The present invention relates generally to an image sensor and, more particularly, to a pixel structure used in an image sensor.
Solid-state image sensors have found widespread use in camera systems. The solid-state imager sensors in some camera systems are composed of a matrix of photosensitive elements in series with switching and amplifying elements. The photosensitive sensitive elements may be, for example, photoreceptors, photo-diodes, phototransistors, charge-coupled device (CCD) gate, or alike. Each photosensitive element receives an image of a portion of a scene being imaged. A photosensitive element along with its accompanying electronics is called a picture element or pixel. The image obtaining photosensitive elements produce an electrical signal indicative of the light intensity of the image. The electrical signal of a photosensitive element is typically a current, which is proportional to the amount of electromagnetic radiation (light) falling onto that photosensitive element.
Of the image sensors implemented in a complementary metal-oxide-semiconductor (CMOS)- or MOS-technology, image sensors with passive pixels and image sensors with active pixels are distinguished. The difference between these two types of pixel structures is that an active pixel amplifies the charge that is collect on its photosensitive element. A passive pixel does not perform signal amplification and requires a charge sensitive amplifier that is not integrated in the pixel.
The pixel structure of
Before that, however, the switch, or pre-charge, transistor briefly unloads the Cmem capacitor. The voltage (Vmem) applied to the back plate of Cmem may be a fixed voltage, but in practice, a varying voltage for Vmem may help to shift the voltage on the memory node, so as to drive the source follower transistor M2 in a more suitable regime. A potential problem with the pixel structure illustrated in
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which:
A pixel structure having reduced leakage is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.
Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. Any of the signals provided over various buses described herein may be time multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit components or blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be one or more single signal lines, and each of the single signal lines may alternatively be buses.
The pixel matrix 1020 may be arranged in N rows of pixels by N columns of pixels (with N≧1), with each pixel (e.g., pixel 300) is composed of at least a photosensitive element and a readout switch (not shown). A pixel matrix is known in the art; accordingly, a more detailed description is not provided.
The Y-addressing scan register(s) 1030 addresses all pixels of a row (e.g., row 1022) of the pixel matrix 1020 to be read out, whereby all selected switching elements of pixels of the selected row are closed at the same time. Therefore, each of the selected pixels places a signal on a vertical output line (e.g., line 1023), where it is amplified in the column amplifiers 1040. An X-addressing scan register(s) 1035 provides control signals to the analog multiplexer 1045 to place an output signal (amplified charges) of the column amplifiers 1045 onto output bus 1046. The output bus 1046 may be coupled to a buffer 1048 that provides a buffered, analog output 1049 from the imaging core 1010.
The output 1049 from the imaging core 1010 is coupled to an analog-to-digital converter (ADC) 1050 to convert the analog imaging core output 1049 into the digital domain. The ADC 1050 is coupled to a digital processing device 1060 to process the digital data received from the ADC 1050 (such processing may be referred to as imaging processing or post-processing). The digital processing device 1060 may include one or more general-purpose processing devices such as a microprocessor or central processing unit, a controller, or the like. Alternatively, digital processing device 1060 may include one or more special-purpose processing devices such as a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Digital processing device 1060 may also include any combination of a general-purpose processing device and a special-purpose processing device.
The digital processing device 1060 is coupled to an interface module 1070 that handles the information input/output (I/O) exchange with components external to the image sensor 1000 and takes care of other tasks such as protocols, handshaking, voltage conversions, etc. The interface module 1070 may be coupled to a sequencer 1080. The sequencer 1080 may be coupled to one or more components in the image sensor 1000 such as the imaging core 1010, digital processing device 1060, and ADC 1050. The sequencer 1080 may be a digital circuit that receives externally generated clock and control signals from the interface module 1070 and generates internal signals to drive circuitry in the imaging core 1010, ADC 1050, etc. In one embodiment, the voltage supplies that generate the control signals used to control the various components in the pixel structure of
It should be noted that the image sensor illustrated in
The reset transistor 310 of the light detecting stage 301 is used to reset the pixel to a high value using a voltage applied to gate 312. The corresponding voltage (i.e., the voltage on gate 312 minus a gate-to-source threshold voltage VT of reset transistor 310) applied to the gate 322 of the source follower transistor M1 starts dropping due to the photocurrent generated in the photodiode 305. The source follower transistor M1320 operates as a unity gain amplifier to buffer the signal from the photodiode 305. The output (i.e., source) of transistor M1320 is coupled to the sample and hold stage 302. The sample and hold stage 302 “sample” loads the voltage signal of source follower transistor M1320, through the sample transistor 330, on the front plate 351 of memory capacitor Cmem 350. The voltage signal from the source 323 of the source follower transistor M1320 will remain on the memory capacitor 350 when the sample transistor 330 is turned off. In one exemplary embodiment, Cmem may have a value approximately in a range of 20 to 100 fF. Alternatively, other values of Cmem may be used.
