Development of long-range UHF-band RFID tag chip using Schottky diodes in standard CMOS technology

We present the design of three key building blocks for UHF-band passive RFID tag chip, i.e., voltage multiplier, ASK demodulator, and internal clock generator. An analysis on a simple equivalent circuit of RFID tag chip for long reading range is presented taking into account the finite turn-on voltage of tag chip. The Schottky diodes used in the passive RFID tag chip were fabricated using Titanium (Ti/Al/Ta/Al)-Silicon (n-type) junction in 0.35 μm CMOS process, and the effect of size of Schottky diode on the turn-on voltage and the input impedance of the voltage multiplier was investigated. For 300 mV RF input voltage, the fabricated voltage multiplier using Schottky diodes generated output voltages of 1.5 V and corresponding voltage conversion efficiency of 45%. In addition, we propose an example circuit for internal oscillator of tag chip with digital calibration, which can generate precise copy of RFID reader timing signals.