Passive tags are the most prominent type in use within RFID today. In general, they are the most simple in design. The tag does not contain a battery and depends on the strength of the reader RF signal to cause the tag to generate a response. In general, the passive tag contains a serial number, typically 96 to 128 bits in length. The serial number will most often be just a serial number with no connection to a particular product or application. The serial number can be read and then used to establish a relationship to a product within an application database.
The semi-passive tag operates similarly to the passive tag, using the reader signal to cause a response from the tag. The primary difference is that the semi-passive tag does have a battery -- not for generating a response, but to power electronics that are used in conjunction with off-board sensors such as a thermal sensor. The sensor reading is incorporated into the tag return signal along with the tag serial number. Unsurprisingly, the semi-passive tag has most of the limitations noted for the passive tag in terms of slow read speeds and short read distances.The price point for semi-passive tags is higher than that for the passive tag, with prices ranging from $10 to $50.
Active tags contain a battery and do not depend on the reader signal to generate a response. As a result, the active tag can be read at much greater distances, with read distances up to 100 yards seeming quite common. Active tags may be either read-only or read/write, thus allowing data modification by the reader. Data storage on active tags has a range up to 8K bytes, a common value. Data rates are also faster in the active tag, thus making electronic toll collection and weigh station bypass highly successful applications of RFID. There are also active tags that provide off-tag communication via RS232/RS485 protocols, or JBUS to provide such information as fuel levels and odometer readings for transportation gate control applications. As one might suspect, the active tag is more expensive, with prices ranging from $20 to over $100 per tag.