CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/760,121, filed Feb. 6, 2013, now allowed, which is a continuation of U.S. application Ser. No. 13/241,346, filed Sep. 23, 2011, now U.S. Pat. No. 8,378,443, which is a continuation of U.S. application Ser. No. 12/754,702, filed Apr. 6, 2010, now U.S. Pat. No. 8,058,699, which is a continuation of U.S. application Ser. No. 11/278,841, filed Apr. 6, 2006, now U.S. Pat. No. 7,786,544, which is a divisional of U.S. application Ser. No. 09/924,108, filed Aug. 8, 2001, now U.S. Pat. No. 7,030,551, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2000-242932 on Aug. 10, 2000, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an area sensor (semiconductor device) having an image sensor function and a display function. In particular, the present invention relates to an area sensor (semiconductor device) that has EL (electroluminescence) elements as a light source and is composed of photoelectric conversion devices provided on a flat surface (insulating surface) and a plurality of thin film transistors (TFTs) arranged in a matrix.

2. Description of the Related Art

In recent years, a solid-state image sensing device is being used, which has diodes, CCDs, or the like for reading an electric signal having image information from a light signal having textural/graphic information, video information, and the like on a sheet of paper. Such a solid-state image sensing device is used for a scanner, a digital camera, and the like.

The solid-state image sensing device having photoelectric conversion devices are classified into a line sensor and an area sensor. In the line sensor, photoelectric conversion devices provided in a line shape are scanned with respect to a subject, whereby image information is captured as an electric signal.

The area sensor is also called a contact-type area sensor, in which photoelectric conversion devices provided on a flat surface are disposed on a subject, and image information is captured as an electric signal. Unlike the line sensor, it is not required to scan photoelectric conversion devices in the area sensor, so that a motor and the like for scanning are not necessary.

FIGS. 23A and 23B show a configuration of a conventional area sensor. FIG. 23A is a perspective view of the area sensor, and FIG. 23B is a cross-sectional view thereof. A sensor substrate 2501 with photoelectric conversion devices formed thereon, a backlight 2502, and a light scattering plate 2503 are provided as shown in FIG. 23B.

Light from the backlight 2502 (light source) is refracted in the light scattering plate 2503, and is radiated to a subject 2504. The radiated light is reflected from the subject 2504, and radiated to the photoelectric conversion devices provided on the sensor substrate 2501. When the photoelectric conversion devices are irradiated with light, a current with a magnitude in accordance with the brightness of light is generated in the photoelectric conversion devices, and image information of the subject 2504 is captured in the area sensor as an electric signal.

In the above-mentioned area sensor, when light is not radiated uniformly to the subject from the backlight 2502, a read image may partially become light or dark, resulting in inconsistencies of the image. This makes it necessary to design the light scattering plate 2503 so that light is radiated uniformly to the subject 2504, and to precisely adjust the position of the backlight 2502, the light scattering plate 2503, the sensor substrate 2501, and the subject 2504.

It is also difficult to minimize the size of the backlight 2502 and the light scattering plate 2503, which prevents the area sensor from becoming small, thin, and light-weight.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the present invention to provide an area sensor that is small, thin, and light-weight, and in which a read image has no inconsistencies in lightness.

An area sensor of the present invention uses a photodiode as a photoelectric conversion device. The area sensor also uses an electroluminescence (EL) element as a light source.

In the present specification, a photodiode (photoelectric conversion device) includes an N-type semiconductor layer, a P-type semiconductor layer, and a photoelectric conversion device provided so as to come into contact with a part of the N-type semiconductor layer and the P-type semiconductor layer.

When a photodiode is irradiated with light, the voltage thereof is decreased due to carriers generated by the light. At this time, as light intensity is higher, the amount of a decrease in voltage becomes larger. Furthermore, by comparing a voltage in the case where a photodiode is irradiated with light, with a voltage in the case where the photodiode is not irradiated with light, a signal is input to a sensor signal line.

An EL element (light-emitting element) is a spontaneous light-emitting element, and is mainly used for an EL display. An EL display is also called an organic EL display (OELD) or an organic light-emitting diode (OLED).

An EL element has a configuration in which an EL layer (organic compound layer) is interposed between a pair of electrodes (positive electrode and negative electrode), and the EL layer usually has a multi-layer configuration. Typically, there is a multi-layer configuration â€œhole transport layer/light-emitting layer/electron transport layerâ€ proposed by Tang of Eastman Kodak. This configuration has a very high light-emitting efficiency, and most of the EL display apparatuses that are being studied and developed adopt this configuration.

Alternatively, an EL layer may have a configuration in which a hole injection layer, a hole transport layer, a light-emitting layer, and an electron transport layer are stacked in this order on an electrode or a configuration in which a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer are stacked in this order on an electrode. A light-emitting layer may be doped with a fluorescent colorant or the like.

In the present specification, all the layers provided between a pair of electrodes are collectively referred to as an â€œEL layer (organic compound layer)â€ . Therefore, the above-mentioned hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, etc. are all included in the EL layer. A predetermined voltage is applied to an EL layer with the above-mentioned configuration through a pair of electrodes, whereby carriers are recombined in a light-emitting layer to emit light.

In the present specification, an EL element (light-emitting element) has a configuration in which an organic compound layer is interposed between a pair of electrodes (positive electrode and negative electrode). The organic compound layer can be made of a known light-emitting material. Furthermore, the organic compound layer can have a single-layer configuration and a multi-layer configuration. According to the present invention the organic compound layer may have either configuration. As luminescence in the organic compound layer, there are light emission (fluorescence) occurring when a singlet excited state is changed to a ground state and light emission (phosphorescence) occurring when a triplet excited state is changed to a ground state. According to the present invention, either light emission may be used.

