In the field of display devices typified by liquid crystal display devices, a brightness of a display screen of a display device is adjusted according to an intensity of ambient light of the display device (hereinafter this light is referred to as “external light”). Therefore, to incorporate a light sensor in the display device has been proposed. The incorporation of the light sensor in the display device can be achieved by mounting a light sensor as a discrete component on a display panel thereof. Further, in the case of a liquid crystal display device, a light sensor can be formed monolithically on an active matrix substrate by utilizing a process for forming an active element (TFT) and a peripheral circuit.
In the field of display devices for mobile terminal devices in particular, the light sensor is required to be formed monolithically on the active matrix substrate, from the viewpoint of reducing the number of components and downsizing a display device. As the light sensor formed monolithically, a photodiode having a lateral structure, for example, is known (see, for example, JP 2006-3857 A).
Here, a conventional photodiode (light sensor) is described with reference to FIG. 6. FIG. 6 is a cross-sectional view showing a configuration of a conventional photodiode. As shown in FIG. 6, a photodiode 51 is a PIN diode having the lateral structure. In the example shown in FIG. 6, the photodiode 51 is formed monolithically on the active matrix substrate included in a liquid crystal display panel.
As shown in FIG. 6, the photodiode 51 has a silicon film 60 provided on a glass substrate 52 serving as a base substrate of an active matrix substrate 50. The silicon film 60 is formed at the same time when the thin film transistor (TFT) functioning as the active element is formed, by utilizing the step for forming the TFT. In the silicon film 60, a p-type semiconductor region (p-layer) 51a, an intrinsic semiconductor region (i-layer) 51b, and an n-type semiconductor region (n-layer) 51c are provided in a plane direction in the stated order. In this photodiode 51, the i-layer 51b serves as a light detection region.
The i-layer 51b is a region electrically almost neutral, as compared with the adjacent p-layer 51a and n-layer 51c, and is formed by an ion implantation step performed in a process for forming active elements (TFT) or peripheral circuits in such a manner that a conductive electron density and a positive hole density are equal to each other. Further, below the photodiode 51, a light blocking film 53 for blocking illumination light from a backlight device is provided, with a basecoat film 54 being interposed between the photodiode 51 and the light blocking film 53. The photodiode 51 is covered with interlayer insulation films 55 and 56.
In FIG. 6, “57” denotes a line connected with the p-layer 51a, “58” denotes a line connected with the n-layer 51c. Further, “59” denotes a flattening film, and “61” denotes a protective film. “62” denotes a liquid crystal layer. Regarding a counter substrate 63, only its outline is shown.
In the photodiode 51 shown in FIG. 6, depletion layers are formed between the p-layer 51a and the i-layer 51b, and between the n-layer 51c and the i-layer 51b. When light enters the i-layer 51b while a reverse bias voltage is being applied across opposite ends of the photodiode 51, free electrons in each of the depletion layers move toward the n-layer 51c, and positive holes move toward the p-layer 51a. As a result, a photoelectric current is outputted from the photodiode 51.