Realization of carbon nanotube (CNT)-based transistors requires preexisting semiconducting CNTs, which are not selectively grown by the conventional synthesis approaches. While CNT heterojunction and cross-junction formed with different chiralities have demonstrated the required performance for CNT transistors, these relied on an junction formation accidentally obtained during sample preparation. We propose that hydrogen functionalization of CNTs can transform the electronic structure systematically from metallic (narrow-gap semiconducting) to semiconducting (large-gap semiconducting). We visualize this phenomenon by fabricating a heterojunction between the pristine and the functionalized CNTs that clearly shows both rectifying and gating effects even at room temperature. (C) 2004 Elsevier B.V. All rights reserved.