JPH1038686A - Charge integration type photodetector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce temperature drift to be caused by changes in temperature of a substrate in operation and to improve the S/N ratio. SOLUTION: This detector is constituted of a bolometer element 1 for detecting signal light, its integrating circuit, and an offset circuit by a bolometer element 8 for reference. The reference element 8 here is provided with a structure which does not generate resistance changes due to signal light by the provision of a light shielding plate, the leaving of a sacrifice layer, etc. As both of the elements 1 and 8 here are formed in the same shape and spatially in close proximity to each other, the only difference between the electrical characteristics of the two elements is the presence or absence of the change in current to incident light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a charge integration type photodetector including a photodetector.

[0002]

2. Description of the Related Art A conventional charge-integrating photodetector will be described mainly with respect to a portion related to the present invention, that is, a photodetector circuit including a photodetector.

In this type of photodetector, a photodetector is connected to a charge storage capacitor, the charge generated by the photodetector is integrated by the capacitor for a certain period of time, and the resulting change in the terminal voltage of the capacitor occurs. As an output signal. In the technical field to which the present invention pertains, a charge accumulating capacitor is charged to a predetermined voltage in advance to accumulate electric charge, and then discharged by a photodetector such as a photodiode connected in series to the capacitor, for example. This is referred to as integrating the charge generated by the photodetector with a capacitor.

FIG. 3 shows an example of a conventional charge integration type photodetector. The conventional charge integration type photodetector includes a photodetection element 1, a constant current element 2, an element selection switch 3, and a charge storage capacitor 4, which are connected in series to form an integration circuit.

The charge generated by the photodetector 1 is integrated by the charge storage capacitor 4 via the constant current element 2 only while the element selection switch 3 is turned on. When the element selection switch 3 is opened and the integration is completed, the terminal voltage of the charge storage capacitor 4 is fixed at a constant voltage. The terminal voltage at this time appears at the output terminal 7 via the output amplifier 6. After reading out the signal, the reset switch 5 is turned on to return the terminal voltage of the charge storage capacitor 4 to the value before the integration again, in preparation for the next integration. Here, an NPN-type bipolar transistor is used as the constant current element 2, and a constant voltage is applied to the gate terminal of the charge storage capacitor 4 so that the discharge is performed with a constant current even if the terminal voltage changes during the integration operation. .

As described above, the conventional charge integration type photodetector has:
The integration, reading, and reset operations are performed as one cycle, and an optical signal incident on the photodetector is integrated and detected.

[0007]

By the way, the following two problems arise in the conventional charge integration type photodetector described here.

First, there is a problem of output fluctuations due to temperature fluctuations of the photodetector 1 during operation, so-called temperature drift. For example, as an application example of such a photodetector, there is an optical imaging device having a configuration in which photodetectors are arranged in a two-dimensional array. In this optical imaging device, in order to correct the deterioration of an output image due to a temperature drift. And image correction such as offset correction. Therefore, when the optical imaging device is used for outdoor imaging in which the environmental temperature changes drastically, this operation is often required, which causes inconvenience.

Second, the photodetector has a multi-element array.
In a two-dimensional array sensor or a two-dimensional array sensor, the offset variation of the output signal due to the resistance variation of each element is large, which limits the dynamic range of the entire system. If the gain is reduced to widen the dynamic range in consideration of variations, the S / N of the photodetector
There is a disadvantage that the ratio is reduced.

[0010]

In order to solve such a problem, a photodetector according to the present invention comprises a photodetector, a charge integrating capacitor, a constant current element, and an element selection switch which are connected to a discharge path. A new reference element is provided in addition to the conventional integration circuit having a configuration connected in series so as to form, and a discharge path is formed with the charge integration capacitor through a second constant current element. I have.

Here, the reference element must satisfy the following two conditions. First, no change in current due to light incidence occurs in the detection wavelength band of the photodetector;
And the electrical characteristics such as the temperature characteristics thereof when no light is incident on the bolometer element 1 used as the light detection element.

[0012]

With this configuration, in the charge integration type photodetector of the present invention, only the current caused by the incident light is integrated by the charge storage capacitor. At this time, the current change due to the temperature fluctuation is equally generated in both the light detection element and the reference element, and is canceled during integration, and does not appear as an output voltage. That is, with the configuration of the present invention, it is possible to reduce the temperature drift due to the temperature change during operation and to improve the S / N ratio.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An integrating photodetector using a bolometer type infrared detecting element as a photodetecting element according to an embodiment of the present invention will be described below with reference to the drawings. The bolometer element is an element for detecting a resistance change due to radiant heating of the incident signal light, and is capable of operating at room temperature, and is one of the infrared detection elements that has been receiving the most attention at present.

FIG. 1 shows an integrating photodetector according to an embodiment of the present invention. In this example, bolometer element 1, NPN
Type bipolar transistor 2, element selection switch 3, and charge storage capacitor 4 constitute an integration circuit.
In addition, a reference element 8, a PNP bipolar transistor 9, and a second element selection switch 10 are provided.

