Study and reduction of waterfall effect caused by dimming backlight in LCD based on amorphous silicon TFT

Abstract

Liquid crystal display (LCD) is one of the most popular displays nowadays, and the amorphous silicon thin film transistors are adopted in LCD as active devices due to the low cost, low process issue and high stability. In the LCD device, the backlight unit consumes almost one third of the power of the LCD device. Therefore, reducing backlight power consumption is important to lower the total power consumption. Dimming backlight, which decreases brightness of the backlight and thereby reduces power consumption without affecting the display quality, is the most popular and effective method. However, because of photon assisted current effect of amorphous silicon, the conductivity of the amorphous silicon is different when backlight unit (BLU) turns on and off, which will cause a waterfall phenomenon. Compared to the traditional 4‐mask process, the difference between AS tail and N+ tail in novel 4‐mask process is smaller. The waterfall phenomenon of novel 4‐mask is not obvious but still visible. In this paper, we analyzed the mechanism of waterfall effect and modified the novel 4‐mask process to solve this issue. By combining capacitance extraction and charging simulations, we simulated the variation of grayscale caused by backlight switching. It was shown that there were about 4 gray levels of brightness change under 24 gray levels. Then we examined the cause of the formation of AS tail and N+ tail, and reduced the difference between AS tail and N+ tail by changing the process, which effectively modified the waterfall issue.