The present invention relates to a display device with a photosensor having a photodetection element such as a photodiode, and in particular to a display device that includes a photosensor inside a pixel region.
Conventionally, there has been proposed a display device with a photosensor that includes a photodetection element such as a photodiode inside a pixel to measure the brightness of external light and pick up an image of an object that is located close to the display. Such a display device with a photosensor is envisioned to be used as a bidirectional communication display device or display device with a touch panel function.
In a conventional display device with a photosensor, when using a semiconductor process to form known constituent elements such as signal lines, scan lines, TFTs (Thin Film Transistors), and pixel electrodes on an active matrix substrate, a photodiode or the like is simultaneously formed on the active matrix substrate (see PTL 1 and NPL 1).
It is known that sensor output in a display device with a photosensor is largely dependent on environmental temperature. Specifically, there is the problem that when the environmental temperature changes, the characteristics of the photodetection element fluctuate, and a change in light intensity can no longer be properly detected.
Such temperature dependency of a photosensor is attributed to dark current (also called “leakage current”). To compensate for this dark current, a configuration is known in which besides a photosensor having a photodetection element (element for photodetection) that measures the intensity of incident light, a light-shielded photodetection element (reference element) for detecting only dark current is provided as a so-called dummy sensor on an active matrix substrate (see PTL 2 and 3 and NPL 2). In this conventional configuration, since output from the reference element reflects the dark current component, the output from the reference element counteracts with the output from the photodetection element in a downstream circuit of the photosensor, thus enabling obtaining sensor output having reduced temperature dependency.
PTL 1: JP 2006-3857A
PTL 2: JP 2007-18458A
PTL 3: JP 2007-81870A
NPL 1: “A Touch Panel Function Integrated LCD Including LTPS A/D Converter”, T. Nakamura et al., SID 05 DIGEST, pp. 1,054-1,055, 2005
NPL 2: “LTPS Ambient Light Sensor with Temperature Compensation”, S. Koide et al., IDW '06, pp. 689-690, 2006
However, in the case where a photodetection element and a reference element are disposed in a pixel region, the storage capacitor of the photodetection element is charged with and discharges both current generated due to incident light and dark current. Accordingly, there is the issue that when the fact that dark current increases at high temperatures is taken into consideration, a wide dynamic range cannot be obtained for the photosensor.
The present invention has been achieved in light of the above-described issues, and an object thereof is to provide a display device having a photosensor with a wide dynamic range and reduced temperature dependency even in the case where a photodetection element and a reference element are disposed in a pixel region.
In order to address the above-described issues, a display device according to the present invention is a display device including a photosensor in a pixel region of an active matrix substrate, the photosensor including; a photodetection element that receives incident light; a reference element connected in series to the photodetection element and having a light shielding layer that blocks incident light; a storage capacitor, one electrode of which is connected to a connection point between the photodetection element and the reference element, that is charged with and discharges output current from the photodetection element and the reference element; reset signal wiring that supplies a reset signal to the photosensor; readout signal wiring that supplies a readout signal to the photosensor; a switching element having a control electrode connected to a connection point between the photodetection element and the reference element; and a capacitor connected between the readout signal wiring and an electrode of the reference element that is not connected to the photodetection element, wherein from when the reset signal is supplied until when the readout signal is supplied, the photosensor charges the storage capacitor or outputs sensor output that is in accordance with the output current discharged from the storage capacitor.
The present invention enables providing a display device having a photosensor with a wide dynamic range and reduced temperature dependency.