Before sample loading, a pre-charge transistor 340 is used to briefly unload the memory capacitor Cmem 350. The pre-charge transistor 340 is coupled between the source 333 of the sample transistor 330 and the back plate 352 of the memory capacitor 350. A voltage (Vmem) 355 is applied to the back plate 352 of Cmem 350. In one embodiment, Vmem may be a fixed voltage that is not zero. Alternatively, a varying voltage may be used to shift the voltage Vmem 355 on the memory node, so as to drive the source follower transistor M2360 in a more suitable regime. As a source follower induces a downward voltage shift of one Vth, the useful input range at the gate of a source follower (i.e., M2360. The signal level that is at the output of M1 is between (VDD−2*Vth) and zero (GND). As VDD is tends to become lower and lower in modern technologies, the following estimation may be made: VDD may be 1.8V, Vth may be 0.5V. Then, the output range of M1 is between 0 and 0.8 (1.8−2*0.5), and the input range of M2 is between 0.5 and 1.8. The overlap (i.e., the practical useful range) is thus from 0.5 to 0.8, which is very small. If we shift the M1 output range at least 0.5V up (to 0.5-1.3, or higher), a much larger overlap of the ranges is created: the whole M1 output range is now acceptable for M2. It should be noted that in other embodiments, the source follower transistor M2360 may not be used or, alternatively, a non-unity gain amplifier may be used.
In this embodiment, the pre-charge transistor 340 is a MOSFET with its source 343 coupled to the back plate 352 of the memory capacitor 350. The pre-charge MOSFET 340 is operated in a fashion that it has a lower (i.e. negative) VGS with respect to the low supply voltage VSS node (e.g., ground) of the pixel 300. In this manner, the drain 344 current of the pre-charge MOSFET 340 is several orders of magnitude lower than with VGS=0. This is realized by coupling the source 343 of the pre-charge transistor 340 to a slightly positive voltage compare to the gate 342. In one embodiment (not illustrated), this achieved by using a signal line that is coupled to the source 343 of the pre-charge transistor 340. However, the use of an additional (i.e., not otherwise used by the pixel structure) interconnection may be expensive in terms of requiring more dies area in which the pixel is implement. In one embodiment, VSS node is configured to receive the low supply voltage from outside of the pixel structure through the substrate (which may constitute the backside of the photodiode and/or the bulk of the n-channel MOSFETS) via the substrate potential. Alternatively, VSS node may be configured to receive the low supply voltage through a dedicated signal trace.
In the embodiment illustrated in
In one embodiment, six transistors may be used to implement the pixel structure described in regards to
An advantage to the pixel structures discussed above in regards to
As noted above, in an alternative embodiment, the pixel structure have other configurations to make VGS 341 of the pre-charge transistor 340 slightly negative with respect to the low supply voltage VSS (e.g., GND) of the pixel 300, for example, a physical line (i.e., trace) may be routed to the source 343 of the pre-charge transistor 340.
Although the low supply voltage VSS has been discussed at times in relation to a ground potential for ease of explanation, in alternative embodiment, other low supply voltage potentials may be used.
Another advantage to the pixel structure discussed herein is that there is a voltage level shift that brings the signal in a range that is more suitable for the source follower M2360.
Embodiments of the present have been illustrated with MOS technology for ease of discussion. In alternative embodiments, other device types and process technologies may be used, for example, Bipolar and BiCMOS. It should be noted that the circuits described herein may be designed utilizing various voltages.
The image sensor and pixel structures discussed herein may be used in various applications including, but not limited to, a digital camera system, for example, for general-purpose photography (e.g., camera phone, still camera, video camera) or special-purpose photography (e.g., in automotive systems, hyperspectral imaging in space borne systems, etc). Alternatively, the image sensor and pixel structures discussed herein may be used in other types of applications, for example, machine and robotic vision, document scanning, microscopy, security, biometry, etc.
Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.
This application claims the benefit of U.S. Provisional Application No. 60/666,124, filed on Mar. 28, 2005.
Number | Name | Date | Kind |
---|---|---|---|
3770968 | Hession et al. | Nov 1973 | A |
3904818 | Kovac | Sep 1975 | A |
4148048 | Takemoto et al. | Apr 1979 | A |
4253120 | Levine | Feb 1981 | A |
4373167 | Yamada | Feb 1983 | A |
4389661 | Yamada | Jun 1983 | A |
4473836 | Chamberlain | Sep 1984 | A |
4484210 | Shiraki et al. | Nov 1984 | A |
4498013 | Kuroda et al. | Feb 1985 | A |
4565756 | Needs et al. | Jan 1986 | A |
4580103 | Tompsett | Apr 1986 | A |
4630091 | Kuroda et al. | Dec 1986 | A |
4647975 | Alston et al. | Mar 1987 | A |
4696021 | Kawahara | Sep 1987 | A |
4703169 | Arita | Oct 1987 | A |
4774557 | Kosonocky | Sep 1988 | A |
4814848 | Akimoto et al. | Mar 1989 | A |
4831426 | Kimata et al. | May 1989 | A |
4914493 | Shiromizu | Apr 1990 | A |
4951105 | Yamada | Aug 1990 | A |
4984044 | Yamamura | Jan 1991 | A |
4984047 | Stevens | Jan 1991 | A |
4998265 | Kimata | Mar 1991 | A |
5084747 | Miyawaki | Jan 1992 | A |
5101253 | Mizutani et al. | Mar 1992 | A |
5128534 | Wyles et al. | Jul 1992 | A |
5144447 | Akimoto et al. | Sep 1992 | A |
5146074 | Kawahara et al. | Sep 1992 | A |
5153420 | Hack et al. | Oct 1992 | A |
5164832 | Halvis et al. | Nov 1992 | A |
5182623 | Hynecek | Jan 1993 | A |
5191398 | Mutoh | Mar 1993 | A |
5258845 | Kyuma et al. | Nov 1993 | A |
5283428 | Morishita et al. | Feb 1994 | A |
5296696 | Uno | Mar 1994 | A |
5307169 | Nagasaki et al. | Apr 1994 | A |
5321528 | Nakamura | Jun 1994 | A |
5329112 | Mihara | Jul 1994 | A |
5335008 | Hamasaki | Aug 1994 | A |
5436949 | Hasegawa et al. | Jul 1995 | A |
5461425 | Fowler et al. | Oct 1995 | A |
5496719 | Miwada | Mar 1996 | A |
5519207 | Morimoto | May 1996 | A |
5528643 | Hynecek | Jun 1996 | A |
5576763 | Ackland et al. | Nov 1996 | A |
5578842 | Shinji | Nov 1996 | A |
5587596 | Chi et al. | Dec 1996 | A |
5608204 | Hofflinger et al. | Mar 1997 | A |
5608243 | Chi et al. | Mar 1997 | A |
5614744 | Merrill | Mar 1997 | A |
5625210 | Lee et al. | Apr 1997 | A |
5668390 | Morimoto | Sep 1997 | A |
5675158 | Lee | Oct 1997 | A |
5710446 | Chi et al. | Jan 1998 | A |
5714753 | Park | Feb 1998 | A |
5721425 | Merrill | Feb 1998 | A |
5754228 | Dyck | May 1998 | A |
5786607 | Ishikawa et al. | Jul 1998 | A |
5812191 | Orava et al. | Sep 1998 | A |
5828091 | Kawai | Oct 1998 | A |
5841126 | Fossum | Nov 1998 | A |
5841159 | Lee et al. | Nov 1998 | A |
5861621 | Takebe et al. | Jan 1999 | A |
5872371 | Guidash et al. | Feb 1999 | A |
5872596 | Yanai | Feb 1999 | A |
5886353 | Spivey et al. | Mar 1999 | A |
5898168 | Gowda et al. | Apr 1999 | A |
5898196 | Hook et al. | Apr 1999 | A |
