Sharp Laboratories of Europe of UK and Sharp Corp. have jointly developed a 2.6-inch LCD panel incorporating an image sensor in a drive circuit, and unveiled it at the ISSCC 2007 event being held in San Francisco. The panel features the VGA resolution (640 x 480) and definition of 300 ppi. The frame rate of sensor output is 30 Hz. The companies aim to use the panel to add a function of touch-panel input and biometrics capability to LCD units for use in mobile devices.
Other manufacturers have also tried to embed the image sensor function in an LCD panel thus far, the two unique features of Sharp's latest panel are (1) integration of image sensor with the drive circuit of the display panel and (2) one-transistor one-capacitor (1T1C) circuit structure employed for the image sensor. As a result, new functions can be added at a low cost and the resolution of sensor output is enhanced.
Thanks to the effect of the feature (2), the aperture ratio is increased to 40%, which is about 1.5 times that of the pervasive three-transistor one-capacitor (3T1C) circuit structure. This ratio is only 18% lower than that of a standard LCD panel without including a sensor.
TFT, photodiodes mounted on the same substrate
The above two features result from Sharp's proprietary TFT fabrication technology called continuous grain (CG)-Silicon TFT process. This technology enables the integration of circuits with multiple functions on a single glass substrate by the same film formation process.
Based on this process, a PIN photodiode that converts incident light into the electrical signal and the TFT that drives the display panel are formed on the Si thin film on the same glass substrate. The photodiode is formed adjacent to the TFT so that they can share the same bus, thereby allowing the function as a sensor circuit even without the resetting and row-selecting transistors.