JP2009204466A - Light quantity detecting circuit and electro-optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light quantity detecting circuit wherein deterioration of characteristics of TFT due to a deviation of polarity is reduced, and which provides characteristics of an output having a linear characteristic in relation to the quantity of light, and improves a light detection accuracy. <P>SOLUTION: The light quantity detecting circuit includes a light detecting part which detects the quantity of natural light by utilizing a current running at the time when reverse bias voltage is impressed on a gate electrode of the TFT. By a gate bias control means, in this light detecting part, the voltage Gv impressed on the gate electrode of the TFT is changed from first reverse bias voltage Gv0 to be impressed on the occasion of detecting the natural light to positive bias voltage Gvon and is changed thereafter to second reverse bias voltage Gvoff which is lower further than the first reverse bias voltage Gv0, and then the voltage Gv is restored to the first reverse bias voltage Gv0. An output brought about by the current running after this restoration is output as the quantity of the natural light. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a light amount detection circuit that is less deteriorated by light irradiation and that has an output characteristic having a linear characteristic with respect to a light amount, and that has improved light detection accuracy, and an electro-optical device including the same.

As a conventional light amount detection circuit, for example, as disclosed in Patent Document 1 below, the fact that the leakage current of a thin film transistor (hereinafter referred to as “TFT”) is proportional to the amount of received light is used to detect the voltage using this leakage current. 2. Description of the Related Art There is known a device that detects the amount of light by charging or discharging an electric capacitor and monitoring a voltage change between both ends of the capacitor. by the way,
The leakage current of a TFT is proportional to the amount of light received, but it has been found that its sensitivity (current value with respect to the amount of light received) decreases with exposure to light. Therefore, in the light amount detection circuit disclosed in Patent Document 1 below, the light amount detection accuracy decreases due to this sensitivity decrease.

In order to prevent this, the following Patent Document 2 discloses a photoelectric conversion element that improves the anti-deterioration characteristics by improving the method of generating a TFT photosensor. However, the photoelectric conversion element disclosed in Patent Document 2 below requires special manufacturing conditions. For example TFT
It is necessary to provide an optical sensor inside the display device using the above. In addition, when manufacturing a display device and an optical sensor in the same device, a manufacturing process different from the characteristics suitable for display is required.
The manufacturing process becomes long, or the condition setting of the manufacturing apparatus is frequently changed.

Further, if a constant reverse bias voltage is continuously applied to the gate electrode of the TFT photosensor, the characteristics of the TFT photosensor are deteriorated due to the polarity bias. On the other hand, the following patent document 3
In the invention disclosed in the above, it is shown that the characteristics of the TFT photosensor are prevented from being deteriorated by applying a reset signal corresponding to the positive bias voltage to the gate electrode of the TFT photosensor.
JP 2006-29832 A Japanese Patent Laid-Open No. 9-232620 JP 2001-169190 A

In the invention disclosed in Patent Document 3 described above, it is possible to suppress the deterioration of the characteristics of the TFT photosensor due to the bias in the polarity of the voltage applied to the gate electrode of the TFT photosensor. However, when the charge charged in the capacitor is discharged after applying a reset signal corresponding to the positive bias voltage to the gate electrode of the TFT photosensor, the obtained discharge characteristics are far from the normal characteristics, so the amount of light However, it has been found that there is a problem that an output result having linearity cannot be obtained.

As a result of various investigations on this cause, when a positive bias voltage is applied to the gate electrode at the time of resetting, it takes time in seconds until the charge trapped in the channel region is completely turned off after the gate is turned off. Because of that.

Accordingly, the present invention provides a light amount detection circuit and an electro-optical device that are less deteriorated by light irradiation and have an output characteristic having a linear characteristic with respect to the light amount and improved in light detection accuracy. Objective.

In order to solve the above problems, a light amount detection circuit according to the present invention includes a light detection unit that detects a light amount of external light using a current that flows when a reverse bias voltage is applied to a gate electrode of a TFT. In the detection circuit, the light detection unit is configured to output the T by gate bias control means.
After the voltage applied to the gate electrode of the FT is changed from the first reverse bias voltage applied when detecting external light to the positive bias voltage, the second reverse bias is lower than the first reverse bias voltage. The output by the current that flows after changing to the bias voltage and then returning to the first reverse bias voltage is output as the amount of external light.

