An image sensor based on charge carrier avalanche

Abstract

Disclosed herein is an image sensor comprising: a plurality of avalanche photodiodes (APDs); wherein each of the APDs comprises a radiation absorption layer comprising an absorption region and an amplification region; wherein the absorption region is configured to generate charge carriers in the absorption region from radiation particles absorbed by the radiation absorption layer; wherein the absorption region comprises an InGaAs layer sandwiched between InP layers; wherein the amplification region has an electric field therein having a field strength sufficient to cause an avalanche of charge carriers in the amplification region.

Description

TECHNICAL FIELD

[0001] The present invention relates to an image sensor, in particular to an image sensor based on avalanche of charge carriers.

BACKGROUND

[0002] An image sensor or imaging sensor is a sensor that can detect a spatial intensity distribution of radiation. An image sensor typically represents the detected image by electrical signals. Image sensors based on semiconductor devices can be classified into several types, which include semiconductor charge-coupled devices (CCD), complementary metal-oxide-semiconductor (CMOS), N-type metal-oxide-semiconductor (NMOS). A CMOS image sensor is an active pixel sensor made using CMOS semiconductor fabrication processes. Light incident on a pixel in a CMOS image sensor is converted to a voltage. The voltage is digitized to a discrete value representing the intensity of light incident on the pixel. An active pixel sensor (APS) is an image sensor that includes pixels with a photodetector and an active amplifier. A CCD image sensor includes a capacitor in a pixel. When light is incident on the pixel, the light generates a charge and the charge is stored on the capacitor. The stored charge is converted to a voltage, and the voltage is digitized to a discrete value representing the intensity of light incident on the pixel.

SUMMARY

[0003] Disclosed herein is an image sensor comprising: a plurality of avalanche photodiodes (APDs); wherein each of the APDs comprises a radiation absorption layer comprising an absorption region and an amplification region; wherein the absorption region is configured to have charge carriers generated therein by radiation particles absorbed by the radiation absorption layer; wherein the absorption region comprises an InGaAs layer sandwiched between InP layers; wherein the amplification region has an electric field therein, the electric field having a field strength sufficient to cause an avalanche of charge carriers in the amplification region.

[0004] In an aspect, the absorption region has a thickness of 10 microns or more.

[0005] In an aspect, an interface between the InGaAs layer and the InP layer is parallel to a radiation receiving surface of the radiation absorption layer.

[0006] In an aspect, an interface between the InGaAs layer and the InP layer is perpendicular to a radiation receiving surface of the radiation absorption layer.

[0007] In an aspect, the doped semiconductor has a non-zero dopant concentration gradient.

[0008] In an aspect, the amplification region comprises a doped semiconductor in electrical contact with a first electrode.

[0009] In an aspect, the first electrode is configured to generate the electric field.

[0010] In an aspect, the first electrode comprises a tip having a conical, truncated conical, prismatic, pyramidal, cuboid, or cylindrical shape.

[0011] In an aspect, the first electrode is configured to collect charge carriers generated directly by the radiation particle or by the avalanche.

[0012] In an aspect, the first electrode is configured to concentrate the electric field.

[0013] In an aspect, the first electrode extends into the radiation absorption layer.

[0014] In an aspect, at least one of the plurality of APDs comprises an electronics layer.

[0015] In an aspect, the image sensor further comprises an outer electrode arranged around the first electrode and electrically insulated from the first electrode; wherein the outer electrode is configured to shape the electric field in the amplification region.

[0016] In an aspect, the outer electrode is configured not to collect charge carriers.

[0017] In an aspect, the outer electrode comprises discrete regions.

[0018] In an aspect, the image sensor further comprises a second electrode on the radiation absorption layer, the second electrode being opposite to the first electrode.

[0019] In an aspect, the second electrode is configured to collect charge carriers in the radiation absorption layer.

[0020] In an aspect, the second electrode is planar.

[0021] In an aspect, the second electrode comprises discrete regions.

[0022] In an aspect, the discrete regions of the second electrode extend into the radiation absorption layer. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 Schematically shown is the current in an APD as a function of the intensity of light incident on the APD when the APD is in linear mode, and the current in an APD as a function of the intensity of light incident on the APD when the APD is in Geiger mode.

