US20080265242A1 - Cmos image sensor with enhanced photosensitivity - Google Patents
Cmos image sensor with enhanced photosensitivity Download PDFInfo
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- US20080265242A1 US20080265242A1 US11/739,650 US73965007A US2008265242A1 US 20080265242 A1 US20080265242 A1 US 20080265242A1 US 73965007 A US73965007 A US 73965007A US 2008265242 A1 US2008265242 A1 US 2008265242A1
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- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14601—Structural or functional details thereof
- H01L27/14632—Wafer-level processed structures
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- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
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- H01L27/14643—Photodiode arrays; MOS imagers
- H01L27/14645—Colour imagers
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- H—ELECTRICITY
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
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- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
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- H01L27/14643—Photodiode arrays; MOS imagers
- H01L27/14658—X-ray, gamma-ray or corpuscular radiation imagers
- H01L27/14663—Indirect radiation imagers, e.g. using luminescent members
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- H—ELECTRICITY
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14683—Processes or apparatus peculiar to the manufacture or treatment of these devices or parts thereof
- H01L27/14696—The active layers comprising only AIIBVI compounds, e.g. CdS, ZnS, CdTe
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- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
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Definitions
- the present invention relates generally to semiconductor image sensors and more particularly to an image sensor structure with enhanced photosensitivity.
- CMOS image sensor which comprises arrays of active MOS image sensor cells that are produced in a CMOS process, is one of the typical image sensing devices that utilize the photoconductive characteristics of the reverse-biased P-N junction structure.
- FIG. 1 illustrates a conventional 3 T CMOS image sensor cell 100 which comprises a P-N junction diode 110 , a reset NMOS transistor 120 , an amplifier NMOS transistor 130 and a row select NMOS transistor 140 .
- the P-N junction diode 110 which serves as a photo-detector, and the reset NMOS transistor 120 are serially connect between a power supply VRST and a ground (GND).
- the reset MOS transistor 120 is turned on by the RST signal, the P-N junction diode 110 is effectively connected to the VRST and reverse biased.
- an additional combination current generated by photon created electron-hole pairs cause a voltage drop at node VC.
- the voltage drop is then amplified by the NMOS transistor 130 , which has a power supply VDD.
- the VDD is traditionally tied to the VRST.
- the row select NMOS transistor 140 is coupled between the amplifier NMOS transistor 130 and a column line (COL).
- a row line (ROW) is connected to a gate of the NMOS transistor 140 . Therefore, the row select NMOS transistor 140 is a switch that allows a signal row of an array the CMOS image sensor cells 100 to be read by a read-out circuit.
- the aforementioned combination current flowing through the P-N junction diode 110 is proportional to the intensity of the light, therefore the read-out voltage and/or current at the COL is also proportional to the intensity of the light.
- FIG. 2 is a cross-sectional view of such CMOS image sensor 100 forming an array of cells 200 in a semiconductor substrate 210 .
- the P-N junction diode 110 and NMOS transistors 120 , 130 and 140 are formed in the substrate 210 .
- a passivation layer 220 is applied on the substrate 210 .
- a planarization layer 230 is processed on top of the passivation layer 220 to make the semiconductor surface flat, for subsequent applications of a color filter 240 , a spacer 250 and micro-lenses 260 . All these layers 220 through 260 merely pass the light to the substrate 210 , where the P-N junction diode 110 is the only device that has the photoconductive effect. Therefore, the conventional CMOS image sensor cell 200 has only mediocre optical sensitivity and signal-to-noise ratio.
- the present invention discloses a CMOS image sensor with enhanced photosensitivity.
- the CMOS image sensor has a photosensitive device, which includes a semiconductor substrate, at least one reverse biased device, such as a P-N junction diode formed in the semiconductor substrate, and at least one photosensitive layer disposed above the semiconductor substrate and substantially covering the reverse biased device, the photosensitive layer releasing electrons and holes when struck by photons, wherein the photon generated electrons and holes in the photosensitive layer reach the reverse biased device and create a combination current therein when a light shines thereon.
- the photosensitive device further includes a transparent insulation layer interposed between the photosensitive layer and the semiconductor substrate.
- FIG. 1 is a schematic diagram illustrating a conventional CMOS image sensor cell.