The conventional light quantity detection circuit utilizes the fact that the leakage current of the TFT is proportional to the amount of light received, and this leakage current charges or discharges the voltage detection capacitor, and monitors the voltage change across the capacitor at this time. It is the light quantity detection circuit of the light detection part which detects light quantity by doing.
In this light amount detection circuit, when a light amount of external light is detected, a predetermined reverse bias voltage is applied to the gate electrode, but if this is done, TFT characteristics are deteriorated due to a bias in polarity. Therefore, a reset operation is performed by periodically applying a predetermined positive bias voltage to the gate electrode by the gate bias control means. At this time, a large amount of charge is trapped in the channel region of the TFT. The large amount of trapped charges is mainly caused by the parasitic capacitance between the gate electrode and the drain electrode, and even if the first reverse bias voltage for measuring the amount of light is simply applied after the reset operation, it is completely eliminated. It takes a few seconds. For this reason, when the light quantity is measured at short time intervals, linear characteristics cannot be obtained with respect to the light quantity due to the charge remaining in the channel region.

Therefore, in the light quantity detection circuit of the present invention, the gate bias control means applies a predetermined positive bias voltage to the gate electrode to perform a reset operation, and then the second lower bias voltage than the first reverse bias voltage. The reverse bias voltage is applied, and then the first reverse bias voltage for measurement is applied. By applying the second reverse bias voltage, the charges trapped in the channel region are discharged in a short time, so that the TFT as the photosensor returns to the normal characteristics in the past in a short time.

Therefore, according to the light amount detection circuit of the present invention, it is possible to suppress the deterioration of the TFT characteristics due to the bias of the polarity of the voltage applied to the gate electrode of the TFT as the optical sensor,
An output having linearity with respect to the amount of light can be obtained. In addition, since the light amount detection circuit of the present invention does not involve a change in the manufacturing conditions of the light detection unit itself, for example, even when the display device and the optical sensor are manufactured in the same manufacturing device, There is no longer a manufacturing process or a forced change of conditions in the manufacturing apparatus.

In the light amount detection circuit of the present invention, the light detection unit is connected between a capacitor connected between a source electrode and a drain electrode of the TFT and between the source electrode and a predetermined constant voltage source. A voltage applied to the gate electrode of the TFT.
After returning to the reverse bias voltage, it is preferable to output the detection time from when the switch element is turned off until the voltage across the capacitor reaches a predetermined threshold voltage.

The light detection unit that detects the amount of light by monitoring the voltage change across the capacitor when the charge of the voltage detection capacitor is discharged by the leakage current of the TFT as described above has good measurement accuracy and facilitates digital processing. It becomes.

In the light amount detection circuit of the present invention, the light detection unit further includes integrated light amount measurement means that is a product of the light amount irradiated to the TFT and the irradiation time, and the gate bias control means includes the gate bias control means. It is preferable that the time for applying the second reverse bias voltage is adjusted by the output of the integrated light quantity measuring means.

In the light detection unit that outputs the amount of external light using the current that flows when a reverse bias voltage is applied to the gate electrode of the TFT, the characteristics of the TFT as a light sensor are temporarily degraded by applying a positive bias voltage. However, the optimum value of the second reverse bias voltage application time changes due to the influence of deterioration over time caused by light irradiation to the TFT. Therefore, in the light quantity detection circuit of the present invention, the deterioration of the TFT as the optical sensor is predicted by the output of the integrated light quantity measuring means, and the time for applying the second reverse bias voltage applied to the gate electrode of the TFT is an optimum value. It is adjusted by the gate bias control means so that According to the light quantity detection circuit of this aspect, output characteristics having linear characteristics with respect to the light quantity can be obtained even when the TFT as the optical sensor deteriorates, and the light detection accuracy can be improved.

In the light amount detection circuit of the present invention, the light detection unit includes a reference TFT having a dimming unit formed on a surface thereof, and the integrated light amount measurement unit includes the TFT and the reference TFT obtained in advance.
The integrated light amount is obtained based on the correlation between the respective output ratios and the integrated light amount, and based on the correlation between the predetermined integrated light amount and the optimum application time of the second reverse bias voltage. It is preferable to adjust the application time of the second reverse bias voltage.

