JP2010164374A - Optical detection apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical detection apparatus which determines accurate optical intensity. <P>SOLUTION: The optical detection apparatus includes: FETs 1, 2; a capacitor 3 connected to the source and gate of the FET 1; a capacitor 4 connected to the source and gate of the FET 2; a switch 5 for opening/closing the gate of the FET 1 and the gate of the FET 2; a switch 6 for opening/closing the source of the FET 1 and a ground node; a switch 7 for opening/closing the source of the FET 1 and an output node of a power supply outputting -5[V]; a switch 8 for opening/closing the gate of the FET 2 and the drain of the FET 2; and a switch 9 for opening/closing the drain of the FET 2 and an output node of a power supply outputting +5[V]. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a photodetection device.

  2. Description of the Related Art Conventionally, there are various configurations of a light detection device that detects light.

  FIG. 7 is a diagram illustrating a circuit example of a conventional photodetector. The drain of the FET 10 is grounded, and the gate and source of the FET 10 are connected to an output node of a power source that outputs -5 volts (hereinafter, [V]). A photocurrent I corresponding to the intensity of the light L flows through the FET 10. The photodetection device is used, for example, for adjusting the luminance of a backlight in a liquid crystal display device according to the photocurrent I.

  Prior art document information related to the invention of this application includes the following.

JP 2008-26688 A

  However, the photocurrent I may not correspond to the intensity of the light L due to variations in the characteristics of the FET 10 or fluctuations due to temperature or the like. For example, in a liquid crystal display device, when adjusting the luminance of the backlight after making the gate voltage of the FET 10 equal to the threshold voltage, the gate voltage becomes the threshold voltage due to variations in characteristics of the FET 10 and fluctuations. On the other hand, if it differs, accurate adjustment according to the light intensity cannot be performed.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a light detection device capable of obtaining accurate light intensity.

  In order to solve the above-described problem, the first aspect of the present invention includes a first FET that sends out a photocurrent according to the intensity of incident light, a capacitor connected between the source and gate of the first FET, Before the first FET sends out a photocurrent, the second FET provided at a position where it does not enter, the capacitor connected between the source and the gate of the second FET, and the capacitor connected to the first FET are charged. The gate of the first FET, the drain of the second FET, and the gate of the second FET are connected in a state where the source of the first FET and the source of the second FET are connected, and the gate of the first FET is connected to the second FET. And means for separating from the drain of the second FET and the gate of the second FET.

  The photodetector according to the first aspect of the present invention is manufactured so that the threshold voltage of the second FET is equal to the threshold voltage of the first FET. First, the capacitor is charged so that the gate voltage of the first FET becomes higher than the threshold voltage. When the gate of the first FET, the drain of the second FET, and the gate of the second FET are connected, the charged capacitor serves as a power source, and a current flows through the second FET. When the capacitor is discharged by the current and the current stops flowing, the gate voltage of the second FET becomes equal to the threshold voltage. That is, the gate voltage of the first FET becomes equal to the threshold voltage. Next, the gate of the first FET is disconnected, and a photocurrent flows through the first FET. If the gate voltage of the first FET is not equal to the threshold voltage due to fluctuations in the characteristics of the first FET, fluctuations due to temperature, etc., accurate control according to the light intensity is performed under the premise that they are equal. However, in the first aspect of the present invention, since the gate voltage becomes equal to the threshold voltage, accurate control according to the light intensity can be performed.

According to a second aspect of the present invention, there is provided a photodetection device comprising: an FET for sending a photocurrent according to the intensity of incident light; a capacitor connected between a source and a gate of the FET; and light incident on the FET. Then, the light control means for making the light incident on the FET, and charging the capacitor while the light incident on the FET is blocked, connecting the gate of the FET and the drain of the FET, And means for separating the gate of the FET from the drain of the FET before or after light is incident on the FET.
In the light detection device according to the second aspect of the present invention, first, light incident on the FET is blocked. Next, the capacitor is charged so that the gate voltage of the FET becomes higher than the threshold voltage. When the gate and drain of the FET are connected, the charged capacitor becomes a power source, and a current flows through the FET. When the capacitor is discharged by the current and the current stops flowing, the gate voltage of the FET becomes equal to the threshold voltage. Next, the gate of the FET is disconnected, light enters the FET, and a photocurrent flows through the FET. If the gate voltage of the FET is not equal to the threshold voltage due to fluctuations in the characteristics of the FET, fluctuations due to temperature, etc., accurate control according to the intensity of light is performed under the assumption that it is equal. However, in the second aspect of the present invention, since the gate voltage becomes equal to the threshold voltage, accurate control according to the light intensity can be performed.

  According to the photodetecting device of the present invention, the gate voltage of the FET can be made equal to the threshold voltage, so that accurate light intensity can be obtained.