A display device according to an embodiment of the present invention is a display device including a photosensor in a pixel region of an active matrix substrate, the photosensor including: a photodetection element that receives incident light; a reference element connected in series to the photodetection element and having a light shielding layer that blocks incident light; a storage capacitor, one electrode of which is connected to a connection point between the photodetection element and the reference element, that is charged with and discharges output current from the photodetection element and the reference element; reset signal wiring that supplies a reset signal to the photosensor; readout signal wiring that supplies a readout signal to the photosensor; a switching element having a control electrode connected to a connection point between the photodetection element and the reference element; and a capacitor connected between the readout signal wiring and an electrode of the reference element that is not connected to the photodetection element, wherein from when the reset signal is supplied until when the readout signal is supplied, the photosensor charges the storage capacitor or outputs sensor output that is in accordance with the output current discharged from the storage capacitor (first configuration).
In this first configuration, one of the electrodes of the storage capacitor that is charged with and discharges output current from the photodetection element and the reference element is connected to a connection point between the photodetection element and the reference element. Also, the control electrode of the switching element for charging the storage capacitor or reading out output current discharged from the storage capacitor from when the reset signal is supplied to when the readout signal is supplied (so-called integration period) is connected to this connection point. Accordingly, in the readout period, the storage capacitor is charged with and discharges the sum of the sum of the photocurrent and the dark current output from the photodetection element (IPHOTO+IDARK), and the dark current (IDARK, having the reversed sign of the dark current IDARK from the photodetection element) output from the reference element. As a result, the storage capacitor is charged with and discharges only the photocurrent IPHOTO component, thus enabling accurately measuring the intensity of external light regardless of the magnitude of the dark current IDARK. Also, a wide dynamic range can be obtained since the storage capacitor neither is charged with nor discharges the dark current IDARK. Accordingly, it is possible to realize a display device including a photosensor that can highly accurately measure the intensity of external light without being influenced by the environmental temperature.
A display device according to an embodiment of the present invention is a display device including a photosensor in a pixel region of an active matrix substrate, the photosensor including: a photodetection element that receives incident light; a reference element connected in series to the photodetection element and having a light shielding layer that blocks incident light; a storage capacitor, one electrode of which is connected to a connection point between the photodetection element and the reference element, that is charged with and discharges output current from the photodetection element and the reference element; reset signal wiring that supplies a reset signal to the photosensor; readout signal wiring that supplies a readout signal to the photosensor; and a switching element having a control electrode connected to a connection point between the photodetection element and the reference element, wherein an electrode of the reference element that is not connected to the photodetection element is connected to the readout signal wiring, and from when the reset signal is supplied until when the readout signal is supplied, the photosensor charges the storage capacitor or outputs sensor output that is in accordance with the output current discharged from the storage capacitor (second configuration).
In this second configuration as well, one of the electrodes of the storage capacitor that is charged with and discharges output current from the photodetection element and the reference element is connected to a connection point between the photodetection element and the reference element. Also, the control electrode of the switching element for charging the storage capacitor or reading out output current discharged from the storage capacitor from when the reset signal is supplied until when the readout signal is supplied (so-called integration period) is connected to this connection point. Accordingly, in the readout period, the storage capacitor is charged with and discharges the sum of the sum of the photocurrent and the dark current output from the photodetection element (IPHOTO+IDARK), and the dark current (IDARK, having the reversed sign of the dark current IDARK from the photodetection element) output from the reference element. As a result, the storage capacitor is charged with and discharges only the photocurrent IPHOTO component, thus enabling accurately measuring the intensity of external light regardless of the magnitude of the dark current IDARK. Also, a wide dynamic range can be obtained since the storage capacitor neither is charged with nor discharges the dark current IDARK. Accordingly, it is possible to realize a display device including a photosensor that can highly accurately measure the intensity of external light without being influenced by the environmental temperature.
The display device according to the first and second configurations can have a configuration in which the switching element is composed of one transistor, and the readout signal wiring is connected to another electrode of the storage capacitor. Alternatively, a configuration is possible in which the switching element is composed of a first transistor and a second transistor, a control electrode of the first transistor is connected to a connection point between the photodetection element and the reference element, one of two electrodes other than the control electrode of the first transistor is connected to wiring that supplies a power supply voltage, the other of the two electrodes other than the control electrode of the first transistor is connected to one of two electrodes other than a control electrode of the second transistor, the readout signal wiring is connected to the control electrode of the second transistor, one electrode of the storage capacitor is connected to the wiring that supplies the power supply voltage, and the other of the two electrodes other than the control electrode of the second transistor is connected to wiring for reading out the output current.