The reference TFT having a dimming means on the surface has the same characteristics as a TFT as an optical sensor having no dimming means on the surface. Therefore, the integrated light quantity can be obtained based on the correlation between the output ratio of the TFT as the photosensor and the reference TFT obtained in advance and the integrated light quantity, and the predetermined integrated light quantity and the optimal The optimum second reverse bias voltage application time can be obtained based on the correlation with the reverse bias voltage application time of 2. Therefore, according to the light quantity detection circuit of this aspect, even if the TFT as the optical sensor is deteriorated, an output characteristic having a more linear characteristic with respect to the light quantity can be obtained, and the light detection accuracy can be further improved. Become.

Furthermore, the electro-optical device according to the present invention corresponds to any one of the light quantity detection circuits, the plurality of scanning lines, the plurality of data lines, and the intersection of the plurality of scanning lines and the plurality of data lines. A display panel having a plurality of pixels provided; an illumination unit that illuminates the display panel; and a dimming circuit that controls the illumination unit based on an output value of the light amount detection circuit. And

According to the electro-optical device of the present invention, an electro-optical device capable of correcting the sensitivity reduction of the light amount detection circuit and detecting the illuminance of external light with high accuracy and appropriately controlling the illumination unit. Can do. In the electro-optical device of the present invention, since the manufacturing conditions of the light detection unit itself are not changed, the manufacturing process is performed even when the display panel and the optical sensor are manufactured by the same manufacturing device. Will not be long, and will not be forced to change the frequent setting of the manufacturing equipment.

The best mode for carrying out the present invention will be described below with reference to the drawings and examples. However,
The embodiments described below exemplify a light amount detection circuit for embodying the technical idea of the present invention, and are not intended to specify the present invention as this light amount detection circuit. It is equally applicable to those of other embodiments within the scope. In each drawing used for the description in this specification, each layer and each member are displayed in different scales so that each layer and each member can be recognized on the drawing. However, it is not necessarily displayed in proportion to the actual dimensions.

FIG. 1 is a block diagram showing a configuration of a light amount detection circuit of the embodiment. FIG. 2 is a circuit diagram of the light detection unit and the reading unit. FIG. 3 is a cross-sectional view of an optical sensor, a capacitor, and a switch element that constitute the optical detection unit. FIG. 4 is a diagram illustrating a timing chart of the first comparative example. FIG. 5 is a diagram illustrating a timing chart of the comparative example 2. FIG. 6 is a diagram showing a change in voltage across the capacitor Cw of Comparative Examples 1 and 2. FIG. 7 is a diagram showing calibration curves of Comparative Examples 1 and 2. FIG. 8 is a diagram showing a timing chart of the embodiment. FIG. 9 is a diagram illustrating a change in voltage across the capacitor Cw of the example. FIG. 10 is a block diagram of a modification of the embodiment. FIG. 11 is a diagram illustrating a configuration of an electro-optical device to which the light amount detection circuit of the embodiment is applied.

As shown in FIG. 1, the light amount detection circuit 10 of the embodiment includes a light detection unit LS ₁ , a reading unit S _H ,
Gate bias control means Vc and timing controller Tc are provided. Reading unit S _H reads the output of the light detecting portion LS ₁ at a predetermined timing, by appropriately signal processing to output a signal P corresponding to the amount of external light. The gate bias control means Vc supplies predetermined gate voltage Gv respect TFT as an optical sensor of the light sensing section LS _1. Further, the timing controller Tc outputs a control signal for causing the light detection unit LS ₁ , the reading unit _SH , and the gate bias control unit Vc to operate at predetermined timings.

Light detecting section LS _1, as shown in FIG. 2, the capacitor Cw between the drain electrode _{D L} and the source electrode _{S L} of the TFT are connected in parallel, one terminal C of the source electrode _{S L} and the capacitor Cw
b and is connected to a reference voltage source for applying a reference voltage Vs through the switching elements SW _1, further, a drain electrode D _L and the other terminal Ca of the capacitor Cw of the TFT another source (e.g., common voltage V _COM ). This TFT operates as an optical sensor.

