In this work, three-dimensional (3-D) p-n junctions were formed for the fabrication of field ionization gas sensors and solar cells. P-Si micro-pillars/ZnO NWs, n-TiO2-nanorod/p-CdTe and n-Si-NW/p-CuInSe2(CIS) material combinations were preferred for the construction of p-n hetero-junction solar cells. Vertically well-aligned Si NWs were synthesized over the surface of n-type silicon wafer by using electroless etching technique. The synthesized Si-NWs embedded into a sputter deposited mono-phase chalcopyrite thin film (CIS) for the realization of nanowire array embedded in thin film type inorganic solar cell, which exhibited a 1.51% power conversion efficiency. In addition to Si nanowires, high aspect ratio vertically well-oriented p-silicon micropillars (MPs) were also synthesized using deep reactive ion Etching (DRIE) process with the BOSCH recipe of cyclical passivation and etching. Three-dimensional (3D) p-Si-MPs/n-ZnO-NWs heterostructures were constructed from hydrothermally grown dense arrays of ZnO nanowires onto these p-type silicon micropillars. The device structures were tested for both the field ionization gas sensor and photovoltaic applications, which showed very promising results. As a final part of this study, TiO2 nanorods (NRs) were grown on FTO glass substrates by using hydrothermal technique, which is sequentially coated with CdTe thin film (sputtering) and subjected to CdCl2 chemical solution treatment to fabricate a core-shell model solar cell with a power conversion efficiency over 0.4% power conversion efficiency.