- FIG. 2 is a cross-sectional view of such conventional CMOS image sensor cells formed in a semiconductor substrate.
- FIGS. 3A and 3B are cross-sectional views of photosensitive P-N junction diodes according to embodiments of the present invention.
- FIGS. 4A ⁇ 4D are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention.
- FIGS. 5A through 5B are cross-sectional views of CMOS image sensor cells with photosensitive layers applied according to embodiments of the present invention.
- This invention describes a CMOS image sensor cell with a photoconductive layer for improving its sensitivity to a change of illumination.
- the following merely illustrates the various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention.
- FIGS. 3A and 3B are cross-sectional views of P-N junction diodes 300 and 350 covered by photo sensitive thin films 330 and 360 according to embodiments of the present invention.
- the photo sensitive or photo conductive means extra carriers are generated when light is radiated into the film 330 and 360 .
- the P-N junction diode comprises an N-type region 310 disposed inside a P-type semiconductor substrate (Psub) 320 .
- An N+ region 312 serves as a pick-up for the N-type region 310 .
- a P+region 322 serves as a pick-up for the Psub 320 . If the P-N junction is equivalent to the diode 110 of FIG.
- the Psub 320 is coupled to a ground (GND) through the P+ region 322
- the N-type region 310 is coupled to node VC through the N+ region 312 .
- light (photon) shines on the P-N junction 300
- electrons and holes will be generated in the N-type region 310 , and then combined in a photon collection region 315 .
- a combination current will then be the CMOS sensor cell 100 shown in FIG. 1 .
- the N-type region 310 is coated by the photo sensitive thin film 330 , which serves two purposes.
- a first purpose is to convert incoming invisible lights, such as X-ray or UV, into visible light, as CMOS image sensors are typically designed to sense only visible lights. When the invisible lights shine on the photo sensitive thin film 330 , electrons in the thin film 330 are incited into higher energy levels by the incoming photons and then emit other photons after falling back into the ground states.
- the thin film 330 is made of phosphor.
- the P-N junction diode 300 can sense the incoming X-ray light.
- One having ordinary skills in the art would choose other materials for the thin film 330 when other incoming lights need to be converted.
- a second purpose of the photo sensitive thin film 330 is to generate electron-hole pairs when light shines on it. These generated electrons and holes will then tunnel through the thin oxide 335 and induce more combinations of electrons and holes in the photon collection region 315 .
- the thin film 330 is made of a semiconductor material, such as phosphor, it is therefore transparent to light. Meanwhile the thin oxide 335 is also transparent. Both the photo sensitive thin film 330 and the P-N junction itself can generate electron-hole pairs when shined by light, therefore current generation efficiency of the P-N junction diode 300 is improved by adding the photo sensitive thin film 330 .
- the thin film 330 may convert photons otherwise invisible to the P-N junction diode 300 to visible wavelengths, there will be more photons in the incoming light to generate electron-hole pairs, and the light sensitivity of the P-N junction diode 300 is further enhanced.
- the thickness of the thin oxide 335 can be adjusted to control the tunneling rate for the electron-hole pairs generated by the photoconductive layer 330 .
- the thickness of the thin oxide 335 is preferably thinner than 100 angstroms.
- One having skills in the art may also recognize other dielectric materials may also be used in place of the thin oxide 335 .
- a bias voltage VP may be applied to the thin film 330 .
- the photo sensitive thin film 300 may be selectively coated with lithograph patterning to cater to various needs.
- FIG. 3B shows another embodiment of the present invention with a photo sensitive thin film 360 overlays the N-type region 310 .
- the overlay alters the silicon surface potential, thus allows more carries to be stored in the depletion regions of the P-N junction discharging.
- the photo sensitive materials used for the thin film 330 or 360 are usually photoconductive semiconductor slab that generates carriers either by band-to-band transitions (intrinsic) or by transitions involving forbidden-gap energy levels (extrinsic).
- the photoconductive material may be selected from the group consisted of CdS, PbS, InSb, HgCdTe, GaAs, nickel-doped germanium (Ge—Ni) and phosphorus-doped silicon (Si—P). They can be pure material or in PN junction form with or without bias.
- CdS is commonly used as light-sensitive material in discrete devices for wavelength near 0.5 um.
- the resistance between two terminals of CdS film changes drastically when light shines on the surface.