With this configuration, when the switch element SW ₁ in each predetermined period is turned on, a predetermined reference voltage Vs from the reference voltage source (e.g., + 2V) is applied to the capacitor Cw, reference to both ends of the capacitor Cw It is charged by the potential difference between the voltage Vs and the second voltage _{V COM.} Incidentally, there is a gate-off state to the opposite polarity of the gate voltage from the gate bias control means Vc Gv (e.g., -5V) is applied to the gate electrode G _L of the TFT. In this state, the switch element SW ₁ is when it is turned off, the voltage across the capacitor Cw is reduced by the leakage current caused by light irradiation to the TFT. Therefore, the detection time t ₀ from when the charging completion of the capacitor Cw until the voltage across the capacitor Cw becomes the threshold voltage Vth of predetermined, for example, read by the reading unit S _H consisting of sampling hold circuit, read it is possible to detect the illuminance of external light by the corresponding signal P to the amount of external light based on the detection time t ₀ has.

The light detection unit LS ₁ includes a TFT, a capacitor Cw, and a switch element SW constituting the light sensor.
₁ are provided on a TFT substrate 11 as shown in FIG. That is, on the surface of the TFT substrate 11, the gate electrode G _L of the TFT as an optical sensor,
The gate electrode Gs of the TFT constituting one terminal Ca and the switch elements SW _l capacitor Cw is formed, the gate insulating film 12 made of silicon oxide or silicon nitride so as to cover these surfaces are stacked.

Further, the surface of the gate electrode Gs of the TFT constituting the surface and the switch elements SW _l of the gate electrode G _L of the TFT as an optical sensor, and the like amorphous silicon and polycrystalline silicon through a respective gate insulation film 12 semiconductor layer 13 _L and 13s are formed comprising. Also,
On the gate insulating film 12, a source electrode S _L and a drain electrode D _L of a TFT as an optical sensor made of a metal such as aluminum or molybdenum, and a source electrode Ss and a drain electrode Ds of a TFT constituting the switch element SW ₁ are respectively provided. It is provided in contact with the semiconductor layer 13 _L and 13s.

Among these, the source electrode S _L of the TFT as the photosensor and the drain electrode Ds of the TFT constituting the switch element SW ₁ are connected to each other so as to form the other terminal Cb of the capacitor Cw. Furthermore, TFT as an optical sensor, so as to cover the surface of the TFT as a capacitor Cw and the switch elements SW _l, for example, protective insulating film 14 made of inorganic insulating material are stacked, also, the switch element composed of TFT The surface of SW ₁ is covered with a light shielding layer 15 so as not to be affected by external light.

Here will be described with reference to FIG. 4 is a timing chart of a light detecting portion LS ₁ of Comparative Example 1 not reset operation on TFT as an optical sensor. In the optical detection section LS ₁ of Comparative Example 1, a constant gate voltage Gv0 of -5V is constantly applied, for example, a reverse bias voltage from the gate bias control means Vc to the gate electrode G _L. Then, the switch element SW ₁ in each predetermined period by the timing controller Tc is turned on, the reference voltage Vs from the reference voltage source for example + 2V is applied to one terminal Cb of the capacitor Cw. Here, since the common potential V _COM is supplied to the other terminal Ca of the capacitor Cw, the capacitor Cw is charged with a voltage corresponding to Vs−V _COM . The time required for charging the capacitor Cw is determined by the capacitance of the capacitor Cw and the output impedance of the reference voltage source.
Generally, it takes only a short time because the output impedance of the reference voltage source is small.

After charging of the capacitor Cw is completed, the switch element SW ₁ by the timing controller Tc is turned off, the voltage across the capacitor Cw by the leakage current caused by the TFT as an optical sensor is irradiated with light is descend. At this time, the voltage at both ends of the capacitor Cw decreases, for example, as shown in “voltage at both ends of Cw” in FIG. Therefore, in the optical detection unit LS ₁ of Comparative Example 1, the detection time t ₀ until the switch elements SW ₁ by the timing controller Tc is discharged to the threshold voltage Vth predetermined since in the off state, for example, a sampling hold circuit read by the reading unit S _H consisting measures the light amount of outside light.

Since the output of the TFT as the optical sensor of Comparative Example 1 has good linearity with the amount of external light, the amount of external light can be measured with high accuracy. However, the sensitivity of the TFT as the optical sensor of Comparative Example 1 has a problem that the characteristics of the TFT deteriorate due to light exposure and polarity bias.

