Image sensor and method of operating an image sensor

Abstract

An image sensor (20) comprises a photosensitive element (110), configured to generate charges from incident electromagnetic radiation ( 15), a transfer transistor (112) electrically coupled to the photosensitive element (110), and a pixel circuit (120) electrically coupled to the transfer transistor (112). The transfer transistor (112) is configured to control a transfer of the charges from the photosensitive element (110) to the pixel circuit (120). The image sensor (20) further comprises a first voltage supply (115) for supplying a first low voltage level (116) higher than 0V to a terminal of the photo- sensitive element (110), and a second voltage supply (117) configured to supply a second low voltage level (118) and a second high voltage level (119) to the transfer transistor (112) and the pixel circuit (120).

Description

[0001] IMAGE SENSOR AND METHOD OF OPERATING AN IMAGE SENSOR

[0002] BACKGROUND

[0003] CMOS image sensors comprising a photosensitive element and a readout circuit are increasingly employed in a variety of applications . Generally, attempts are made to reduce the area needed for the image sensor .

[0004] It is an obj ect of the present invention to provide an improved image sensor and an improved method for operating an image sensor .

[0005] SUMMARY

[0006] According to embodiments , the above obj ect is achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

[0007] An image sensor may comprise a photosensitive element , configured to generate charges from incident electromagnetic radiation, a trans fer transistor electrically coupled to the photosensitive element , and a pixel circuit electrically coupled to the trans fer transistor . The trans fer transistor is configured to control a trans fer of the charges from the photosensitive element to the pixel circuit . The image sensor further comprises a first voltage supply for supplying a first low voltage level higher than OV to a terminal of the photosensitive element , and a second voltage supply configured to supply a second low voltage level and a second high voltage level to the trans fer transistor and the pixel circuit . For example , the second voltage supply may be configured to generate a second low voltage level of OV or more than OV . Further, the first voltage supply may be configured to generate a first low voltage level of at least 0 . 6V . For example , the second low voltage level may be less than the first low voltage level .

[0008] According to embodiments , the image sensor may further comprise a first semiconductor substrate and a second semiconductor substrate , wherein the first semiconductor substrate is vertically stacked over the second semiconductor substrate . For example , the photosensitive element and the first voltage supply may be formed in the first semiconductor substrate . Further, the trans fer transistor, the pixel circuit and the second voltage supply may be formed in the second semiconductor substrate .

[0009] The image sensor may further comprise a floating di f fusion and a source follower . An output of the trans fer transistor may be electrically coupled to the floating di f fusion, and the floating di f fusion may be coupled to a gate electrode of the source follower .

[0010] According to embodiments , the image sensor further comprises a selection transistor configured to electrically couple an output of the source follower to a column output .

[0011] For example , the image sensor may further comprise a reset transistor configured to reset a state of the floating di f fusion .

[0012] Further embodiments are directed to a method of operating an image sensor comprising a photosensitive element , configured to generate charges from incident electromagnetic radiation, a trans fer transistor electrically coupled to the photosensitive element , and a pixel circuit electrically coupled to the trans fer transistor . The method may comprise connecting a terminal of the photosensitive element to a first low voltage level higher than 0 V, and controlling a trans fer of the charges from the photosensitive element to the pixel circuit . For example , controlling the trans fer comprises switching a voltage applied to a gate electrode of the trans fer transistor between a second low voltage level of 0V or higher and a second high voltage level .

[0013] For example , the second low voltage level may be less than the first low voltage level .

[0014] Further embodiments relate to an electronic device comprising the image sensor as described above . The electronic device may be selected from a video camera, a still camera and a smart phone .

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this speci fication . The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detai led description . The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts .

[0017] Fig . 1 is a schematic equivalent circuit diagram of a portion of an image sensor according to embodiments . Fig. 2 is a cross-sectional view of an image sensor according to embodiments.

[0018] Fig. 3 is a schematic equivalent circuit diagram of an image sensor according to embodiments.

[0019] Fig. 4 summarizes a method according to embodiments.