It is a circuit diagram of the photon detection device concerning a 1st embodiment. It is a figure which shows the mode in the time before the light detection apparatus shown in FIG. 1 detects light. It is a figure which shows a mode in the time of the light detection apparatus shown in FIG. 1 detecting light. It is a circuit diagram of the photon detection apparatus which concerns on 2nd Embodiment. It is a figure which shows the mode in the time before the photon detection apparatus shown in FIG. 4 detects light. It is a figure which shows a mode in the time of the light detection apparatus shown in FIG. 4 detecting light. It is a figure which shows the circuit example of the conventional photon detection apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a circuit diagram of the photodetecting device according to the first embodiment.

  The light detection device includes field effect transistors (hereinafter referred to as field effect transistors (FETs)) 1 and 2, a capacitor 3 connected between the source and gate of FET 1, and between the source and gate of FET 2. , A switch 5 that opens and closes between the gate of FET1 and the gate of FET2, a switch 6 that opens and closes between the source of FET1 and the ground node, and outputs -5 [V] of the source of FET1 A switch 7 that opens and closes between the output nodes of the power source to be switched, a switch 8 that opens and closes between the gate of the FET 2 and the drain of the FET 2, and an output node of the power source that outputs +5 [V]. And a switch 9.

  The light detection device is provided on a substrate constituting a display device using a liquid crystal display device or an organic EL, for example, and adjusts the luminance of a light emitting element using a backlight or EL in accordance with the intensity of light from the outside Used for. In the first embodiment, the FET may be either an N-channel type or a P-channel type, and an output point of current flowing through the FET is referred to as a source, and an input point is referred to as a drain. Each FET is, for example, a thin film transistor formed in a polysilicon film on a glass substrate. Each capacitor is formed by, for example, a metal layer formed on such a glass substrate. Each switch includes, for example, an analog switch constituted by such a thin film transistor and a control circuit thereof. Each capacitor is formed by a metal layer formed on such a glass substrate. These are the same in the second embodiment described later.

  The FET 1 is provided at a position where the light L is incident, and the FET 2 is provided at a position where the light is not incident. The drain of FET1 and the source of FET2 are connected to the ground node. When the gate voltage (gate voltage) with respect to the drain of the FET 1 is equal to or lower than a predetermined voltage (referred to as a threshold voltage), the FET 1 is turned off. The same applies to FET2. The photodetector is manufactured so that the threshold voltage of FET2 is equal to the threshold voltage of FET1. The capacitance of the capacitor 3 is set so large that the stray capacitance between the source and gate of the FET 1 can be ignored. The capacitance of the capacitor 4 is set so large that the stray capacitance between the source and gate of the FET 2 can be ignored.

(Operation of photodetection device)
First, as shown in FIG. 1, when the switches 5, 6, 8 and 9 are turned on and the switch 7 is turned off, the capacitor 3 is charged by the current i1 flowing from the power source of +5 [V]. As a result, the gate voltage of the FET 1 becomes +5 [V]. +5 [V] is a voltage higher than the threshold values of the FETs 1 and 2.

  Next, as shown in FIG. 2, only the switch 9 is turned off. At this time, the source of FET1 and the source of FET2 are connected via the ground node. Thereby, the charged capacitor 3 serves as a power source, and a current i2 due to the discharge of the capacitor 3 flows through the switches 5 and 8. The current i2 flows from the drain to the source of the FET2. At this time, the capacitor 4 is charged.

  When the capacitor 3 is discharged by the current i2 and the current i2 stops flowing, the gate voltage of the FET2 becomes equal to the threshold voltage of the FETs 1 and 2. That is, the gate voltage of the FET 1 connected to the FET 2 by the switch 5 becomes equal to the threshold voltage.

  Next, as shown in FIG. 3, the switches 5 and 6 are turned off and the switch 7 is turned on. At this time, the gate of FET1 is disconnected from the drain and gate of FET2. A photocurrent I corresponding to the intensity of the light L flows through the FET 1. As described above, the gate voltage of the FET 1 is equal to the threshold voltage (Vth), and the capacitor 3 maintains this state.

  Therefore, according to the photodetector according to the first embodiment, the first FET (1) that sends out a photocurrent according to the intensity of incident light, and the capacitor (between the source and gate of the first FET) 3), the second FET (2) provided at a position where light does not enter, the capacitor (4) connected between the source and gate of the second FET, and the first FET before sending the photocurrent, The capacitor (3) connected to the 1FET is charged, and the gate of the first FET, the drain of the second FET, and the gate of the second FET are connected in a state where the source of the first FET and the source of the second FET are connected. By providing means (each switch) for separating the gate of the second FET from the drain of the second FET and the gate of the second FET, the gate voltage of the first FET can be made equal to the threshold voltage. . If the gate voltage of the first FET is not equal to the threshold voltage due to fluctuations in the characteristics of the first FET, fluctuations due to temperature, etc., the brightness of the backlight or the like is adjusted under the assumption that the gate voltage is equal. However, in the first embodiment, since the gate voltage is equal to the threshold voltage, accurate control according to the light intensity can be performed. .