Alternatively, the display device according to the first and second configurations may have a configuration in which the switching element is composed of a first transistor, a second transistor, and a third transistor, a control electrode of the first transistor is connected to a connection point between the photodetection element and the reference element, one of two electrodes other than the control electrode of the first transistor is connected to wiring that supplies a power supply voltage, the other of the two electrodes other than the control electrode of the first transistor is connected to one of two electrodes other than a control electrode of the second transistor, the storage capacitor is connected in parallel to the photodetection element, the readout signal wiring is connected to the control electrode of the second transistor, the other of the two electrodes other than the control electrode of the second transistor is connected to wiring for reading out the output current, a control electrode of the third transistor is connected to the reset signal wiring, one of two electrodes other than the control electrode of the third transistor is connected to a connection point between the photodetection element and the reference element, and the other of the two electrodes other than the control electrode of the third transistor is connected to the wiring that supplies the power supply voltage.
Note that in the display device according to the first and second configurations, it is preferable that in a case where no light is incident on the photosensor, an output current from the photodetection element and an output current from the reference element are equivalent. This is because if the dark current of the photodetection element and the dark current of the reference element are equivalent, temperature dependency can be substantially reliably eliminated when the environmental temperature has changed.
Also, in the display device according to the first and second configurations, it is preferable that the photodetection element and the reference element are each a photodiode, and the length and width of a gap between a p layer and an n layer are substantially equivalent between the photodetection element and the reference element. Note that “substantially equivalent” as referred to here includes the case where even if the lengths and widths are the same in terms of design, the lengths and widths do not strictly take the design values due to variability in processes such as etching and exposure. According to this configuration, although there is the possibility of slight differences due to self-parasitic storage capacitance, the characteristics of the photodetection element and the reference element are substantially equivalent. As a result, the dark current of the photodetection element and the dark current of the reference element are equivalent, and therefore temperature dependency can be substantially reliably eliminated when the environmental temperature has changed.
Furthermore, the display device according to the first and second configurations can be, but is not limited to being, suitably implemented as a liquid crystal display device further including a common substrate opposing the active matrix substrate, and liquid crystal sandwiched between the active matrix substrate and the common substrate.
Below is a description of more specific embodiments of the present invention with reference to the drawings. Note that although the following embodiments show examples of configurations in which a display device according to the present invention is implemented as a liquid crystal display device, the display device according to the present invention is not limited to a liquid crystal display device, and is applicable to any display device that uses an active matrix substrate. It should also be noted that due to having a photosensor, the display device according to the present invention is envisioned to be used as, for example, a display device with a touch panel that performs input operations after detecting an object that is close to the screen, or a bidirectional communication display device that is equipped with a display function and an image capture function.
Also, for the sake of convenience in the description, the drawings that are referred to below show simplifications of, among the constituent members of the embodiments of the present invention, only relevant members that are necessary for describing the present invention. Accordingly, the display device according to the present invention may include any constituent members that are not shown in the drawings referred to in this specification. Also, regarding the dimensions of the members in the drawings, the dimensions of the actual constituent members, the ratios of the dimensions of the members, and the like are not shown faithfully.
First, a configuration of an active matrix substrate included in a liquid crystal display device according to Embodiment 1 of the present invention will be described with reference to
Note that the above constituent members on the active matrix substrate 100 can also be formed monolithically on the glass substrate by a semiconductor process. Alternatively, a configuration is possible in which the amplifier and various drivers among the above constituent members are mounted on the glass substrate by COG (Chip On Glass) technology or the like. As another alternative, it is possible for at least some of the above constituent members shown on the active matrix substrate 100 in
The pixel region 1 is a region in which a plurality of pixels are formed in order to display an image. In the present embodiment, a photosensor for picking up an image is provided in each pixel in the pixel region 1.
