A self-aligned and localized nanoforest structure is constructed in a top-down fabricated silicon nanowire (SiNW). The Surface-to.:Volume ratio (SVR) of the SiNW is enhanced due to the local; nanoforest formation: The conductance modulation property Of the SiNWs, which is an important characteristic in sensor and charge transfer based applications, can be largely enhanced. For the selective modification of the channel region, localized. Joule heating and subsequent metal-assisted chemical etching (mac etch) are employed: The nanoforest is formed only in the channel-region:without misalignment due to the self aligned process of Joule heating The modified SiNW is applied to a porphyrin-silicon hybrid device to verify the enhanced conductance modulation. The charge transfer efficiency between the porphyrin and the SiNW, which is caused by external optical excitation, is clearly increased compared to the initial SiNW. The effect of the local nanoforest formation is enhanced when longer etching times and larger widths are used.