Since circuits are pushed to operate at millimeter-wave and terahertz frequencies, the conventional approaches for chip-level connections such as wire bondings, vias, flip chips are being challenged by high losses, low bandwidth, and fabrication difficulties, especially for applications above 250 GHz. The goal is to have a fully-integrated chip which does not require sophisticated packaging or integration processes. To this end, on-chip antennas are under intensive study and it becomes one of the key components for realizing chip-to-chip wireless communications at such high frequencies.