For this reason, as shown in
Thin-film transistors (TFTs) M1 are provided as switching elements for the pixels at intersections between the gate lines GL and the source lines COL. Note that in
In
Note that in the example in
As shown in
In the example in
The anode of the photodiode D1 is connected to reset signal wiring RST, which is for supplying a reset signal. The photodiode D1 and the photodiode D2 are connected in series, and the gate of the thin-film transistor M2 and one of the electrodes of the capacitor CINT are connected between the cathode of the photodiode D1 and the anode of the photodiode D2. The cathode of the photodiode D2 is connected to one of the electrodes of a capacitor Cref. The other electrode of the capacitor Cref is connected to the readout signal wiring RWS. The capacitor Cref can be formed by the silicon film forming the cathode of the photodiode D2, the metal film forming the readout signal wiring RWS, and an insulating film therebetween.
The drain of the thin-film transistor M2 is connected to the wiring VDD, and the source is connected to the wiring OUT. The reset signal wiring RST and the readout signal wiring RWS are connected to the sensor row driver 5. Since the reset signal wiring RST and the readout signal wiring RSW are provided in each row, the notations reset signal wiring RSTi and readout signal wiring RWSi (i=1 to M) are used hereinafter when there is a need to distinguish between the pairs of wiring.
The sensor row driver 5 successively selects a reset signal wiring RSTi/readout signal wiring RWSi pair shown in
Note that as shown in
The following describes operations of the photosensor according to the present embodiment with reference to
In the example shown in
First, when the reset signal supplied from the sensor row driver 5 to the reset signal wiring RST rises from the low level (−4 V) to the high level (0 V), the photodiode D1 becomes forward biased. Since the potential VINT of the gate electrode of the thin-film transistor M2 is lower than the threshold voltage of the thin-film transistor M2 at this time, the thin-film transistor M2 is in a non-conducting state. Note that the capacitor Cref is connected between the cathode of the photodiode D2 and the readout signal wiring RWS. Accordingly, the photodiode D2 is also reset by the reset signal.
Next, the reset signal returns to the low level VRST.L, and thus the photocurrent integration period (period TINT shown in
When the integration period ends, the readout signal rises as shown in
Note that in
As described above, periodically performing initialization with a reset pulse, integration of the photocurrent in the integration period, and reading out of sensor output in the readout period enables obtaining photosensor output for each pixel.
As described above, in the photosensor provided in each pixel of the display device according to the present embodiment, the capacitor CINT is charged with and discharges only the photocurrent IPHOTO component of the photodiode D1, thus enabling accurately measuring the intensity of external light regardless of the magnitude of the dark current IDARK. Also, a wide dynamic range can be obtained since the dark current IDARK is not discharged from the capacitor CINT. Accordingly, it is possible to realize a photosensor that can highly accurately measure the intensity of external light without being influenced by the environmental temperature.
Note that although a photosensor including the capacitor Cref is disclosed in the present embodiment, a configuration is possible in which, as shown in
However, the configuration shown in
Note that in the present embodiment, as previously described, the source lines COLr and COLg are also used as the photosensor wiring VDD and the wiring OUT, and therefore as shown in
As shown in
The following describes operations of the sensor column driver 4 and the buffer amplifier 6 that are performed after the sensor output VSOUT has been read out from the pixel region 1, with reference to
Next is a description of operations of the sensor column amplifier 42 with reference to
Note that although the sensor column scan circuit 43 may scan the photosensor columns one column at a time as described above, there is no limitation to this, and a configuration is possible in which the photosensor columns are interlace-scanned. Also, the sensor column scan circuit 43 may be formed as a multi-phase drive scan circuit that has, for example, four phases.
