We investigated the effects of gate voltage (V-gate) on the source-drain current (I-DS) measured using either poly(dG)-poly(dC) or poly(dA)-poly(dT) DNA molecules attached on the Au electrodes with a nanometer-sized gap of similar to 20 nm. We confirmed that the IDS curves of both poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA molecules can be modulated by simply controlling the magnitude and/or polarity of gate voltage under given humidity level (i.e., 70 similar to 80% in this work). However, different types of I-DS modulation were observed from poly(dG) poly(dG)- and poly(dA)-poly(dT), suggesting that the content of DNA molecules has an important role in current formation under the influence of gate voltage. Furthermore, we suggested that the effective diameter of DNA wires is the important factor for deciding the absolute value of IDS in addition to the types of DNA. Based on the current observation, we suggested that poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA molecules might be treated as p- and n-type semiconductor materials, respectively. (c) 2005 Elsevier B.V All rights reserved.