The light detection unit L of Comparative Example 2 that performs a reset operation on the TFT as an optical sensor for the purpose of preventing a decrease in sensitivity due to the deterioration of the TFT characteristics due to such a bias in polarity.
The timing chart of S ₁ will be described with reference to FIG. In Comparative Example 2, the gate voltage G is applied from the gate bias control means Vc to the gate electrode G _L of the TFT as an optical sensor
In general, v is a constant reverse bias voltage Gv0 of, for example, −5V as in the case of Comparative Example 1, but a positive bias voltage Gvon of, for example, + 15V is applied for a predetermined time, for example, 1 ms immediately before measurement, and then −5V. To the constant reverse bias voltage Gv0. Then, the timing controller Tc, the predetermined time after the application of a positive bias voltage Gvon is completed, the switch element SW ₁ is turned on for example, after 100ms elapses, the reference voltage for example from source +2
A reference voltage Vs of V is applied to the capacitor Cw, and the capacitor Cw is charged with a voltage corresponding to Vs−V _COM .

After charging of the capacitor Cw is completed, the switch element SW ₁ is turned off, the capacitor C by the leakage current caused by light irradiation to the TFT as an optical sensor
The voltage across w decreases. The voltage across the capacitor Cw at this time is, for example, “
It decreases as shown in “Voltage across Cw”. Therefore, the switch element SW ₁ by the timing controller Tc is the detection time t ₀ until it is discharged to the threshold voltage Vth predetermined since in the off state, the reading by the reading unit S _H, to measure the amount of external light. According to the configuration of the light detecting portion LS ₁ of Comparative Example 2, the effect of reduced TFT of sensitivity as the light sensor is suppressed it is obtained.

However, in Comparative Example 2, since the discharge characteristics obtained when the voltage charged in the capacitor Cw is discharged are far from those in Comparative Example 1, an output result having linearity with respect to the amount of light can be obtained. There is a problem of not. For each of Comparative Examples 1 and 2, Figure 6 the variation of the voltage across the capacitor Cw from the switch element SW ₁ is turned off, in Figure 7 the detection time, are summarized respectively. As is apparent from FIGS. 6 and 7, in the light detection unit LS ₁ of Comparative Example 2, since the voltage change of the capacitor Cw immediately after the switch element SW ₁ is turned off is small, the detection time is light amount. It turns out that it becomes nonlinear with respect to.

This causes, particularly parasitic capacitance between the gate electrode G _L and the drain electrode D _L of the TFT as an optical sensor in a state that joined in parallel with the capacitor Cw, the charge trapped in the reset channel region This is because a time in seconds is required until it comes out.
In order to prevent such a phenomenon from occurring, the gate electrode G of the TFT as an optical sensor
_The gate voltage Gv applied to _L is changed from the positive bias voltage Gvon to the constant reverse bias voltage Gv0.
After changing to, a predetermined reference voltage Vs may be applied from the reference voltage source to the capacitor Cw after several seconds to several tens of seconds. However, when such a configuration is adopted, the measurement period of the amount of external light becomes as long as several seconds to several tens of seconds, which is not practical.

Therefore, in the embodiment, the timing controller Tc performs the positive bias voltage Gvo.
A constant reverse bias voltage Gv during measurement for a certain time after n is applied, for example, 10 ms.
A reverse bias voltage Gvoff lower than 0 is applied to discharge charges trapped in the channel region in a short time during reset. The Figure 8 is a timing chart of this embodiment, also, the variation of the voltage across the capacitor C _W when changing the time to apply a lower reverse bias voltage Gvoff than a certain reverse bias voltage Gv0 during measurement Tgoff This will be described with reference to FIG.

In this embodiment, as the gate voltage Gv to be applied to the gate electrode G _L of the TFT as an optical sensor, but in terms of 1ms applying a positive bias voltage Gvon of just before the measurement example + 15V is similar to that of Comparative Example 2 Immediately thereafter, a reverse bias voltage Gvoff lower than the reverse bias voltage Gv0 at the time of measurement is applied for a predetermined time Tgoff, for example, −10 V for 10 ms. Further, in this embodiment, it is similar to that the time of the switching elements SW ₁ to be turned on state of Comparative Example 2 after the application of a positive bias voltage Gvon. In addition,
The reverse bias voltage Gv0 at the time of measurement in this embodiment corresponds to the first reverse bias voltage of the present invention, and the reverse bias voltage Gvoff lower than the reverse bias voltage at the time of measurement is the second reverse bias voltage of the present invention.
This corresponds to the reverse bias voltage.

