ZnO nanowires/p-Si heterojunction diodes are fabricated by employing low temperature double-step chemical bath deposition technique. The grown samples are initially annealed at different temperatures in the range of 300 degrees C to 600 degrees C at 10 psi argon atmosphere for 30 min. Field emission scanning electron microscope images confirmed the growth of vertically oriented ZnO nanowires with average nanowire diameter and height of 183-190 nm and similar to 1.4 mu m, respectively. The x-ray diffraction study shows that the samples are poly-crystalline with hexagonal wurtzite structure and ZnO nanowires are vertically c-axis oriented. The photoluminescence study indicates the presence of oxygen vacancy in as-grown and 300 degrees C annealed sample, whereas, the 500 degrees C annealed ZnO nanowires show the existence of oxygen interstitials. Heterojunction diodes are fabricated on the as-grown, 300 degrees C, 400 degrees C, 450 degrees C, 460 degrees C, 475 degrees C, 490 degrees C, 500 degrees C, 550 degrees C and 600 degrees C annealed nanowires for the measurements of current-voltage and capacitance-voltage characteristics. Both the results indicate an electron dominated transport for the as-grown to 490 degrees C samples and hole transport for >= 500 degrees C annealed sample. The acceptor or p-type dopant formation temperature is observed to be 500 degrees C, which shows an effective acceptor concentration of 1.15 x 10(15) cm(-3) (at 10 kHz) and 1.74 x 10(15) cm(-3) (at 1 MHz). Thus the work indicates a possible route for converting the n-type ZnO nanowires to p-type of nature.