200 with an n layer 210 is provided the structure is essentially similar to the structure shown in fig3 a referring to fig4 b an intrinsic or lightly doped layer i layer 220 is epitaxially grown on the n layer 210 alternatively the structure in fig4 b can be formed by implanting n type dopants into an intrinsic or lightly doped substrate to form the n layer 210 referring to fig4 c a hardmask layer 240 is deposited and patterned the pattern is then transferred through the i layer to form silicon pillars 230 referring to fig4 d gaps between pillars are filled with an insulating material such as oxide 250 the oxide is then recessed to expose the upper portion of pillars 230 b the lower portion of pillar 230 a is surrounded by oxide 250 referring to fig4 e the exposed top pillars 230 b in fig4 d are doped with a p type dopant to form p regions 260 processes for forming the p regions include but are not limited to ion implantation plasma doping plasma immersion ion implantation infusion doping gas phase doping in situ doped epitaxial growth solid phase doping by depositing a dopant source layer such as arsenic doped oxide driving dopants into.