1. Field of the Invention
The present invention relates to a photoelectric conversion element, an image reading device, and an image forming apparatus.
2. Description of the Related Art
For a photoelectric conversion element used in a scanner, a charge-coupled device (CCD) has conventionally been used. However, with recent demands for high speed processing, a complementary metal oxide semiconductor (CMOS) linear image sensor (CMOS sensor) has been attracting attention. The CMOS sensor is the same as CCDs in the point that photoelectric conversion of incident light is performed by a photodiode (PD). However, the CMOS sensor differs from CCDs in a point that charge-voltage conversion is performed near a pixel to be output to a subsequent stage. Moreover, because the CMOS process is used, the CMOS sensor can include therein a circuit such as an analog digital converter (ADC), and is more advantageous in high speed processing.
However, in a CMOS linear sensor, it is necessary to configure multiple circuits such as a transfer transistor that transfers a charge of a PD to a floating diffusion (FD), a reset transistor that resets an FD, and an amplifier transistor (source follower) that outputs a signal to a subsequent stage, for example, in a pixel.
Therefore, in a CMOS sensor, wirings such as a driving signal of respective circuits, and a power source and a ground (GND) are arranged in layers inside a pixel, and an opening (region through which light can enter) of a pixel is limited by these multilayer wirings. Particularly, because the opening of a pixel of a CMOS linear image sensor affects an incidence angle of light, it can cause unevenness in light amount (shading) in a scanner in which incidence angles of light is different at each pixel position, and there has been a problem that unevenness in an signal-to-noise (S/N) ratio in a main scanning direction is caused.
To tackle the above problem, for example, in Japanese Laid-open Patent Publication No. 2002-170944, a solid-state imaging device is disclosed in which positions of the opening and a micro lens are shifted toward a center of each imaging area from a corresponding photoelectric converting region.
However with a configuration in which an opening position is varied for each pixel, the opening position is fixed according to a light incidence angle, and accordingly, there has been a problem that an optical system used for incidence of light is limited.
In view of the above problem, there is a need to provide a photoelectric conversion element, an image reading device, and an image forming apparatus that can reduce unevenness in an amount of light received at each pixel without limiting an optical system that is used for incidence of light.