This article presents an ultra-small, high-securityidentification tag that is entirely built in a CMOS chip withoutexternal components. The usage of backscatter communica-tions at 260 GHz enables full integration of a 2×2patchantenna array. For chip compactness and minimum interfer-ence caused by direct wave reflection, the backscatter signal isfrequency-shifted by 2 MHz and radiated with cross polarizationfrom the same antenna array. Such a configuration also, for thefirst time for RF tags, enables beamsteering for enhanced linkbudget. For authentication and secure wireless data transmission,the tag also integrates a compact elliptic-curve-cryptography(ECC) dedicated processor, which is based on a narrow-strongprivate identification protocol. The presented tag has a peakpower consumption of 21μW and can be powered by a chip-widearray of photodiodes and a DC–DC converter. Using a low-cost65-nm bulk CMOS technology, the terahertz (THz) ID chiphas an area of only 1.6 mm2and demonstrates the measureddownlink speed of 100 kb/s and the upload speed of 2 kb/sacross 5-cm distance from the reader. The tag-reader authentica-tion/communication protocol is fully demonstrated using externaltag power and partially demonstrated using the tag-integratedphoto-voltaic powering. The tag size is the smallest amongall prior radio-frequency identifications (RFIDs) using far-fieldcommunications.