Ambient Light Sensing Circuit with Low-Temperature Polycrystalline Silicon p–Intrinsic–n Diode and Source Follower for Auto Brightness Control

Abstract

An ambient light sensing function, which is one of several value-added functions in system-on-panel (SoP), can contribute to low power consumption and improve visibility by auto brightness control. However, it is very challenging to read the photoleakage current of p-intrinsic-n (PIN) diode directly, which is generally used as a photodetector in SoP. Therefore, we propose an ambient light sensing circuit using low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) and the PIN diode, which can output analog voltage converted from the photo-leakage current of the PIN diode. Through the measurement of the proposed circuits under ambient light variations from 0 to 2000 lx, we confirmed that the proposed ambient light sensing circuits can perform sensing and readout operations accurately. The dynamic range of the proposed circuit is 35 dB and the maximum variation of the output voltage among eight samples is AE45 mV.

Current‐mode ambient‐light sensing circuit using p‐type low‐temperature polycrystalline‐silicon TFTs and p‐i‐m diodes

“…Figure 3(b) shows a comparison of the output signal range between the proposed sensing circuit and the previously reported source followertype sensing circuit. 5 Because it is meaningless to compare the output signal range between both sensing circuits through a direct comparison, the output current of the proposed sensing circuit with the output voltage of the source follower-type sensing circuit, the current signal of the proposed sensing circuit is just converted into a voltage signal using an arbitrary resistor for comparing the output signal range quantitatively. In fact, the output node of the proposed sensing circuit can be connected to the current ADC or the voltage ADC with an input capacitor.…”

“…An ambient-light sensing function can contribute to lower power consumption and improved visibility by detecting ambient light and controlling the brightness of the display panel accordingly. [1][2][3][4][5] Several ambient-light sensors using polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) and p-intrinsic-n (p-i-n) diodes have been reported. [1][2][3][4][5] However, the photoleakage current range of a p-i-n diode is several picoamps to several hundreds of picoamps in the low-ambient-light range because the p-i-n diode has thin thickness and low absorption efficiency compared to a photodiode used in a CMOS image sensor (CIS).…”

“…[1][2][3][4][5] Several ambient-light sensors using polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) and p-intrinsic-n (p-i-n) diodes have been reported. [1][2][3][4][5] However, the photoleakage current range of a p-i-n diode is several picoamps to several hundreds of picoamps in the low-ambient-light range because the p-i-n diode has thin thickness and low absorption efficiency compared to a photodiode used in a CMOS image sensor (CIS). Therefore, it is very difficult to design a readout circuit that performs a very accurate readout operation with the small current of the p-i-n diode.…”

“…Recently, an ambient-light sensing circuit with a source follower, which converts photoleakage current to analog voltage signal and buffers the converted voltage signal to an analog-to-digital converter (ADC), was reported. 4,5 However, this circuit has the drawback of being sensitive to the coupling noise because the readout line becomes a floating node during the readout operation. In addition, processing the p-i-n diode incurs a high manufacturing cost.…”

“…The experimental results of the proposed sensing circuit exhibit a wide dynamic range of 56 dB, while the output signal range is 1.8 times wider than that of a previously reported sensing circuit. 5…”