Imaging device, control method of imaging device, and pixel structure

1. Field of the Invention

The disclosures discussed herein relate to an imaging device, a control method of the imaging device, and a pixel structure.

2. Description of the Related Art

There are disclosed solid-state imaging devices having improved sensitivity known in the art. Such solid-state imaging devices employ phototransistors configured to amplify an optical current to output the amplified optical current, thereby serving as a photoelectric converters. The solid-state imaging devices having the above-described phototransistors have residual electric charges remaining in a base. The residual electric charges are obtained by reset operations after the output of the amplified optical current. The solid-state imaging devices may thus be able to improve their sensitivity by discharging the residual electric charges remaining in the base (accumulated electric charges in the base that have not been completely discharged when reading).

For example, Japanese Laid-open Patent Publication No. 2013-187527 (hereinafter referred to as “Patent Document 1”) discloses an imaging device with a reduced size and improved light utilization efficiency. The imaging device disclosed in Patent Document 1 includes implanted electrodes insulated by an oxide film to separate pixels. Hence, the imaging device increases optical current amplification by applying a voltage to the implanted electrodes to thereby achieve the reduction in size and the improved light utilization efficiency.

However, the related art solid-state imaging devices have the optical current amplified by the application of the voltage to the implanted electrodes when reading signals, which may increase a time to discharge residual electric charges when resetting relative to the amplification of the optical current when reading.

Patent Document

Patent Document 1

Japanese Laid-open Patent Publication No. 2013-187527

Accordingly, it is a general object in one embodiment of the present invention to provide an imaging device, a control method of the imaging device, and a pixel structure capable of reducing a reset time that substantially obviate one or more problems caused by the limitations and disadvantages of the related art.

According to an aspect of embodiments, there is disclosed an imaging device having phototransistors in photodetectors of pixels. The imaging device includes an implanted electrode configured to separate the pixels; a first emitter disposed at a position adjacent to the implanted electrode; and a second emitter disposed such that a distance from the implanted electrode to the second emitter is longer than a distance from the implanted electrode to the first emitter.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 is a diagram illustrating an example of an imaging device according to an embodiment;

FIGS. 2A and 2B are diagrams illustrating an example of the imaging device according to an embodiment;

FIG. 3 is a diagram illustrating a related art imaging device; and

FIG. 4 is a diagram illustrating a related art imaging device.

In the following, a description is given of an embodiment of the present invention with reference to accompanying drawings. In the drawings, identical components are provided with the same reference numbers to omit duplicated illustrations.

Configuration of Imaging Device

FIG. 1 , and FIGS. 2A and 2B illustrate examples of an imaging device according to an embodiment. FIG. 1 is a cross-sectional diagram cut along a broken line A-A illustrated in FIG. 2A .

An imaging device (a solid-state imaging device) 100 includes pixels in an array. Each pixel has a photodetector 201 and a transistor part 202 , and the photodetector 201 has a phototransistor. The phototransistor includes implanted electrodes 101 ( 101 a and 101 b ), a first emitter 102 a (an output emitter), a second emitter 102 b (a discharge emitter), a collector 103 , and a base 104 . The imaging device 100 may be able to take an image with high sensitivity by utilizing the phototransistor amplification effect. The phototransistor amplification effect is generated in an electrode surface by the application of a voltage to the implanted electrodes 101 ( 101 a and 101 b ).

The implanted electrodes (trenches) 101 are formed such that the implanted electrodes 101 surround the pixels and penetrate the first emitter 102 a and the base 104 to separate the pixels. Further, the implanted electrodes 101 are formed such that the implanted electrodes 101 surround the photodetector 201 and the transistor part 202 , respectively, to separate the photodetector 201 and the transistor part 202 (see FIGS. 2A and 2B ).

The collector 103 is formed in a deep area of a semiconductor layer, and the base 104 is formed in a shallow area of the semiconductor layer. The collector 103 and the base 104 are formed in contact with each other. The emitters 102 ( 102 a and 102 b ) are formed in a shallow area of the base 104 , and the base 104 and the emitters 102 are formed in contact with one another.