5903021 | Lee et al. | May 1999 | A |
5904493 | Lee et al. | May 1999 | A |
5933190 | Dierickx | Aug 1999 | A |
5952686 | Chou et al. | Sep 1999 | A |
5953060 | Dierickx | Sep 1999 | A |
5955753 | Takahashi | Sep 1999 | A |
5956570 | Takizawa | Sep 1999 | A |
5973375 | Baukus et al. | Oct 1999 | A |
5977576 | Hamasaki | Nov 1999 | A |
5990948 | Sugiki | Nov 1999 | A |
6011251 | Dierickx et al. | Jan 2000 | A |
6040592 | McDaniel et al. | Mar 2000 | A |
6043478 | Wang | Mar 2000 | A |
6051857 | Miida | Apr 2000 | A |
6100551 | Lee et al. | Aug 2000 | A |
6100556 | Drowley et al. | Aug 2000 | A |
6107655 | Guidash | Aug 2000 | A |
6111271 | Snyman et al. | Aug 2000 | A |
6115066 | Gowda et al. | Sep 2000 | A |
6133563 | Clark et al. | Oct 2000 | A |
6133954 | Jie et al. | Oct 2000 | A |
6136629 | Sin | Oct 2000 | A |
6137100 | Fossum et al. | Oct 2000 | A |
6166367 | Cho | Dec 2000 | A |
6188093 | Isogai et al. | Feb 2001 | B1 |
6194702 | Hook et al. | Feb 2001 | B1 |
6204524 | Rhodes | Mar 2001 | B1 |
6225670 | Dierickx | May 2001 | B1 |
6239456 | Berezin et al. | May 2001 | B1 |
6316760 | Koyama | Nov 2001 | B1 |
6459077 | Hynecek | Oct 2002 | B1 |
6545303 | Scheffer | Apr 2003 | B1 |
6570618 | Hashi | May 2003 | B1 |
6631217 | Funatsu et al. | Oct 2003 | B1 |
6636261 | Pritchard et al. | Oct 2003 | B1 |
6778214 | Toma | Aug 2004 | B1 |
6812539 | Rhodes | Nov 2004 | B1 |
6815791 | Dierickx | Nov 2004 | B1 |
6825455 | Schwarte | Nov 2004 | B1 |
6836291 | Nakamura et al. | Dec 2004 | B1 |
6906302 | Drowley | Jun 2005 | B2 |
6975356 | Miyamoto | Dec 2005 | B1 |
7199410 | Dierickx | Apr 2007 | B2 |
7230289 | Komori | Jun 2007 | B2 |
7253019 | Dierickx | Aug 2007 | B2 |
7402881 | Kuwazawa | Jul 2008 | B2 |
7557845 | Kuwazawa | Jul 2009 | B2 |
20020022309 | Dierickx | Feb 2002 | A1 |
20030011694 | Dierickx | Jan 2003 | A1 |
20070145503 | Dierickx | Jun 2007 | A1 |
Number | Date | Country |
---|---|---|
2132629 | Sep 1993 | CA |
0260954 | Mar 1988 | EP |
0548987 | Jun 1993 | EP |
0635973 | Jan 1995 | EP |
0657863 | Jun 1995 | EP |
0739039 | Oct 1996 | EP |
0773669 | May 1997 | EP |
0632930 | Jul 1998 | EP |
0858111 | Aug 1998 | EP |
0858212 | Aug 1998 | EP |
0883187 | Dec 1998 | EP |
0903935 | Mar 1999 | EP |
0978878 | Feb 2000 | EP |
2324651 | Oct 1998 | GB |
01-204579 | Aug 1989 | JP |
02-050584 | Feb 1990 | JP |
04088672 | Mar 1992 | JP |
04-207589 | Jul 1992 | JP |
05-030433 | Feb 1993 | JP |
06-284347 | Oct 1994 | JP |
07-072252 | Mar 1995 | JP |
09321266 | Dec 1997 | JP |
11-313257 | Sep 1999 | JP |
9304556 | Mar 1993 | WO |
9319489 | Sep 1993 | WO |
9810255 | Mar 1998 | WO |
9916268 | Apr 1999 | WO |
WO 9930368 | Jun 1999 | WO |
0055919 | Sep 2000 | WO |
Number | Date | Country | |
---|---|---|---|
60666124 | Mar 2005 | US |