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

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 10⁴ for TFTs having channel lengths of and light intensity up to ~ 8 mW/cm². 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.