[Second Embodiment]
FIG. 4 is a circuit diagram of the photodetecting device according to the second embodiment.

  The photodetector includes a FET 11, a capacitor 12 connected between the source and gate of the FET 11, a switch 13 that opens and closes between the gate of the FET 11 and the output node of the power source that outputs +5 [V], and the source of the FET 11 A switch 14 that opens and closes between the ground nodes, a switch 15 that opens and closes between the source of the FET 11 and the output node of the power supply that outputs −5 [V], a switch 16 that opens and closes between the gate and the drain of the FET 11, A switch 17 that opens and closes between the drain of the FET 11 and the ground node, and a light control unit 18 that makes the light L incident on the FET 11 or shields the FET 11 are provided. The capacitance of the capacitor 12 is set so large that the stray capacitance between the source and gate of the FET 11 can be ignored.

  The light control unit 18 includes, for example, a liquid crystal layer 181 provided in the path of light incident on the FET 11 and a liquid crystal control circuit 182 that controls the orientation of molecules of the liquid crystal layer 181. When the photodetector is provided in a liquid crystal display device, for example, the liquid crystal layer 181 is shared with the liquid crystal layer of the liquid crystal display device.

(Operation of photodetection device)
First, as shown in FIG. 4, the light control means 18 shields the FET 11. For example, the liquid crystal control circuit 182 controls the liquid crystal layer 181 so that the molecules block the light L.

  Next, the switches 13 and 14 are turned on and the switches 15, 16 and 17 are turned off, so that the capacitor 12 is charged by the current i11. As a result, the gate voltage of the FET 11 becomes +5 [V]. +5 [V] is a voltage higher than the gate voltage threshold of the FET 11.

  Next, as shown in FIG. 5, the switch 13 is turned off and the switch 16 is turned on. The charged capacitor 12 serves as a power source, and a current i12 generated by discharging the capacitor 12 flows through the switch 16. The current i12 flows from the drain of the FET 11 to the source.

  When the capacitor 12 is discharged by the current i12 and the current i12 stops flowing, the gate voltage of the FET 11 becomes equal to the threshold voltage.

  Next, as shown in FIG. 6, the light control unit 18 causes the light L to enter the FET 11. For example, the liquid crystal control circuit 182 controls the liquid crystal layer 181 so that the molecules do not block the light L. At that time, that is, before or after light is incident on the FET 11, the switches 14 and 16 are turned off and the switches 15 and 17 are turned on.

  A photocurrent I corresponding to the intensity of the light L flows through the FET 11. As described above, the gate voltage of the FET 11 is equal to the threshold voltage (Vth), and the capacitor 12 maintains its state.

  Therefore, according to the photodetector according to the second embodiment, the FET (11) that sends out a photocurrent according to the intensity of incident light, and the capacitor (12) connected between the source and gate of the FET. The light control means (18) for blocking the light incident on the FET and then the light incident on the FET, and charging the capacitor while the light incident on the FET is blocked, the gate of the FET and the FET The gate voltage of the FET can be made equal to the threshold voltage by providing means (each switch) for connecting the drain and disconnecting the gate of the FET from the drain of the FET before or after light is incident on the FET. If the FET gate voltage is not equal to the threshold voltage due to fluctuations in FET characteristics, temperature fluctuations, etc., the brightness of the backlight etc. can be adjusted under the assumption that they are equal. However, in the second embodiment, since the gate voltage becomes equal to the threshold voltage, accurate control according to the light intensity can be performed.

1,2,10,11 ... FET
3, 4, 12 ... capacitors 5, 6, 7, 8, 9, 13, 14, 15, 16, 17 ... switch 18 ... light control means 181 ... liquid crystal layer 182 ... liquid crystal control circuit I ... photocurrent L ... light

Claims (3)

A first FET that sends out a photocurrent according to the intensity of incident light;
A capacitor connected between the source and gate of the first FET;
A second FET provided at a position where light is not incident;
A capacitor connected between the source and gate of the second FET;
Prior to the first FET sending a photocurrent, a capacitor connected to the first FET is charged, and the gate of the first FET and the source of the first FET are connected in a state where the source of the first FET and the source of the second FET are connected. And a means for connecting the drain of the second FET and the gate of the second FET and separating the gate of the first FET from the drain of the second FET and the gate of the second FET. FET that sends out photocurrent according to the intensity of incident light,
A capacitor connected between the source and gate of the FET;
Light control means for blocking light incident on the FET and then allowing light to enter the FET;
While the light incident on the FET is blocked, the capacitor is charged, the gate of the FET is connected to the drain of the FET, and the gate of the FET is connected before or after the light is incident on the FET. And a means for separating from the drain of the FET. The light control means includes
A liquid crystal layer provided in a path of light incident on the FET;
The photodetection device according to claim 2, further comprising: a liquid crystal control circuit that controls the molecules of the liquid crystal layer to block light and then controls the molecules of the liquid crystal layer not to block light.

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