[0020] Fig. 5 is a schematic drawing of an electronic device according to embodiments.

[0021] DETAILED DESCRIPTION

[0022] In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims.

[0023] The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments.

[0024] Fig. 1 shows an equivalent circuit diagram of a pixel 10 forming part of an image sensor according to embodiments. As is shown, the pixel 10 illustrated in Fig . 1 comprises a photosensitive element 110 that is configured to generate charges from incident electromagnetic radiation 15 . For example , the photosensitive element 110 may comprise a photodiode , for example , a pinned photodiode or any other suitable photosensitive element .

[0025] The image sensor further comprises a pixel circuit 120 and a trans fer transistor 112 . The trans fer transistor 112 is electrically coupled to the photosensitive element 110 . Moreover, the pixel circuit is electrically coupled to the trans fer transistor 112 . The trans fer transistor 112 is configured to control a trans fer of the charges from the photosensitive element 110 to the pixel circuit 120 . The image sensor 20 further comprises a first voltage supply 115 for supplying a first low voltage level 116 which is higher than 0 V . The first low voltage level 116 i s supplied to a terminal of the photosensitive element 110 .

[0026] The image sensor further comprises a second voltage supply 117 which is configured to supply a second low voltage level 118 and a second high voltage level 119 . The second low voltage level 118 and the second high voltage level 119 are configured to be supplied to the trans fer transistor 112 and to further elements of the pixel circuit 120 . According to embodiments , the pixel circuit 120 comprises a floating di f fusion 121 which is electrically coupled to the trans fer transistor 112 . According to implementations , the floating di f fusion 121 may be implemented as or may comprise a capacitor and may implement a sense node . By applying a corresponding voltage to the gate electrode 113 of the trans fer transistor 112 , charges generated by the photosensitive element 110 may be trans ferred to the floating di f fusion 121 .

[0027] Generally, a low voltage level applied to the gate electrode 113 of the trans fer transistor 112 needs to be lower than a low level applied to a terminal of the photosensitive element, in order to appropriately control the transfer transistor, for providing a certain full well in electrons and for providing a certain low dark current. According to embodiments, since the first low level 116 provided by the first voltage supply 115 is higher than 0 V, a second low level 118 supplied by the second voltage supply 117 may be larger than 0 V. In other words, the voltage levels applied as the low voltage level are shifted to a range above 0 V. As a result, for example, a charge pump for supplying a negative low voltage level to the gate electrode 113 of the transfer transistor 112 may be dispensed with.

[0028] As is indicated in Fig. 1, a voltage applied to the gate electrode 113 of the transfer transistor 112 is ramped between the second low level 118 and the second high level. As is further illustrated in Fig. 1, for example a terminal of a capacitor forming part of the floating diffusion 121 may also be electrically coupled to the second low level 118.

[0029] For example, the second voltage supply 117 may be configured to generate a second low level voltage of 0 V or more than 0 V.

[0030] Moreover, the first voltage supply 115 may be configured to generate a first low voltage level 116 of at least 0.6 V or at least 0.8 V. For example, the first voltage supply 115 may be implemented as a small LDO ("low-dropout regulator") or as an on-chip DAC (digital / analog converter) . Usually, no current flows when the low voltage level 116 is applied. Consequently, the first voltage supply 115 may be designed at a comparatively small size. The second low voltage level 118 supplied by the second voltage supply 117 may be smaller than the first low voltage level 116 supplied by the first voltage supply 115. The pixel circuit 120 may further comprise a source follower 122. For example, a gate electrode 123 of the source follower 122 may be electrically coupled to the floating diffusion 121.

[0031] The image sensor 20 may further comprise a selection transistor 124. A voltage applied to a gate electrode of the selection transistor 124 may be ramped between the second low voltage level 118 and the second high voltage level 119. The selection transistor 124 may be configured to electrically couple an output of the source follower 122 to a column output 125. The pixel circuit 120 may further comprise a reset transistor 126. A voltage applied to a gate electrode of the reset transistor 126 may be ramped between the second low level 118 and the second high level 119. The reset transistor 126 may be configured to reset a state of the floating diffusion 121.