The first emitter 102 a is formed adjacent to the implanted electrodes 101 . The second emitter 102 b is surrounded by the first emitter 102 a , and disposed in the center of the photodetector 201 (see FIG. 2A ). Note that the second emitter 102 b is not necessarily surrounded by the first emitter 102 a . For example, the first emitter 102 a may be formed adjacent only to one side of the implanted electrode 101 that surrounds the photodetector 201 , and the second emitter 102 b is disposed in the center of the photodetector 201 (see FIG. 2B ). The second emitter 102 b may at least be disposed at a position having a distance farther from the implanted electrode 101 than the position at which the first emitter 102 a is disposed.

The phototransistor includes a multi-emitter structure in which the collector 103 is shared between all the pixels, and each of the pixels has two emitters 102 . Detailed illustration of the phototransistor is given later. However, the phototransistor is configured to output, when reading, from the first emitter 102 a the optical current amplified at a surface of the implanted electrode 101 , and configured to discharge, when resetting, from the second emitter 102 b the residual electric charges remaining in the base 104 . That is, the imaging device 100 utilizes the first emitter 102 a exhibiting increased current amplification when reading, and utilizes the second emitter 102 b exhibiting stable (unchanging) current amplification when resetting. Hence, the imaging device 100 may be able to shorten the discharge time when resetting despite the fact that the optical current is amplified when reading.

Note that the current amplification of the phototransistor is determined based on a distance (i.e., a width of the base 104 ) from a junction capacitance between the collector 103 and the base 104 to a junction capacitance between the base 104 and the emitter 102 . The current amplification of the phototransistor increases as the width of the base 104 decreases. Further, the current amplification of the phototransistor may be determined based on the impurity concentration of the emitter 102 , the impurity concentration of the collector 103 , the impurity concentration of the base 104 , the voltage between the collector 103 and the emitter 102 , and the like, in addition to the width of the base 104 .

Note that an illustration is given of the amplification effect of the implanted electrodes. When a voltage (e.g., a positive voltage) is applied to the implanted electrode 101 , electrons gather in the surfaces of the implanted electrodes 101 (see upwards thick Arrows in FIG. 1 ), and the electrons present near the implanted electrode 101 surfaces and the positive holes present in the base 104 are recoupled. Accordingly, an electrically neutral area is formed in the surfaces of the implanted electrodes 101 to shorten the width of the base 104 . The current amplification near the electrode 101 surfaces increases as the optical current preferentially flows near the surfaces of the electrodes 101 , which may further increase the current amplification of the phototransistor.

By contrast, when no voltage is applied to the implanted electrode, the current amplification does not change in the surfaces of the implanted electrodes 101 . Hence, the optical current flows from the collector 103 to the emitters 102 in an entire surface of the phototransistor.

That is, the application of a voltage to the implanted electrodes 101 may increase the current amplification (e.g. several hundreds×hFE times) of the first emitter 102 a adjacent to the implanted electrode 101 higher than the current amplification (e.g. hFE times) of the second emitter 102 b . Accordingly, the imaging device 100 may be able to perform imaging with high sensitivity by utilizing the voltage applied to the implanted electrodes 101 .

Output Emitter (First Emitter) and Discharge Emitter (Second Emitter)

Next, an illustration is given of the amplification of the optical current when reading and the reduction in the discharge time when resetting for performing high sensitivity imaging by applying the voltage to the implanted electrodes 101 by using the output emitter (first emitter) and the discharge emitter (second emitter). The current amplification of the phototransistor is represented by hFE.

Each of the first emitter 102 a and the second emitter 102 b is connected to not-illustrated wiring of the transistor part 202 . Whether to output or not to output the optical current from the first emitter 102 a , or whether to discharge or not to discharge the residual electric charges from the second emitter 102 b is controlled based on an on or off of the transistors disposed on the transistor part 202 .

