Active RFID uses an internal power source, such as a battery, within the tag to continuously power the tag and its RF communication circuitry. Active RFID allows extremely low-level RF signals to be received by the tag (since the reader/interrogator does not power the tag), and the tag can generate high-level signals back to the reader/interrogator. Active RFID tags are continuously powered, whether in the reader/interrogator field or not, and are normally used when a longer tag read distance is desired.
Passive RFID relies on RF energy transferred from the reader/interrogator to the tag to power the tag. Passive RFID tags reflect energy from the reader/interrogator or receive and temporarily store a small amount of energy from the reader/interrogator signal in order to generate the tag response. Passive RFID requires strong RF signals from the reader/interrogator, and the RF signal strength returned from the tag is constrained to very low levels by the limited energy. Passive RFID tags are best used when the tag and interrogator will be close to one another.
Semi-passive RFID uses an internal power source to monitor environmental conditions, but requires RF energy transferred from the reader/interrogator similar to passive tags to power a tag response. Semi-passive RFID tags use a process to generate a tag response similar to that of passive tags. Semi-passive tags differ from passive in that semi passive tags possess an internal power source (battery) for the tag's circuitry which allows the tag to complete other functions such as monitoring of environmental conditions (temperature, shock) and which may extend the tag signal range.