Charge-Gated Thin-Film Transistors for High Resolution Digital Imaging Applications

A field-effect thin-film transistor (TFT) with two gates, which are charge and voltage gates, is introduced. By applying a proper voltage to the voltage gate, the device is biased at a desired operating point or turned on and off, whereas the amount of charge deposited on the charge gate, which is embedded inside the dielectric, shifts the threshold voltage and therefore modulates the drain-source current. Such device finds application in sensor circuits, particularly high resolution sensor arrays, where it replaces both the transconducting amplifier and the addressing switch transistor, thus reducing transistor count per pixel. Device structure, operation, and characteristics are derived and discussed. A planar configuration of the charge-gated TFT was fabricated by using top-gate amorphous silicon TFTs and implemented in a 100-mum -pitch two-transistor active pixel sensor test structure.