For example, when reading, a transistor connected to the first emitter 102 a is turned on and a transistor connected to the second emitter 102 b is turned off. That is, when reading, the imaging device 100 utilizes only the first emitter 102 a exhibiting the high current amplification. The imaging device 100 is provided with the amplification effect generated in the surfaces of the implanted electrodes 101 of the phototransistor, and capable of amplifying the optical current by (hFE) to (several hundreds×hFE) times to output the amplified optical current.

Further, when resetting, the transistor connected to the first emitter 102 a is turned off and the transistor connected to the second emitter 102 b is turned on. That is, when resetting, the imaging device 100 utilizes only the second emitter 102 b exhibiting no current amplification change. The imaging device 100 is not provided with the amplification effect generated in the implanted electrode 101 surfaces of the phototransistor, and is hence capable of discharging the residual electric charges remaining in the base 104 at a (1/hFE) times speed. Hence, the imaging device 100 is capable of reducing the discharge time. Note that the residual electric charges remaining in the base 104 are uniquely determined based on the electric potential or resistance of a circuit; however, the imaging device 100 may be able to reduce the discharge time regardless of the quantity of the residual electric charges.

The imaging device 100 according to the embodiment includes the two emitters having different current amplifications in the phototransistor to switch the two emitters between reading and resetting. This configuration enables the imaging device 100 to perform high sensitivity imaging and reduce a resetting time, simultaneously. Further, the imaging device 100 may expand a dynamic range as well as maintaining high sensitivity by switching between the emitters in use based on imaging subjects.

Related Art Phototransistor Structure

FIG. 3 illustrates an imaging device 400 having a phototransistor of a related art structure, and FIG. 4 illustrates an imaging device 500 having a phototransistor of a related art structure.

The imaging device 400 includes implanted electrodes 401 ( 401 a and 401 b ), an emitter 402 , a collector 403 , a base 404 , and the like.

As illustrated in FIG. 3 , the imaging device 400 includes the emitter 402 formed in an entire surface of the phototransistor. In this case, when reading, the imaging device 400 is provided with an amplification effect generated in surfaces of an implanted electrodes 401 , and capable of amplifying an optical current by (hFE) to (several hundreds×hFE) times to output the amplified optical current. However, when resetting, the imaging device 400 discharges the residual electric charges at a (1/several hundreds×hFE) times speed, and hence the discharge time is extremely long.

Accordingly, it appears difficult to reduce the reset time in the imaging device 400 having the structure illustrated in FIG. 3 .

The imaging device 500 includes implanted electrodes 501 ( 501 a and 501 b ), an emitter 502 , a collector 503 , a base 504 , and the like.

As illustrated in FIG. 4 , in the imaging device 500 , the emitter 502 is not formed on surfaces of the implanted electrodes 501 . In this case, the residual electric charges are discharged at a (1/hFE) times speed when resetting, indicating that the imaging device 500 may reduce the discharge time compared to the discharge time in the imaging device 400 illustrated in FIG. 3 . However, the imaging device 500 merely includes a current path between the collector 503 , the base 504 , and the emitter 502 when reading. Hence, the imaging device 500 may fail to amplify the optical current by (hFE) to (1/several hundreds×hFE) times. Accordingly, it appears difficult for the imaging device 500 having the structure illustrated in FIG. 4 to perform high sensitivity imaging.

The imaging device 100 according to the embodiment may be combined with the phototransistor structure illustrated in FIG. 3 or the phototransistor structure illustrated in FIG. 4 to reduce the reset time while performing high sensitivity imaging. That is, the imaging device 100 according to the embodiment having a pixel structure capable of having benefit alone may be obtained by appropriately switching between the emitters to utilize the amplification effect generated by the application of the voltage to the implanted electrodes merely when reading, and not to utilize such amplification effect when resetting.

The imaging device 100 according to the embodiment may be able to reduce the reset time.

The embodiments of the present invention are described above. However, the present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the gist of the embodiment recited in the scope of the present invention.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

The present application is based on and claims the benefit of priority of Japanese Priority Application No. 2014-149445 filed on Jul. 23, 2014, the entire contents of which are hereby incorporated herein by reference.