[0032] As is clearly to be understood, the described example of the pixel circuit 120 is by way of example. According to further embodiments, different types of pixel circuits 120 may be employed. Moreover, the pixel circuit 120 may comprise further elements for performing further functions. Moreover, according to further embodiments, pixel circuits 120 may be shared among e.g. adjacent photosensitive elements 110.

[0033] For example, according to embodiments, the first low voltage level 116 supplied by the first voltage supply 115 may be about 0.8 V. As a consequence, in comparison with conventional image sensors, the second high voltage level 119 supplied by the second voltage supply 117 which is e.g. the upper limit of the ramping process may be shifted by 0.8 V. Accordingly, instead of e.g. 2.5 V which may be usually used, 3.3 V may be employed as the second high voltage level 119. Moreover, instead of a negative voltage of e.g. -0.8 V, e.g. 0 V may be taken as the second low voltage level 118 supplied by the second voltage supply 117. Due to the use of a higher external voltage supplied by the second voltage supply 117 , there is no need to supply a negative voltage to the trans fer transistor .

[0034] Fig . 2 shows a cross-sectional view of an image sensor 20 according to embodiments .

[0035] The image sensor 20 comprises a first semiconductor substrate 100 and a second semiconductor substrate 102 , wherein the first semiconductor substrate 100 is vertically stacked over the second semiconductor substrate 102 . For example , the first and the second semiconductor substrates may be made of the same materials . According to further embodiments , the first and the second semiconductor substrates 100 , 102 may be made of di f ferent materials . For example , the first and the second semiconductor substrates 100 , 102 may be made of silicon . For example , the first photosensitive element 110 or an array of photosensitive elements 110 may be formed in the first semiconductor substrate 100 . Further, the first voltage supply 115 may be formed in the first semiconductor substrate 100 .

[0036] Moreover, the pixel circuit 120 or an array of pixel circuits 120 may be arranged in the second semiconductor substrate 102 . For example , the photosensitive element 110 may be electrically connected to a corresponding one of the pixel circuits 120 by a suitable wiring 105 . For example , the first semiconductor substrate 100 and the second semiconductor substrate 102 may be bonded, e . g . using hybrid bonding . The second voltage supply 117 is formed in the second semiconductor substrate 102 . Accordingly, each of the two semiconductor substrates 100 , 102 has its own ground potential as is indicated in Fig . 2 . The ground potential is supplied by the first voltage supply 115 and the second voltage supply 117 , respectively . Fig. 3 shows a schematic diagram of an image sensor 20. The image sensor 20 comprises an array of pixels 10. For example, each of the pixels 10 may be implemented in the manner as has been described with reference to Fig. 1. Electrical signals generated e.g. in the pixel circuits 120 of an associated pixel 10 may be output using the column output 125. The image sensor 20 may further comprise a row selection line driver 150 for selecting a line to be read out. The image sensor 20 may further comprise a column processing device 152, e.g. including ADCs and a pixel data processing device 153. As a result of the processing by the column processing device 152 and the pixel data processing device 153, image data 17 may be output.

[0037] Fig. 4 summarizes a method of operating an image sensor according to embodiments. The image sensor comprises a photosensitive element which is configured to generate charges from incident electromagnetic radiation and a transfer transistor electrically coupled to the photosensitive element. The image sensor further comprises a pixel circuit electrically coupled to the transfer transistor. The method comprises connecting (S100) a terminal of the photosensitive element to a first low voltage level higher than 0 V and controlling (S110) a transfer of the charges from the photosensitive element to the pixel circuit. Controlling the transfer (S110) comprises switching (S115) a voltage applied to a gate electrode of the transfer transistor between a second low level of 0 V or higher and a second high voltage level.

[0038] Fig. 5 is a schematic drawing of an electronic device 30 according to embodiments. The electronic device 30 comprises the image sensor 20 that has been described herein above. For example, the electronic device 30 may be a still camera, a video camera or a smart phone . As has been described, due to the use of a first voltage supply and a second voltage supply, wherein the first voltage supply is configured to supply a first low voltage level of larger than 0 V, a charge pump for supplying a negative voltage to the gate terminal of the trans fer transistor may be dispensed with . As a result , a si ze of the image sensor may be further reduced .