By changing the time Tgoff for applying the reverse bias voltage Gvoff lower than the reverse bias voltage Gv0 at the time of measurement, the discharge amount of the charges trapped in the channel region at the time of resetting can be changed. Therefore, in this embodiment, the discharge characteristic obtained when the voltage charged in the capacitor Cw is discharged by changing the predetermined time Tgoff for applying the reverse bias voltage Gvoff lower than the reverse bias voltage Gv0 at the time of measurement. For example, as shown in FIG. 9, various changes can be made. Referring to FIG. 9, the discharge characteristics when the reverse bias voltage Gvoff lower than the reverse bias voltage Gv0 is constant at −10 V and Tgoff <10 ms and Tgoff> 10 ms can be seen. It is possible to see the discharge characteristics when Tgoff = 10 ms, where it has been confirmed that a discharge characteristic almost similar to the original discharge characteristic can be obtained.

Therefore, according to the light quantity detection circuit 10 of the above embodiment, the deterioration of the characteristics of the TFT as the photosensor due to the bias in the polarity of the gate voltage Gv applied to the gate electrode _GL of the TFT as the photosensor is suppressed. In addition, a light quantity detection circuit with improved light detection accuracy can be obtained. In addition, since the light amount detection circuit 10 of the embodiment does not involve a change in manufacturing conditions of the light detection unit LS ₁ itself, for example, the display device and the optical sensor are manufactured by the same manufacturing apparatus. However, the manufacturing process is not lengthened and the condition setting of the manufacturing apparatus is frequently changed.

[Modification]
In the above-described embodiment, as in the case of the comparative example 2, the reset operation for applying the positive bias voltage Gvon is performed, thereby suppressing the deterioration of the TFT characteristics due to the polarity deviation as the optical sensor. However, even if the reset operation is actually performed, deterioration due to exposure of the TFT as the light sensor to external light cannot be suppressed, and the deterioration of the TFT as the light sensor gradually proceeds according to the accumulated amount of light exposed. The optimum Tgoff value varies. Therefore, as a modification of the above embodiment, a means for measuring the integrated light quantity is provided to compensate for deterioration caused by exposure of the TFT as an optical sensor to external light, so that an optimum Tgoff value can be obtained. The detection circuit 10 ′ will be described with reference to FIG.

The light quantity detection circuit 10 ′ of this modification has a reference light detection unit LS in addition to the light detection unit LS ₁ described above.
₂ is provided. The reference light detection unit LS ₂ is the same as the light detection unit LS ₁ shown in FIG. 3 except that the surface of the TFT serving as the light sensor is covered with a color filter having a transmittance of 1 / n as a light reducing means. has the same configuration as the light detecting section LS _1, the optical detection unit LS ₁ and the reference light detection section LS ₂ has equal photoelectric conversion characteristics. Here, the color filters, the incident light amount of the reference light detection section LS ₂ is intended for light attenuation such that 1 / n of the amount of light incident on the light detecting section LS ₁ (n> 0).

That is, the TFT as an optical sensor of the light sensing section LS _l external light is incident directly as the incident light, the TFT as an optical sensor of the reference light detection section LS ₂ is dimmed to 1 / n by the color filter External light is incident as incident light. FIG timing controller Tc shown in 10, the light detection unit LS _l of switching elements SW _l and the reference light detection section LS ₂
ON / OFF control of the switch element (not shown) is performed, and the capacitor Cw of the light detection unit LS ₁ is controlled.
And to charge the reference light detection section LS ₂ capacitor (not shown).

At this time, the charging timing and one discharge period of the light detection unit LS ₁ and the reference light detection unit LS ₂ are set to be equal. In addition, 1 discharge period shows the period from the charge start time to the capacitor to the next charge start time. Further, the threshold voltage Vth determined in advance from the end of charging of the capacitor is set equal in the light detection unit LS ₁ and the reference light detection unit LS ₂ . In this way, based on the reading section S _H, the reading Ia obtained from the light detecting section LS _l, reads a reading Ib obtained from the reference light detection section LS _2, these readings Ia and Ib In addition, the following calculation processing is performed by the calculation means Op to obtain an optimum Tgoff value.

That is, as described above, in this modification, the light detecting portion LS _l and the reference light detection section LS ₂ is provided, a reference light detecting section LS _2, the light amount of incident light of the reference light detection section LS ₂ are provided dimming means to 1 / n of the amount of light incident on the detection unit LS _l. Therefore, when irradiated with a certain amount of external light P, the reading values Ia and Ib of the light detection unit LS ₁ and the reference light detection unit LS ₂ are also
The following simultaneous equations are obtained by the correlation with the amount of external light P.