Photodiodes and EL elements are provided on the same sensor substrate in a matrix. The photodiodes and the EL elements, are controlled for operation, respectively, using thin film transistors (TFTs) similarly provided on the substrate in a matrix.

Light emitted from the EL elements is reflected from a subject and radiated to the photodiodes. A current is generated by the light radiated to the photodiodes, and an electric signal (image signal) having image information of the subject is captured by an area sensor.

According to the present invention, due to the above-mentioned configuration, light is radiated uniformly to a subject, so that no inconsistencies in lightness are caused in a read image. Furthermore, it is not required to provide a backlight and a light scattering plate separately from a sensor substrate. Therefore, unlike a conventional example, an area sensor can be made small, thin, and light-weight without precisely adjusting the position of a backlight, a light scattering plate, a sensor substrate, and a subject. Furthermore, the mechanical strength of an area sensor is increased.

The area sensor of the present invention is also capable of displaying an image, using the EL elements. The EL elements in the present invention have a function as a light source for reading an image and a function as a light source for displaying an image. Therefore, even when an electronic display is not provided separately on the area sensor, an image can be displayed.

Examples of a film made of silicon include a single crystal silicon film, a polycrystalline silicon film (polysilicon film), an amorphous silicon film (amorphous silicon film), etc. In the photodiode according to the present invention, a photoelectric conversion layer is made of an amorphous silicon film (amorphous silicon film), an N-type semiconductor layer is made of an N-type polycrystalline silicon film (polysilicon film), and a P-type semiconductor layer is made of a polycrystalline silicon film (polysilicon film). The amorphous silicon film is thicker than the polycrystalline silicon film, and the ratio in thickness therebetween is preferably (1 to 10):1. In the photodiode used in the present invention, a photoelectric conversion layer can receive more light when the thickness of the amorphous silicon film is larger than that of the polycrystalline silicon film.

According to the present invention, a photoelectric conversion layer is made of an amorphous silicon film due to its high light absorptivity.

In a photodiode, a dark current (i.e., current flowing at a light intensity of 0) may flow even when light is not radiated to the photodiode. However, due to a high resistance of an amorphous silicon film, a current does not flow even under the condition of dark light, whereby a dark current can be decreased. More specifically, when a dark current is small, a range of lightness and darkness of light which a photodiode can receive is enlarged in the case of dark light.

As shown in FIG. 16, a metal film 280 can also be formed so as to cover a first interlayer insulating film 250 provided on a photoelectric conversion layer 248.

Light is radiated to a subject 270 from an EL element, and light reflected form the subject 270 is radiated to a photodiode 306. However, in this case, among light passing through the photodiode 306, there exists light that is not radiated to the photoelectric conversion layer 248. If the metal film 280 is present as shown in FIG. 16, such light is reflected from the metal film 280, whereby the photoelectric conversion layer 248 can receive it. Because of this, the photoelectric conversion layer 248 can receive more light.

Hereinafter, the constitution of the present invention will be described.

According to the present invention, there is provided an area sensor, characterized in that:

the sensor comprises a sensor portion provided with a plurality of pixels each including a photodiode, an EL element, and a plurality of thin film transistors;

the photodiode includes a photoelectric conversion layer that is in contact with a part of a P-type semiconductor layer and an N-type semiconductor layer, and is made of an amorphous semiconductor film; and

the photoelectric conversion layer is thicker than the P-type semiconductor layer and the N-type semiconductor layer.

According to the present invention, there is provided an area sensor, characterized in that:

the sensor comprises a sensor portion provided with a plurality of pixels each including a photodiode, an EL element, and a plurality of thin film transistors;

the pixel includes a photodiode, an EL element, a switching TFT, an EL driving TFT, a reset TFT, a buffer TFT, and a selective TFT;

the switching TFT and the EL driving TFT control light emission of the EL element;

light emitted from the EL element is reflected from a subject to be radiated to the photodiode;

the photodiode, the reset TFT, the buffer TFT, and the selective TFT generate an image signal from the light radiated to the photodiode;

the photoelectric conversion layer is thicker than the P-type semiconductor layer and the N-type semiconductor layer.

According to the present invention, there is provided an area sensor, characterized in that:

the area sensor comprises a sensor portion provided with a plurality of pixels each including a photodiode, an EL element, and a plurality of thin film transistors;

the pixel includes a photodiode, an EL element, a switching TFT, an EL driving TFT, a reset TFT, a buffer TFT, a selective TFT, a source signal line, a gate signal line, a power supply line kept at a constant potential, a reset gate signal line, a sensor gate signal line, a sensor output line connected to a constant current power source, and a sensor power source line kept at a constant potential;

a gate electrode of the switching TFT is connected to the gate signal line;

one of a source region and a drain region of the switching TFT is connected to the source signal line, and the other is connected to a gate electrode of the EL driving TFT;

a source region of the EL driving TFT is connected to the power supply line, and a drain region of the EL driving TFT is connected to the EL element;

a source region of the reset TFT is connected to the sensor power source line;

a drain region of the reset TFT is connected to a gate electrode of the buffer TFT and the photodiode;

a drain region of the buffer TFT is connected to the sensor power source line;

one of a source region and a drain region of the selective TFT is connected to the sensor output line, and the other is connected to a source region of the buffer TFT;

a gate electrode of the selective TFT is connected to the sensor gate signal line,

light emitted from the EL element is reflected from a subject to be radiated to the photodiode,

an image signal generated from the light radiated to the photodiode is input to the sensor output line,