- an HgCdTe photoconductor is preferred.
- a GaAs photoconductor is a better choice because of its higher detectivity.
- photoconductivity When light shines on a photoconductive material, such as Cds, amorphous silicon a-Si:H, etc., electron-hole pairs will be generated accordingly.
- the photoconductivity ⁇ ph is determined by the product of the free-carrier lifetime ⁇ and free-carrier mobility ⁇ :
- f is an average optical generation rate, which is the number of carriers generated by the photons absorbed per second and per unit volume.
- the ⁇ product depends on the property of photoconductive material. In general, the ⁇ product depends on the position of Fermi level Ef to the bandgap. The further the Fermi level is away from the midgap and closer to the conduction band edge Ec, the larger the ⁇ product.
- the mobility ⁇ was found independently of Fermi level, the photoconductivity ⁇ ph is therefore proportional to the recombination lifetime ⁇ .
- FIGS. 4A through 4D are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention.
- FIG. 4A illustrates a structure 400 in which addition photon generated electron-hole pairs come from a P-N junction formed by two layers 410 and 415 on top of an insulation layer 403 over a semiconductor substrate 410 . If layer 410 is an N-type, then layer 415 is a P-type, or vice versa. Photon generates electron-hole pairs in a depletion region at an interface of layers 410 and 415 . Traditional P-N junction is built in the semiconductor substrate 410 under the layers 410 and 415 .
- FIG. 4B illustrates a P-Intrinsic-N (P-I-N) structure 420 which is slightly different from the structure 400 in that a thin intrinsic layer 425 is deposited between two doped layers 430 and 435 . If layer 430 is an N-type, then layer 435 is a P-type, or vice versa. Here, the depletion region is the intrinsic layer 425 , the thickness of which can be tailored to optimize the photo sensitivity.
- P-I-N P-Intrinsic-N
- FIG. 4C illustrates a heterojunction structure 450 deposed on top of the insulator 403 over the semiconductor substrate 410 for generating additional electron-hole pairs.
- the heterojunction structure 450 comprises an intrinsic semiconductor layer 455 being sandwiched between two semiconductor blocking layers 460 and 465 of different material composition, such as GaAs or GaAsInP. These layers 455 , 460 and 465 have non-equal band gaps.
- the intrinsic semiconductor layer 455 generates electron-hole pairs when light shines on the structure 450 . These electrons and holes may tunnel through the thin insulation layer 403 and reach the semiconductor substrate 410 .
- FIG. 4D illustrates a Schottkey barrier formed by a photoconductor layer 475 , e.g., a-Si:H, being sandwiched between two layers 480 and 485 biased as electrodes.
- the electrode layers 480 and 485 provide a Schottkey barrier lowering effect for electron-hole pairs being generated in the photoconductor layer 475 by light. ⁇ If this is a Schottkey barrier. Just say so. ⁇ Then these electrons and holes may tunnel through the thin insulation layer 403 .
- the insulation layer 403 has to be thin enough to allow carriers to tunnel through and reach the semiconductor substrate 410 where the conventional P-N junction is formed therein. As a result of these tunneled-in carriers, a combination current in the photosensitive structure 400 , 420 , 450 or 470 will be larger than in a case that has only the conventional P-N junction.
- FIGS. 5A and 5B are cross-sectional views of CMOS image sensor cells 500 and 550 with photosensitive layers 505 and 555 applied, respectively, according to embodiments of the present invention.
- the CMOS image sensor cells 500 and 550 are very similar to the CMOS image sensor cells 200 as shown in FIG. 2 , where a conventional P-N junction diode (not shown) is formed in the substrate 210 .
- a passivation layer 220 is applied on the substrate 210 .
- a planarization layer 230 is processed on top of the passivation layer 220 to make the semiconductor surface flat for subsequent applications of a color filter 240 , a spacer 250 and micro-lenses 260 . All these layers 220 through 260 are transparent to light. Referring to FIG.
- the photosensitive layer 505 is disposed between the color filter 240 and the spacer 250 . Photon generated carriers will have to travel through the color filter 240 , the planarization layer 230 and the passivation layer 220 to reach the semiconductor substrate.
