Modern RFID passive tags all share certain basic characteristics. They are composed of an antenna and a microchip, and a battery for active and semi-passive tags. They work in the same manner:

The passive tag remains dormant until activated by energy from an RFID reader’s antenna. The tag then sends the data stiored on the microchip as radio waves to the reader, powered by energy derived from the reader’s electromagnetic field. The reader interprets the radio waves as data, and can also update the data on the chip.

With decreasing production costs, and without the need for their own power source, passive RFID tags have now reach the point where they can be used disposably.

While the word “tag” is generally used when talking about RFID systems, an RFID tag is specifically an RFID transponder (the microchip and antenna) mounted on an underlying layer. While this is the cheapest option, other possibilities exist:

• the transponder can be placed within a printed adhesive label to form a smart label, which can be used like a barcode, but with the ability to be read without line of sighth and have its data modified;
• it can be embedded in a plastic card, and used in access control or transport pass systems;
• it can be integrated into a container to enable tracking of the contents;
• the transponder can be protected by special packaging to withstand extremes of temperature or other harsh conditions which would otherwise damage it.

The production cost will obviously increase if the transponder is packaged in such a manner.

We will continue to use the term “tag” as an inclusive reference to any of these alternatives.

Whether tag data can be updated is another point of differentiation.

• Read only tags can’t be updated and store a unique serial number, which can be used to uniquely identify them but will need to reference an external database to find more information about the tag.
• Read/write tags can have their information updated many times.
• Write once, read many, or WORM, tags can only be written to once, after which the stored data cannot be updated.

Thus there are a variety of passive RFID tag types, with the most appropriate dependent on the nature of the application in question.