[0024] Figure 2A , Figure 2B , Figure 2C and Figure 2DOperation of an APD comprising a sandwich structure in the absorption layer according to embodiments is schematically illustrated.

[0025] Figure 3A A cross-sectional view of an image sensor comprising a plurality of APDs according to embodiments is schematically illustrated.

[0026] Figure 3B A variant of an image sensor according to embodiments is shown.

[0027] Figure 3C A variant of an image sensor according to embodiments is shown.

[0028] Figures 4A to 4D A process of forming an image sensor according to embodiments is schematically illustrated.

[0029] Figure 5 A system comprising an imaging sensor as described herein is schematically illustrated.

[0030] Figure 6 An X-ray computed tomography (X-ray CT) system is schematically illustrated.

[0031] Figure 7 An X-ray microscope or X-ray micro-CT 700 is schematically illustrated.

[0032] Figure 8 A system suitable for laser scanning according to embodiments is schematically illustrated.

[0033] Figure 9A A top view of an image sensor having a pixel array according to embodiments is schematically illustrated.

[0034] Figure 9B A cross-sectional view of an image sensor according to embodiments is schematically illustrated.

[0035] Figure 10A and Figure 10B Each shows a component diagram of an electronic system of an APD in Figure 3A , Figure 3B and Figure 3C according to embodiments.

[0036] Figure 11 A time variation of the current flowing through the electrode caused by the charge carriers generated by the avalanche of charge carriers by incident radiation particles or in the radiation absorption layer (upper curve) and a corresponding time variation of the voltage of the electrode (lower curve) are schematically illustrated.

DETAILED DESCRIPTION

[0037] A charge carrier avalanche is a process in which free charge carriers in a material are accelerated by an electric field, then collide with other atoms of the material, ionizing them (impact ionization) and releasing additional charge carriers, which are accelerated and collide with additional atoms, releasing more charge carriers—a chain reaction. Impact ionization is a process in which a high-energy charge carrier in a material loses energy by creating other charge carriers. For example, in a semiconductor, an electron (or hole) with sufficient kinetic energy can knock a bound electron out of its bound state (in the valence band) and elevate it to a state in the conduction band, creating an electron-hole pair. An example of an electronic device that uses charge carrier avalanches is an avalanche photodiode (APD), which uses a charge carrier avalanche to generate a current when exposed to light. Charge carrier avalanches will be described using an APD as an example, but the description can apply to other electronic devices that use charge carrier avalanches.

[0038] An APD can operate in either Geiger mode or linear mode. When an APD operates in Geiger mode, it can be referred to as a single-photon avalanche diode (SPAD) (also referred to as a Geiger-mode APD or G-APD). A SPAD is an APD that operates at a reverse bias that is higher than the breakdown voltage. Here, the word "higher" means that the absolute value of the reverse bias is greater than the absolute value of the breakdown voltage. A SPAD can be used to detect low-intensity light (e.g., as low as a single photon) and signal the arrival time of a photon with jitter on the order of tens of picoseconds. A SPAD can be in the form of a p-n junction at a reverse bias that is higher than the breakdown voltage of the p-n junction (i.e., the p-type region of the p-n junction is biased at a lower potential than the n-type region). The breakdown voltage of a p-n junction is the reverse bias above which the current in the p-n junction increases exponentially. An APD that operates at a reverse bias that is lower than the breakdown voltage is operating in linear mode, because the current in the APD is proportional to the intensity of light incident on the APD.

[0039] Figure 1 The current in an APD as a function of the intensity of light incident on the APD 112 when the APD is in linear mode, and the current in an APD as a function of the intensity of light incident on the APD 111 when the APD is in Geiger mode (i.e., when the APD is a SPAD) are shown schematically. In Geiger mode, the current exhibits a sharp increase with light intensity, then saturates. In linear mode, the current is substantially proportional to the light intensity.