[0039] While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

[0040] LIST OF REFERENCES pixel electromagnetic radiation image data image sensor electronic device first semiconductor substrate second semiconductor substrate wiring photosensitive element trans fer transistor gate electrode of trans fer transistor first voltage supply first low voltage level second voltage supply second low voltage level second high voltage level pixel circuit floating di f fusion source follower gate electrode of source follower selection transistor column output reset transistor row selection line driver column processing device pixel data processing device

Claims

CLAIMS1. An image sensor (20) comprising: a photosensitive element (110) , configured to generate charges from incident electromagnetic radiation (15) ; a transfer transistor (112) electrically coupled to the photosensitive element (110) ; and a pixel circuit (120) electrically coupled to the transfer transistor (112) , wherein the transfer transistor (112) is configured to control a transfer of the charges from the photosensitive element (110) to the pixel circuit (120) , the image sensor (20) further comprising a first voltage supply (115) for supplying a first low voltage level (116) higher than 0V to a terminal of the photosensitive element (110) ; and a second voltage supply (117) configured to supply a second low voltage level (118) and a second high voltage level(119) to the transfer transistor (112) and the pixel circuit(120) .

2. The image sensor (20) according to claim 1, wherein the second voltage supply (117) is configured to generate a second low voltage level (118) of 0V or more than 0V.

3. The image sensor (20) according to claim 1 or 2, wherein the first voltage supply (115) is configured to generate a first low voltage level (116) of at least 0.6V.

4. The image sensor (20) according to any of the preceding claims, wherein the second low voltage level (118) is less than the first low voltage level (116) .

5. The image sensor (20) according to any of the preceding claims, further comprising a first semiconductor substrate (100) and a second semiconductor substrate (102) , wherein the first semiconductor substrate (100) is vertically stacked over the second semiconductor substrate (102) , wherein the photosensitive element (110) and the first voltage supply (115) are formed in the first semiconductor substrate (100) , and the transfer transistor (112) , the pixel circuit (120) and the second voltage supply (117) are formed in the second semiconductor substrate (102) .

6. The image sensor (20) according to any of the preceding claims, further comprising a floating diffusion (121) and a source follower (122) , wherein an output of the transfer transistor (112) is electrically coupled to the floating diffusion (121) , and the floating diffusion (121) is coupled to a gate electrode of the source follower (122) .

7. The image sensor (20) according to claim 6, wherein the floating diffusion (121) is implemented by a capacitor, one terminal of the capacitor being coupled to the gate electrode of the source follower (122) , another terminal of the capacitor being disconnected from the first voltage supply (115) .

8. The image sensor (10) according to claim 6 or 7, further comprising a selection transistor (124) configured to electrically couple an output of the source follower (112) to a column output (125) .

9. The image sensor (10) according to any of claims 6 to 8, further comprising a reset transistor (126) configured to reset a state of the floating diffusion (121) .

10. A method of operating an image sensor (20) comprising a photosensitive element (110) , configured to generate charges from incident electromagnetic radiation (15) ; a transfer transistor (112) electrically coupled to the photosensitive element (110) ; and a pixel circuit (120) electrically coupled to the transfer transistor (112) , the method comprising connecting (S100) a terminal of the photosensitive element (110) to a first low voltage level (116) higher than 0 V, and controlling (S110) a transfer of the charges from the photosensitive element (110) to the pixel circuit (120) , wherein controlling (S110) the transfer comprises switching (S115) a voltage applied to a gate electrode of the transfer transistor (112) between a second low voltage level (118) of 0V or higher and a second high voltage level (119) .

11. The method according to claim 10, wherein the second low voltage level (118) is less than the first low voltage level (116) .

12. An electronic device (30) comprising the image sensor (20) according to any of claims 1 to 9.

13. The electronic device (30) according to claim 12, being selected from a video camera, a still camera and a smart phone .

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