Poly-Si TFT Photo-Sensing Circuits for Multi-Functional Displays

Abstract

Ambient-light sensors and optical touch screens, which are smart functions of multi-functional displays, can be incorporate photo-sensing circuits using low-temperature polycrystalline silicon (LTPS) technology. This allows both low manufacturing cost and a thin form factor. These photo-sensing circuits consist of LTPS thin-film transistors (TFTs) and thin-film photo diodes such as p-intrinsic-n (p-i-n) or p-intrinsic-metal (p-i-m) diodes. However, LTPS TFTs have non-uniform electrical characteristics such as threshold-voltage and mobility variations, and the thin-film photo diodes are sensitive to temperature and very low photo currents due to their low absorption coefficient and the thinness of the polycrystalline silicon (poly-Si) film. These limitations cause output errors in the photo-sensing circuits, making it difficult to implement them into ambient-light sensor and optical touch screen. In this dissertation, the implementations of ambient-light sensors and optical touch screens are proposed for multi-functional displays. The ambient-light sensor with multi-frame operation is proposed for very wide dynamic range and temperature compensation in the p-i-m diode. Display pixel circuits for the photo-sensing circuit are proposed for the optical touch screen. Moreover, a single-slope analog-to-digital converter (ADC) using LTPS TFTs is proposed for the integration readout circuits on the display panel. The trans-impedance type and the current amplifier type of photo-sensing circuits are proposed for the ambient-light sensor on the display panel. The proposed photo-sensing circuits consist of p-type only LTPS TFTs and p-i-m diodes for their low manufacturing cost, and they are fabricated by using LTPS technology. The proposed trans-impedance type of photo-sensing circuit has a dynamic range of 35 dB, and the maximum variation of the output voltage among eight samples is ± 45 mV. The proposed current amplifier type of photo-sensing circuit has the dynamic range of 56 dB while the output signal range is improved by 1.8 times over that of the trans-impedance type of photo-sensing circuit. The maximum variation of the output current among the three samples is ± 112 nA. A multi-frame driving method to achieve a wide dynamic range and a temperature compensation method for the thin-film photo diode are proposed. The experimental results show that the proposed methods can improve dynamic range while the temperature characteristics of the thin-film photo diode are compensated. The dynamic range of the trans-impedance type of photo-sensing circuit is improved to 79.7 dB and the maximum linearity error is ± 9% under a temperature variation from - 20 to 70 ºC. Therefore, the proposed photo-sensing circuits and the proposed driving methods are suitable for an ambient-light sensor with a wide dynamic range and temperature compensation. The trans-impedance type of photo-sensing circuit, which can be integrated with an active-matrix liquid-crystal display (AMLCD) pixel circuit or active-matrix organic light-emitting diode (AMOLED) pixel circuit, is proposed for an optical touch screen on the display panel. The proposed photo-sensing circuit can output analog voltage which is converted from the photo current of the p-i-m diode. Moreover, it adds only two p-type TFTs, one p-i-m diode, one capacitor, and one signal line to the display pixel circuit for the optical touch screen or image-scanning function. The experimental results show that the proposed photo-sensing circuit can perform sensing and readout operations accurately, as expected. The dynamic range of the proposed circuit is 30 dB while the maximum variation and range of the output voltage are ± 39 mV and 1.5 V, respectively. The single-slope ADC using a comparator with a Schmitt trigger circuit is proposed to integrate the readout circuits of the ambient-light sensor and optical touch screen. Although the proposed comparator operates in two phases, it can operate in a single-slope ADC due to the proposed circuit techniques, such as including an inverter with a low logic threshold-voltage, an edge-trigger switch, and one stage of the Schmitt trigger circuit. The simulated results of the proposed 5-bit single-slope ADC show that the simulated DNL and INL are under ± 0.3 LSB and ± 0.7 LSB, respectively. Therefore, the proposed single-slope ADC can be used in LTPS TFT readout circuits in multi-functional displays.