[0040] Figure 2A 、 Figure 2B and Figure 2C Operation of an APD according to an embodiment is shown schematically. The APD has a radiation absorption layer with an absorption region 210 and an amplification region 220. Figure 2AIt is shown that when a radiation particle (e.g., an X-ray photon) is absorbed by the absorption region 210, one or more (100 to 10,000 for X-ray photons) electron-hole pairs can be generated. The absorption region 210 has a sufficient thickness, and thus a sufficient absorption rate (e.g., greater than 80% or greater than 90%) for the incident radiation particles. The absorption region 210 can include a sandwich structure of a stack of layers of different semiconductor materials bonded together, e.g., as shown in Figure 2A the InGaAs layer 211 sandwiched between InP layers 212. The absorption region 210 can include one or more sandwich structures formed of InGaAs layers and InP doped layers. In the example shown in Figure 2A , the interface 213 between the InGaAs layer and the InP layer is parallel to the radiation receiving surface 214 of the radiation absorption layer. In one embodiment, as shown in the example of Figure 2D , the interface 213 between the InGaAs layer and the InP layer is perpendicular to the radiation receiving surface 214 of the radiation absorption layer. The absorption region 210 can have a thickness of 10 microns or more for X-ray soft photons. The electric field in the absorption region 210 is not high enough to induce an avalanche effect in the absorption region 210. Figure 2B It is shown that the electrons and holes drift in opposite directions in the absorption region 210. Figure 2C It is shown that an avalanche effect occurs in the amplification region 220 when an electron (or hole) enters the amplification region 220, resulting in more electrons and holes. The electric field in the amplification region 220 is high enough to induce an avalanche of charge carriers that enter the amplification region 220, but not so high that the avalanche effect is self-sustaining. A self-sustaining avalanche is an avalanche that continues after the external trigger (e.g., a radiation particle incident on the APD or a charge carrier drifted into the APD) is removed. The electric field in the amplification region 220 can be a result of the doping profile in the amplification region 220 or the structure of the amplification region 220. For example, the amplification region 220 can include a p-n junction or a heterojunction having an electric field in its depletion region. The threshold electric field for the avalanche effect (i.e., the electric field above which an avalanche effect occurs and below which an avalanche effect does not occur) is a property of the material of the amplification region 220. The amplification region 220 can be located on one side or on both opposite sides of the absorption region 210.

[0041] Figure 3AA cross-sectional view of an image sensor 300 including a plurality of APDs according to an embodiment is shown schematically. The image sensor 300 can include a radiation absorption layer 311 and one or more electrodes 304 on the radiation absorption layer 311. The radiation absorption layer 311 can be configured to generate charge carriers therein from radiation particles absorbed by the radiation absorption layer 311. The one or more electrodes 304 can be configured to generate an electric field 306 in the radiation absorption layer 311. Each of the one or more electrodes 304 can have a geometry (e.g., a small tapered tip) that shapes the electric field 306 such that the electric field 306 in one or more portions of the radiation absorption layer 311 (i.e., one or more amplification regions 320) has a field strength sufficient to induce an avalanche of charge carriers (e.g., electrons or holes) in the one or more amplification regions 320. The charge carriers generated from the avalanche or directly from the radiation particles drift to and are collected by the one or more electrodes 304 or different electrodes. The image sensor 300 can also include a passivation material 303 configured to passivate a surface of the radiation absorption layer 311 to reduce recombination of charge carriers at the surface. The image sensor 300 can also include an opposing electrode 301 on the radiation absorption layer 311, the opposing electrode 301 opposing the one or more electrodes 304. The opposing electrode 301 can be configured to collect charge carriers in the radiation absorption layer 311.

[0042] In one embodiment, part or all of the radiation absorption layer 311 includes a sandwich structure made of a stack of InGaAs layers 212 sandwiched by InP layers 211 as shown. The radiation absorption layer 311 can have a thickness sufficient to have a sufficient absorption (e.g., greater than 80% or greater than 90%) for the incident radiation particles (e.g., X-ray photons) of interest. The radiation absorption layer 311 can have a thickness of 10 microns or more. Figure 3A

[0043] In one embodiment, the radiation absorption layer 311 can include a doped region 312 lightly doped with a dopant. A semiconductor is considered to be lightly doped when the ratio of dopant to semiconductor atoms is small enough that the dopant’s electronic states at the Fermi level are localized (i.e., the dopant’s energy bands can not overlap with the semiconductor’s conduction or valence bands). For example, the ratio of dopant to silicon atoms for lightly doped silicon can be on the order of 1 / 1011. The doped region 312 can extend from the surface into an interior region of the radiation absorption layer 311 and can have a non-zero dopant concentration gradient. In the example shown, the concentration of the dopant gradually decreases from the surface to the interior region of the radiation absorption layer 311. The doped region 312 can be in electrical contact with the electrodes 304. In embodiments, the doped region 312 can include discrete regions, each of which surrounds one of the electrodes 304. Figures 3A to 3C In one embodiment, the radiation absorption layer 311 includes a doped region 312 lightly doped with a dopant. The doped region 312 can be in electrical contact with the electrodes 304. In embodiments, the doped region 312 can include discrete regions, each of which surrounds one of the electrodes 304.​

