A Novel 2T Pixel Topology Enabling High-Quality Optical Fingerprints

Fingerprint detection requires high resolution for a detailed and robust fingerprint verification. In large-area devices, fingerprint detection is limited by the tradeoff between speed and output dynamic range. Following the guidelines of the fingerprint acquisition profiles (FAP) of the FBI, this work targets a 500-ppi resolution and high dynamic range fingerprint detection, maintaining a fast operation. A novel 2T voltage-mode (VM) active pixel sensor (APS) topology using dual-gate (DG) amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) is proposed to address the stringent specifications by introducing a very compact APS. An analysis of the topology is implemented considering the biasing, gain, and speed in comparison with the conventional 3T VM APS topology and a 3T VM DG APS topology. A $128\times128$ array of the proposed topology is designed and fabricated with $50~\mu \text{m}$ pitch. A measurement setup is implemented using discrete readout circuits and double delta sampling (DDS) to prove the operation of the pixel. The pixel topology presents a linear output within a range of 1.25 V, a high output voltage resolution of 4.8 mV for an 8-bit dynamic range, and a fast DDS readout speed of $40~\mu \text{s}$ for a ~45-pF data line capacitance. From the analysis, this result corresponds to 13.8 frames/s for an imager with FAP 60 size. The speed of the 2T VM APS is much higher than the conventional VM topology and can compete with current-mode (CM) APS topologies.