- the photosensitive layer 555 is disposed between the planarization layer 230 and the color filter 240 . Photon generated carriers will only have to travel through the planarization layer 230 and the passivation layer 220 to reach the semiconductor substrate 210 . The additional carriers that reach the semiconductor substrate 210 will increase the combination current therein and hence the photosensitivity of the CMOS image sensor cells 500 and 550 . Thicknesses of the passivation layers 220 and the planarization layer 230 may be used to adjust the sensitivity level of the CMOS image sensor cells 500 and 550 .
- the P-N junction diode is used for collecting the photon generated carriers in above embodiments of the present invention, one having skills in the art would appreciate other types of semiconductor devices may also serve that purpose as long as the device is reverse biased with little or no current of itself, yet, photon generated electrons and holes may combine therein and create a combination current with a magnitude proportional to an incoming photon density.
Abstract
Description
- The present invention relates generally to semiconductor image sensors and more particularly to an image sensor structure with enhanced photosensitivity.
- In the 1950s, researchers found that a fully charged reverse-biased P-N junction would discharge at a rate proportional to the light it received. This is because photons (light) can assist electrons and holes overcome the energy gap. These electron-hole pairs incur discharging current when they recombine after their lifetimes expire. As a result, the P-N junctions can be used as a solid-state image sensor to replace vacuum tube devices with photomultipliers to detect radiations. A CMOS image sensor, which comprises arrays of active MOS image sensor cells that are produced in a CMOS process, is one of the typical image sensing devices that utilize the photoconductive characteristics of the reverse-biased P-N junction structure.
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FIG. 1 illustrates a conventional 3T CMOSimage sensor cell 100 which comprises aP-N junction diode 110, areset NMOS transistor 120, anamplifier NMOS transistor 130 and a rowselect NMOS transistor 140. TheP-N junction diode 110, which serves as a photo-detector, and thereset NMOS transistor 120 are serially connect between a power supply VRST and a ground (GND). When thereset MOS transistor 120 is turned on by the RST signal, theP-N junction diode 110 is effectively connected to the VRST and reverse biased. When light shines on theP-N junction diode 110, an additional combination current generated by photon created electron-hole pairs cause a voltage drop at node VC. The voltage drop is then amplified by theNMOS transistor 130, which has a power supply VDD. However, the VDD is traditionally tied to the VRST. The row selectNMOS transistor 140 is coupled between theamplifier NMOS transistor 130 and a column line (COL). A row line (ROW) is connected to a gate of theNMOS transistor 140. Therefore, the row selectNMOS transistor 140 is a switch that allows a signal row of an array the CMOSimage sensor cells 100 to be read by a read-out circuit. The aforementioned combination current flowing through theP-N junction diode 110 is proportional to the intensity of the light, therefore the read-out voltage and/or current at the COL is also proportional to the intensity of the light. -
FIG. 2 is a cross-sectional view of suchCMOS image sensor 100 forming an array ofcells 200 in asemiconductor substrate 210. TheP-N junction diode 110 andNMOS transistors substrate 210. Apassivation layer 220 is applied on thesubstrate 210. Then aplanarization layer 230 is processed on top of thepassivation layer 220 to make the semiconductor surface flat, for subsequent applications of acolor filter 240, aspacer 250 and micro-lenses 260. All theselayers 220 through 260 merely pass the light to thesubstrate 210, where theP-N junction diode 110 is the only device that has the photoconductive effect. Therefore, the conventional CMOSimage sensor cell 200 has only mediocre optical sensitivity and signal-to-noise ratio. - As such, what is needed is an improved image sensor cell structure that has enhanced photosensitivity.
- The present invention discloses a CMOS image sensor with enhanced photosensitivity. In one embodiment of the present invention, the CMOS image sensor has a photosensitive device, which includes a semiconductor substrate, at least one reverse biased device, such as a P-N junction diode formed in the semiconductor substrate, and at least one photosensitive layer disposed above the semiconductor substrate and substantially covering the reverse biased device, the photosensitive layer releasing electrons and holes when struck by photons, wherein the photon generated electrons and holes in the photosensitive layer reach the reverse biased device and create a combination current therein when a light shines thereon. In another embodiment of the present invention, the photosensitive device further includes a transparent insulation layer interposed between the photosensitive layer and the semiconductor substrate.
