Hybrid Sensor and Display Pixel Circuits for Mobile Near-to-Eye Applications

Abstract

Hybrid sensor/display pixels and their operating schemes are proposed, simulated, and characterized. The circuit design implemented a phototransistor connected to a drive thin-film transistor (TFT) and an organic light-emitting diode (OLED). The circuits were fabricated using hydrogenated amorphous-silicon (a-Si:H) TFTs as a hybrid photosensor/switching device. The light sensitivity of the phototransistors and response to incident illumination were modeled and verified experimentally for different white-light intensities. The pixel gray scale was determined by the pulse-height voltage modulation from the phototransistor, which was varied by changing the light intensity. The resulting circuits have a wide dynamic range with a light to dark output current ratio of $10^{4}$ for TFTs having channel lengths of $ and light intensity up to $\sim {\hbox{8}} {\hbox{mW/cm}}^{2}$ . A 256-RGB-level imaging scale is possible with 5-nA steps for each grey scale in a pixel array matrix having a refresh rate of more than 200 Hz.