According to the above configuration, the display device according to the present embodiment obtains panel output VOUT that is in accordance with the amount of light received by the photodiode D1 formed in each pixel in the pixel region 1. The panel output VOUT is sent to the signal processing circuit 8, subjected to A/D conversion, and then stored in a memory (not shown) as panel output data. Specifically, the same number of panel output data pieces as the number of pixels (number of photosensors) in the pixel region 1 are stored in this memory. With use of the panel output data stored in the memory, the signal processing circuit 8 performs various types of signal processing such as image pickup and the detection of a touch area. Note that although the same number of panel output data pieces as the number of pixels (number of photosensors) in the pixel region 1 are stored in the memory of the signal processing circuit 8 in the present embodiment, due to constraints such as memory capacity, there is no need to necessarily store the same number of panel output data pieces as the number of pixels.
Below is a description of a display device according to Embodiment 2 of the present invention. Note that the same reference numerals are used for constituent elements that have functions similar to those of the constituent elements described in Embodiment 1, and detailed descriptions thereof will be omitted.
In the photosensor of the present embodiment, one of the electrodes of the capacitor CINT is connected to the gate electrode of the thin-film transistor M2 between the cathode of the photodiode D1 and the anode of the photodiode D2, and the other electrode of the capacitor CINT is connected to the wiring VDD. Also, the drain of the thin-film transistor M2 is connected to the wiring VDD, and the source is connected to the drain of the thin-film transistor M4. The gate of the thin-film transistor M4 is connected to the readout signal wiring RWS. The source of the thin-film transistor M4 is connected to the wiring OUT. Note that although a configuration in which one of the electrodes of the capacitor CINT and the drain of the thin-film transistor M4 are both connected in common to constant voltage wiring (wiring VDD) is shown in this example, a configuration is possible in which they are connected to mutually different constant voltage wiring.
The following describes operations of the photosensor according to the present embodiment with reference to
The high level VRST.H of the reset signal is set to a potential at which the thin-film transistor M2 enters the on state. In the example shown in
First, when the reset signal supplied from the sensor row driver 5 to the reset signal wiring RST rises from the low level (0 V) to the high level (8 V), the photodiode D1 becomes forward biased. At this time, although the thin-film transistor M2 enters the on state, there is no output to the wiring OUT since the readout signal is at the low level and the thin-film transistor M4 is in the off state.
Next, the reset signal returns to the low level VRST.L, and thus the photocurrent integration period (period TINT shown in
When the integration period ends, the readout signal rises as shown in
Note that in
As described above, according to the photosensor of the present embodiment as well, periodically performing initialization with a reset pulse, integration of the photocurrent in the integration period, and reading out of sensor output in the readout period enables obtaining photosensor output for each pixel.
In other words, in the photosensor provided in each pixel of the display device according to the present embodiment as well, the capacitor CINT discharges only the photocurrent IPHOTO component of the photodiode D1 similarly to Embodiment 1, thus enabling accurately measuring the intensity of external light regardless of the magnitude of the dark current IDARK. Also, a wide dynamic range can be obtained since the dark current IDARK is not discharged from the capacitor CINT. Accordingly, it is possible to realize a photosensor that can highly accurately measure the intensity of external light without being influenced by the environmental temperature.
In the present embodiment as well, a configuration is possible in which, as shown in
Below is a description of a display device according to Embodiment 3 of the present invention. Note that the same reference numerals are used for constituent elements that have functions similar to those of the constituent elements described in Embodiments 1 and 2 above, and detailed descriptions thereof will be omitted.