- The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
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FIG. 1 is a schematic diagram illustrating a conventional CMOS image sensor cell. -
FIG. 2 is a cross-sectional view of such conventional CMOS image sensor cells formed in a semiconductor substrate. -
FIGS. 3A and 3B are cross-sectional views of photosensitive P-N junction diodes according to embodiments of the present invention. -
FIGS. 4A˜4D are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention. -
FIGS. 5A through 5B are cross-sectional views of CMOS image sensor cells with photosensitive layers applied according to embodiments of the present invention. - This invention describes a CMOS image sensor cell with a photoconductive layer for improving its sensitivity to a change of illumination. The following merely illustrates the various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention.
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FIGS. 3A and 3B are cross-sectional views ofP-N junction diodes thin films film FIG. 3A , the P-N junction diode comprises an N-type region 310 disposed inside a P-type semiconductor substrate (Psub) 320. An N+region 312 serves as a pick-up for the N-type region 310. A P+region 322 serves as a pick-up for the Psub 320. If the P-N junction is equivalent to thediode 110 ofFIG. 1 , then thePsub 320 is coupled to a ground (GND) through theP+ region 322, and the N-type region 310 is coupled to node VC through theN+ region 312. When light (photon) shines on theP-N junction 300, electrons and holes will be generated in the N-type region 310, and then combined in aphoton collection region 315. A combination current will then be theCMOS sensor cell 100 shown inFIG. 1 . - Referring to
FIG. 3A , through athin oxide layer 335, the N-type region 310 is coated by the photo sensitivethin film 330, which serves two purposes. A first purpose is to convert incoming invisible lights, such as X-ray or UV, into visible light, as CMOS image sensors are typically designed to sense only visible lights. When the invisible lights shine on the photo sensitivethin film 330, electrons in thethin film 330 are incited into higher energy levels by the incoming photons and then emit other photons after falling back into the ground states. In case of converting X-ray, thethin film 330 is made of phosphor. When the incoming X-ray light (photon) strikes thephosphor layer 330, it will be converted into a visible wavelength, so that theP-N junction diode 300 can sense the incoming X-ray light. One having ordinary skills in the art would choose other materials for thethin film 330 when other incoming lights need to be converted. - A second purpose of the photo sensitive
thin film 330 is to generate electron-hole pairs when light shines on it. These generated electrons and holes will then tunnel through thethin oxide 335 and induce more combinations of electrons and holes in thephoton collection region 315. According to the embodiments of the present invention, thethin film 330 is made of a semiconductor material, such as phosphor, it is therefore transparent to light. Meanwhile thethin oxide 335 is also transparent. Both the photo sensitivethin film 330 and the P-N junction itself can generate electron-hole pairs when shined by light, therefore current generation efficiency of theP-N junction diode 300 is improved by adding the photo sensitivethin film 330. Besides, as described in above paragraph, thethin film 330 may convert photons otherwise invisible to theP-N junction diode 300 to visible wavelengths, there will be more photons in the incoming light to generate electron-hole pairs, and the light sensitivity of theP-N junction diode 300 is further enhanced. - Here the thickness of the
thin oxide 335 can be adjusted to control the tunneling rate for the electron-hole pairs generated by thephotoconductive layer 330. For example, the thickness of thethin oxide 335 is preferably thinner than 100 angstroms. One having skills in the art may also recognize other dielectric materials may also be used in place of thethin oxide 335. - As shown in
FIG. 3A , in order to assist the generated electrons to tunnel through thethin oxide 335, a bias voltage VP may be applied to thethin film 330. Besides, in a semiconductor manufacturing process, the photo sensitivethin film 300 may be selectively coated with lithograph patterning to cater to various needs. -
FIG. 3B shows another embodiment of the present invention with a photo sensitivethin film 360 overlays the N-type region 310. The overlay alters the silicon surface potential, thus allows more carries to be stored in the depletion regions of the P-N junction discharging. - The photo sensitive materials used for the
thin film - CdS is commonly used as light-sensitive material in discrete devices for wavelength near 0.5 um. The resistance between two terminals of CdS film changes drastically when light shines on the surface. Whereas at 10 um, an HgCdTe photoconductor is preferred. In the wavelength from 100 to 400 um, a GaAs photoconductor is a better choice because of its higher detectivity.