Ia = R _Non [p] KP (1)
Ib = R _CF [p] KP / n (2)
Here, K represents the initial sensitivity, R _Non [p] is a deterioration function of the sensitivity of the photosensor when there is no dimming means, and R _CF [p] is the deterioration of the sensitivity of the photosensor when there is a dimming means. Each of which is a function of the integrated light quantity p, which is the time integration of the received light quantity. These deterioration functions are obtained by empirical rules based on experiments and the like.

Here, from the equations (1) and (2),
Ia / Ib = (R _Non [p] KP) / (R _CF [p] KP / n)
= N (R _Non [p] / R _CF [p]) (3)
The relationship is obtained.

Therefore, since the above expression (3) is expressed in a form that does not include the amount of external light P, the light detection unit LS
_A first look-up table is obtained experimentally using the reading values Ia and Ib of _l and the reference light detection unit LS ₂ as input variables, and the accumulated light quantity of external light is referred to with reference to the first look-up table. p can be obtained. Further, the second look showing the relationship between the accumulated amount of external light p obtained in this way and the value of the optimum predetermined time Tgoff for applying the reverse bias voltage Gvoff lower than the reverse bias voltage Gv0 at the time of measurement. An up table is experimentally obtained, and the reverse bias voltage Gv0 at the time of measurement is referred to with reference to the second look up table.
It is possible to obtain the optimum predetermined time Tgoff value for applying the reverse bias voltage Gvoff lower than that. Thus, in the above-described modification, the low reverse bias voltage Gv
Since the off application time Tgoff can be controlled to an optimum value in consideration of the integrated light amount, the amount of external light can be detected more accurately. In this modification, the light detecting section LS _1, reference light detecting section LS _2, reading unit S _H, computing means Op constitute the integrated light quantity measuring means of the present invention.

In the above-described embodiments and modifications, the fact that the leakage current of the TFT is proportional to the amount of received light makes it possible to measure the voltage across the capacitor when the charge charged in the capacitor is discharged by this leakage current. An example of measuring the amount of external light was shown. However, in the present invention, it is also possible to measure the amount of external light by measuring the amount of leakage current of the TFT itself from a constant voltage source.

Next, an electro-optical device 100 to which the light quantity detection circuit 10 ′ according to the modification described above is applied will be described with reference to FIG. As shown in FIG. 11, the electro-optical device 100 includes a liquid crystal panel 101,
A light control circuit 102, a backlight 103 as an illumination unit, and a control circuit 104 are provided. The liquid crystal panel 101 includes an image display area A, a scanning line driving circuit 201, a data line driving circuit 202, and a light amount detection circuit 10 ′ on the element substrate. Here, the liquid crystal panel 101 is generally T
The element substrate on which the FT is formed and the counter substrate on which the color filter is formed have a structure in which the liquid crystal layer is sandwiched. Therefore, 'if provided on the element substrate, the light quantity detection circuit 10' light quantity detection circuit 10 rather than on the light detecting section LS ₂ dimming for filter light detecting section LS ₂ in, provided the corresponding parts of the opposing substrate it is possible, it becomes the light incident on the light detecting section LS ₂ without particularly increasing the manufacturing process can be dimmed.

The control circuit 104 generates various signals to generate the scanning line driving circuit 201 and the data line driving circuit 2.
02 is supplied. In the image display area A, a plurality of scanning lines Y1 to Ym and a plurality of data lines X1 to Xn are formed so as to be orthogonal to each other, and a plurality of pixels Pl are formed in a matrix at each of these intersections. Therefore, the transmittance can be controlled for each pixel Pl. Light from the backlight 103 is emitted through the pixel Pl. Thereby, gradation display by light modulation becomes possible. The dimming circuit 102 adjusts so that the backlight 103 emits light with luminance according to illuminance data indicating the illuminance of external light (ambient light) output from the light amount detection circuit 10 ′.

The visibility of the display image depends on the brightness of the surrounding environment. Therefore, the dimming circuit 102
Compares the illuminance value of the ambient light output from the light amount detection circuit 10 ′ with a predetermined illuminance threshold value. For example, when the illuminance of the environmental light corresponds to natural light during the day, the light emission of the backlight 103 is performed. Set the brightness higher and display a bright screen. On the other hand, when the ambient light illuminance corresponds to a dark environment at night, the light emission luminance of the backlight 103 is set low.