In the photosensor of the present embodiment, one of the electrodes of the capacitor CINT is connected between the cathode of the photodiode D1 and the anode of the photodiode D2, and the other electrode of the capacitor CINT is connected to GND. Also, the gate of the thin-film transistor M2 is connected between the cathode of the photodiode D1 and the anode of the photodiode D2. The drain of the thin-film transistor M2 is connected to the wiring VDD, and the source thereof is connected to the drain of the thin-film transistor M4. The gate of the thin-film transistor M4 is connected to the readout signal wiring RWS. The source of the thin-film transistor M4 is connected to the wiring OUT The gate of the thin-film transistor M5 is connected to the reset signal wiring RST, the drain thereof is connected to the wiring VDD, and the source thereof is connected between the cathode of the photodiode D1 and the anode of the photodiode D2. Note that although a configuration in which the drains of the thin-film transistors M4 and M5 are both connected in common to constant voltage wiring (wiring VDD) is shown in this example, a configuration is possible in which each of them is connected to different constant voltage wiring.
The following describes operations of the photosensor according to the present embodiment. Note that the waveforms of the reset signal supplied from the reset signal wiring RST to the photosensor of the present embodiment and the readout signal supplied from the readout signal wiring RWS are the same as in
The high level VRST.H of the reset signal is set to a potential at which the thin-film transistor M5 enters the on state. In the example shown in
First, when the reset signal supplied from the sensor row driver 5 to the reset signal wiring RST rises from the low level (VRST.L=0 V) to the high level (VRST.H=8 V), the thin-film transistor M5 enters the on state. Accordingly, the potential VINT at the connection point between the cathode of the photodiode D1 and the anode of the photodiode D2 is reset to VDD.
Next, the reset signal returns to the low level VRST.L, and thus the photocurrent integration period (period TINT shown in
When the integration period ends, the readout signal rises as shown in
As described above, according to the photosensor of the present embodiment as well, periodically performing initialization with a reset pulse, integration of the photocurrent in the integration period, and reading out of sensor output in the readout period enables obtaining photosensor output for each pixel.
In other words, in the photosensor provided in each pixel of the display device according to the present embodiment as well, the capacitor CINT discharges only the photocurrent IPHOTO component of the photodiode D1 similarly to Embodiments 1 and 2, thus enabling accurately measuring the intensity of external light regardless of the magnitude of the dark current IDARK. Also, a wide dynamic range can be obtained since the dark current IDARK is not discharged from the capacitor CINT. Accordingly, it is possible to realize a photosensor that can highly accurately measure the intensity of external light without being influenced by the environmental temperature.
In the present embodiment as well, a configuration is possible in which, as shown in
Although the present invention has been described based on Embodiments 1 to 3, the present invention is not limited to the above-described embodiments only, and it is possible to make various changes within the scope of the invention.
For example, Embodiments 1 to 3 describe an example of a configuration in which the wiring VDD and the wiring OUT that the photosensor is connected to are also used as the source wiring COL. This configuration has the advantage that the pixel aperture ratio is high. However, even with a configuration in which the photosensor wiring VDD and the wiring OUT are provided separately from the source wiring COL, it is possible to achieve effects similar to those in the above-described Embodiments 1 and 2. In particular, in Embodiment 2, with a configuration in which the photosensor wiring VDD and the source wiring COL are provided separately, and the thin-film transistor M2 and the capacitor CINT are connected to this wiring VDD, there is the advantage of preventing the potential of the capacitor CINT from becoming unstable due to the influence of a video signal input to the source wiring COL.
Note that as an alternative to the above description, a configuration is possible in which transistors M3, M6, and M7 provided in an IC chip, for example, are used instead of the thin-film transistors M3, M6, and M7 formed on the active matrix substrate.
The present invention is industrially applicable as a display device having a photosensor in a pixel region of an active matrix substrate.
1 pixel region
2 display gate driver
3 display source driver
4 sensor column driver
41 sensor pixel readout circuit
42 sensor column amplifier
43 sensor column scan circuit
5 sensor row driver
6 buffer amplifier
7 FPC connector
8 signal processing circuit
9 FPC
100 active matrix substrate
Number | Date | Country | Kind |
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2009-048399 | Mar 2009 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2009/068118 | 10/21/2009 | WO | 00 | 9/1/2011 |