Next, specific examples of the reference element will be described.
First, a bolometer element provided with a shielding plate on the light incident surface can be used as a reference element. At this time, the reference element has the same structure as that of the bolometer element 1 used as the light detection element except for the provision of the shielding plate, and is formed in the same manufacturing process.

As another example, according to experiments, a bolometer element having no absorption layer on the incident surface could be used as a reference element. Also in this case, the reference element has the same structure as the bolometer element 1 used as the light detection element except for the absorption layer, and is formed in the same manufacturing process as in the previous example.

In each of the reference elements, there is no change in current caused by an optical signal, and the electrical characteristics are the same as those of the bolometer element 1 used as the light detection element. did.

As another embodiment of the reference element, there is a method using a bolometer element which does not have a thermal isolation structure. The bolometer element has a structure in which a layer called a sacrificial layer is formed below the light receiving element, and is etched later to thermally separate the light receiving element from the substrate. In the reference element, the light receiving element and the substrate are brought into thermal contact by leaving the sacrificial layer to the end without performing etching. As a result, the heat generated by the radiation of the incident light diffuses to the substrate, so that a change in current due to the incident light can be suppressed.

Next, a specific operation of the present embodiment will be described. The two element selection switches 3 and 10 are turned on at the same time, and the integration operation is started. During this time, the terminal voltage of the charge storage capacitor changes in accordance with the current generated in the light detection element 1 and the reference element 8. Here, the gate voltage of the NPN-type bipolar transistor 2 and the gate voltage of the PNP-type bipolar transistor 9 are adjusted so that the voltage between both ends of the reference element and the voltage between the photodetectors are equal. As a result, the reference element supplies and cancels the current generated by the light detection element due to factors other than the incident light. After a certain time, the two element selection switches 3
10 are simultaneously opened, and the terminal voltage of the charge storage capacitor 4 is fixed. The terminal voltage at this time is read from the output terminal 7 via the output amplifier 6.

This circuit can be applied to a linear array sensor and a two-dimensional array sensor by being integrated on the same substrate. FIG. 2 shows an XY address selection type 2
An embodiment applied to a dimensional array sensor is shown. The unit cell 11 includes a bolometer element 1 and an element selection switch 3
It is composed of Here, each unit cell includes an element selection switch 3 controlled by a row shift register 13 and a column selection switch 1 controlled by a column shift register 14.
2 are sequentially selected. The selected unit cell includes a circuit including the NPN type bipolar transistor 2 and the charge storage capacitor 4, and a reference element 8.
And a circuit composed of a PNP-type bipolar transistor 9 and a second element selection switch 10, and the integration operation is performed at the same time. Here, the second element selection switch 10 is turned on in synchronization with an arbitrary element selection switch only while it is turned on. The signal detected in an arbitrary unit cell in this manner appears from the output terminal 7 via the output amplifier 6 in order.

In the above-described embodiment of the charge integration type photodetector of the present invention, no drift of the output voltage due to the fluctuation of the substrate temperature occurred. In the embodiment of the infrared imaging apparatus applied to the two-dimensional array, no image was generated for an image requiring offset correction for a long time. This is because the change in the operating current due to the change in the substrate temperature is caused by the photodetector 1
And the reference element 8 are equally generated and canceled during the integration.

In the embodiment in which the photodetectors of the present invention are two-dimensionally arranged, the dark current component, that is, the offset component, including the characteristic variation of each element, is removed, and the S detector of the photodetector is removed.
/ N ratio improved.

[0023]

According to the structure of the photodetector of the present invention, only the current caused by the signal light is integrated by the charge integrating capacitor. At this time, a change in current due to a temperature change is equal in both the photodetector and the reference element, and therefore is canceled during integration and does not appear as an output voltage. That is, with the configuration of the present invention, it is possible to reduce the temperature drift caused by the temperature change during operation. Further, since the dark current component, that is, the offset component, including the characteristic variation for each element, is automatically removed, the S / N ratio of the photodetector is improved.

[Brief description of the drawings]

FIG. 1 is an embodiment of a photodetector of the present invention.

FIG. 2 is an example in which the photodetector of the present invention is applied to a two-dimensional array sensor.

FIG. 3 is an example of a conventional integrating photodetector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photodetection element 2 NPN type bipolar transistor 3 Element selection switch 4 Charge storage capacitor 5 Reset switch 6 Output amplifier 7 Output terminal 8 Reference element 9 PNP type bipolar transistor 10 Reference element selection switch 11 Unit cell 12 Column selection switch 13 Column shift Register 14 Row shift register

Claims (3)

[Claims] 1. A charge integration type photodetector for detecting an optical signal by a charge integration capacitor for integrating a charge generated by a photodetector, wherein the charge generated irrespective of an incident light amount is integrated during the charge integration. And a means for replenishing and canceling the charge integrating capacitor. 2. The photodetector according to claim 1, wherein all or a part of the photodetection element and the charge replenishment / cancellation means are formed close to the same wafer by the same process. Charge integrating photodetector. 3. A charge-integrating photodetector, wherein a part or the whole of the photodetector according to claim 1 or 2 is formed on the same silicon substrate and integrated. .