I am looking for a small chip that can fit in a shoe or mobile phone.
There are many types of RFID transponders small enough to be put into a shoe or a mobile phone. Electronic component manufacturer Murata Manufacturing Co., for example, produces a high-frequency (HF) tag measuring 3.2 millimeters (0.13 inch) in width and length and 0.7 millimeter (0.03 inch) in thickness. The tag (model LXMS33HCNG-134) is one-tenth the size of most other HF tags, and was designed to be both durable and small, with a ceramic substrate enclosing a layered circuit board and an antenna (see Murata Mass-Produces ‘World’s Smallest HF Tag’).
Hitachi, the Japanese semiconductor company, produces the µ-chip (pronounced mu-chip), which measures just 0.3 millimeters square. The chip operates at 2.45 GHz and stores a 128-bit number based on the µ-chip ID number criterion developed by Hitachi, which issues the numbers (see Hitachi Unveils Smallest RFID Chip).
Hitachi Chemical is marketing an EPC Gen 2 passive ultrahigh-frequency (UHF) RFID tag that is one of the smallest tags on the market, measuring just 2.5 millimeters (0.098 inch) square and 0.3 millimeter (0.012 inch) thick. Consisting of an Impinj Monza 5 chip and an antenna embedded in epoxy resin, the Ultra-Small Package tag is designed to be durable enough that it could be applied via injection molding or incorporated into printed circuit boards (see Hitachi Chemical Markets Tiny UHF Tag).
Keep in mind that as tag size gets smaller, read range shortens. That’s because a small antenna cannot harvest much energy from the reader antenna, so it cannot reflect back a strong signal to a reader.
—Mark Roberti, Founder and Editor, RFID Journal