- One of the mechanisms to achieve multiple carrier generation is through photoconductivity. When light shines on a photoconductive material, such as Cds, amorphous silicon a-Si:H, etc., electron-hole pairs will be generated accordingly. The photoconductivity σph is determined by the product of the free-carrier lifetime τ and free-carrier mobility μ:
-
σph=q·μ·τ·f (Eq. 1) - where f is an average optical generation rate, which is the number of carriers generated by the photons absorbed per second and per unit volume. The μ·τ product depends on the property of photoconductive material. In general, the τ product depends on the position of Fermi level Ef to the bandgap. The further the Fermi level is away from the midgap and closer to the conduction band edge Ec, the larger the μ·τ product. The mobility μ was found independently of Fermi level, the photoconductivity σph is therefore proportional to the recombination lifetime τ.
-
FIGS. 4A through 4D are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention.FIG. 4A illustrates astructure 400 in which addition photon generated electron-hole pairs come from a P-N junction formed by twolayers insulation layer 403 over asemiconductor substrate 410. Iflayer 410 is an N-type, then layer 415 is a P-type, or vice versa. Photon generates electron-hole pairs in a depletion region at an interface oflayers semiconductor substrate 410 under thelayers -
FIG. 4B illustrates a P-Intrinsic-N (P-I-N)structure 420 which is slightly different from thestructure 400 in that a thinintrinsic layer 425 is deposited between twodoped layers layer 430 is an N-type, then layer 435 is a P-type, or vice versa. Here, the depletion region is theintrinsic layer 425, the thickness of which can be tailored to optimize the photo sensitivity. -
FIG. 4C illustrates aheterojunction structure 450 deposed on top of theinsulator 403 over thesemiconductor substrate 410 for generating additional electron-hole pairs. As an example, theheterojunction structure 450 comprises anintrinsic semiconductor layer 455 being sandwiched between two semiconductor blocking layers 460 and 465 of different material composition, such as GaAs or GaAsInP. Theselayers intrinsic semiconductor layer 455 generates electron-hole pairs when light shines on thestructure 450. These electrons and holes may tunnel through thethin insulation layer 403 and reach thesemiconductor substrate 410. -
FIG. 4D illustrates a Schottkey barrier formed by aphotoconductor layer 475, e.g., a-Si:H, being sandwiched between twolayers photoconductor layer 475 by light. {If this is a Schottkey barrier. Just say so.} Then these electrons and holes may tunnel through thethin insulation layer 403. In all the cases ofFIG. 4A through 4D , theinsulation layer 403 has to be thin enough to allow carriers to tunnel through and reach thesemiconductor substrate 410 where the conventional P-N junction is formed therein. As a result of these tunneled-in carriers, a combination current in thephotosensitive structure -
FIGS. 5A and 5B are cross-sectional views of CMOSimage sensor cells photosensitive layers image sensor cells image sensor cells 200 as shown inFIG. 2 , where a conventional P-N junction diode (not shown) is formed in thesubstrate 210. Apassivation layer 220 is applied on thesubstrate 210. Then aplanarization layer 230 is processed on top of thepassivation layer 220 to make the semiconductor surface flat for subsequent applications of acolor filter 240, aspacer 250 and micro-lenses 260. All theselayers 220 through 260 are transparent to light. Referring toFIG. 5A , thephotosensitive layer 505 is disposed between thecolor filter 240 and thespacer 250. Photon generated carriers will have to travel through thecolor filter 240, theplanarization layer 230 and thepassivation layer 220 to reach the semiconductor substrate. Referring toFIG. 5B , thephotosensitive layer 555 is disposed between theplanarization layer 230 and thecolor filter 240. Photon generated carriers will only have to travel through theplanarization layer 230 and thepassivation layer 220 to reach thesemiconductor substrate 210. The additional carriers that reach thesemiconductor substrate 210 will increase the combination current therein and hence the photosensitivity of the CMOSimage sensor cells planarization layer 230 may be used to adjust the sensitivity level of the CMOSimage sensor cells - Although the P-N junction diode is used for collecting the photon generated carriers in above embodiments of the present invention, one having skills in the art would appreciate other types of semiconductor devices may also serve that purpose as long as the device is reverse biased with little or no current of itself, yet, photon generated electrons and holes may combine therein and create a combination current with a magnitude proportional to an incoming photon density.
- The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.
- Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.
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