Here, the case where the light amount detection circuit of the present invention is applied to an electro-optical device using liquid crystal has been described, but it can also be applied to an electro-optical device using an electro-optical material other than liquid crystal. For example, electrophoresis using a display panel using an OLED element such as an organic EL or a light emitting polymer as an electro-optical material, or a microcapsule containing a colored liquid and white particles dispersed in the liquid as the electro-optical material Display panels, twist ball display panels using twist balls that are painted in different colors for areas of different polarity as electro-optical materials, toner display panels using black toner as electro-optical materials, high pressure such as helium and neon The present invention can be applied to various electro-optical devices such as a plasma display panel using a gas as an electro-optical material.

It is a block diagram which shows the structure of the light quantity detection circuit of an Example. It is a circuit diagram of a light detection part and a reading part. It is sectional drawing of the optical sensor, a capacitor | condenser, and switch element which comprise a photon detection part. 6 is a diagram illustrating a timing chart of Comparative Example 1. FIG. 10 is a diagram illustrating a timing chart of Comparative Example 2. FIG. It is a figure which shows the change of the voltage of the both ends of the capacitor | condenser _CW of the comparative examples 1 and 2. FIG. It is a figure which shows the calibration curve of the comparative examples 1 and 2. FIG. It is a figure which shows the timing chart of an Example. It is a figure which shows the change of the voltage of the both ends of the capacitor | condenser _{CW of} an Example. It is a block diagram of the modification of an Example. It is a figure which shows the structure of the electro-optical apparatus to which the light quantity detection circuit of an Example is applied.

Explanation of symbols

10, 10 ': light quantity detecting circuit 11: TFT substrate 12: gate insulating film 13 _L, 13s
: Semiconductor layer 14: protective insulating film 15: light-shielding layer LS _1: light detecting section LS _2: reference light detecting portion S _H: reading section Vc: the gate bias control means Tc: the timing controller S _L, Ss: the source electrode G _L , Gs: gate electrode D _L , Ds: drain electrode V
s: the reference voltage Cw: capacitor SW _1, SW _2: switching element V _COM: common potential Gv: gate voltage GV0: constant reverse bias voltage at the time of measurement Gvon: positive bias voltage Gvoff: low reverse bias voltage Op: calculating means

Claims (5)

In a light amount detection circuit including a light detection unit that detects the amount of external light using a current that flows when a reverse bias voltage is applied to the gate electrode of the TFT,
The light detection unit changes the voltage applied to the gate electrode of the TFT from a first reverse bias voltage applied when detecting external light to a positive bias voltage by a gate bias control unit, and then The light quantity detection is characterized in that the second reverse bias voltage lower than the first reverse bias voltage is changed to a second reverse bias voltage, and then the output due to the current flowing after returning to the first reverse bias voltage is output as the quantity of external light. circuit. The light detection unit includes a capacitor connected between a source electrode and a drain electrode of the TFT and a switching element connected between the source electrode and a predetermined constant voltage source, and the gate of the TFT After the voltage applied to the electrode is returned to the first reverse bias voltage, a detection time from when the switch element is turned off until the voltage at both ends of the capacitor is discharged to a predetermined threshold voltage is output. The light quantity detection circuit according to claim 1. The light detection unit further includes a unit for measuring an integrated light amount, which is a product of the light amount irradiated to the TFT and the irradiation time, and the gate bias control unit applies time for applying the second reverse bias voltage. The light amount detection circuit according to claim 1, wherein the light amount is adjusted by an output of the integrated light amount measuring means. The light detection unit includes a reference TFT having a dimming unit formed on a surface thereof, and the integrated light amount measurement unit has a correlation between a predetermined output ratio of the TFT and the reference TFT and the integrated light amount. And calculating the second reverse bias voltage application time based on a correlation between the predetermined integrated light amount and the optimum second reverse bias voltage application time. The light amount detection circuit according to claim 3. A light amount detection circuit according to any one of claims 1 to 4,
A display panel having a plurality of scanning lines, a plurality of data lines, and a plurality of pixels provided corresponding to intersections of the plurality of scanning lines and the plurality of data lines;
An illumination unit that illuminates the display panel;
A light control circuit for controlling the illumination unit based on an output value of the light amount detection circuit;
An electro-optical device comprising:

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