[0044] The one or more electrodes 304 can comprise an electrically conductive material, such as a metal (e.g., gold, copper, aluminum, platinum, etc.), or any other suitable electrically conductive material (e.g., heavily doped semiconductor). The one or more electrodes 304 can have a small size or a suitable shape such that the electric field 306 near the one or more electrodes 304 is concentrated. For example, the one or more electrodes 304 can comprise a tip having a shape of a cone, a truncated cone, a prism, a pyramid, a cuboid, or a cylinder, etc. In Figure 3A The tip in the example is flat, cylindrical. Figure 3A The flat tip of the electrode 304 in the example has a contact area with the radiation absorption layer 311 that is small enough such that the electric field 306 near the tip becomes strong enough to cause an avalanche of charge carriers near the tip. In other words, the strength of the electric field 306 increases as one approaches the electrode 304, and Figure 3A The amplification region 320 in the example is a region around the tip of the electrode 304 where the electric field 306 is strong enough to cause an avalanche of charge carriers. In embodiments, the one or more amplification regions 320 respectively correspond to the one or more electrodes 304. The amplification region 320 corresponding to one electrode 304 can not be joined with another amplification region 320 corresponding to another electrode 304. In embodiments, the electric field 306 is not high enough to cause a self-sustaining avalanche; that is, the electric field 306 in the amplification region 320 should cause an avalanche when there is an incident radiation particle in the radiation absorption layer 311, but the avalanche should stop when there is no further radiation particle in the radiation absorption layer 311.

[0045] When radiation hits the radiation absorption layer 311, it can be absorbed and generate one or more charge carriers through various mechanisms. A radiation particle can generate 10 to 100,000 charge carriers. One charge carrier (an electron or a hole) drifts towards the amplification region 320. The charge carriers can drift in various directions, such that substantially all (greater than 98%, greater than 99.5%, greater than 99.9%, or greater than 99.99%) of the charge carriers generated by a radiation particle incident around the footprint 330 of one electrode 304 flow to the amplification region 320 corresponding to the electrode 304. That is, less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of the charge carriers flow through the amplification region 320 corresponding to the electrode 304. When the charge carriers enter the amplification region 320, an avalanche effect occurs and causes amplification of the charge carriers. The amplified charge carriers can be collected as a current through the corresponding electrode 304. In a linear mode, the current is proportional to the number of incident radiation particles per unit time around the footprint 330 of the electrode 304 (i.e., proportional to the radiation intensity). The current at the electrode 304 can be compiled to represent the spatial intensity distribution of the radiation, i.e., an image.

[0046] Figure 3B A variation of the image sensor 300 is shown, in which the electrodes 304 can extend into the radiation absorption layer 311. The portion of each electrode 304 that extends into the radiation absorption layer 311 can have a small size or a suitable shape such that the electric field 306 near this portion is concentrated. For example, this portion can include a tip having a shape of a cone, a truncated cone, a prism, a pyramid, a cuboid, or a cylinder, among others. In Figure 3B the example, the tip is conical, and the electric field 306 near the conical tip becomes strong enough to cause an avalanche of charge carriers near the tip. In other words, the strength of the electric field 306 increases as one approaches this portion of the electrode 304, and Figure 3B the amplification region 320 in is the region around this portion where the electric field 306 is strong enough to cause an avalanche of charge carriers.

[0047] Figure 3C A variation of the image sensor 300 is shown, in which the image sensor 300 can further include one or more outer electrodes 305. The one or more outer electrodes 305 respectively correspond to and are located around one or more electrodes 304. The outer electrodes 305 are electrically insulated from the electrodes 304. For example, an insulating region (e.g., a portion of the passivation material 303) can exist between the outer electrodes 305 and their corresponding electrodes 304.

[0048] In Figure 3C the example, the outer electrodes 305 and their corresponding electrodes 304 are coaxial. The one or more outer electrodes 305 can include an electrically conductive material, such as a metal (e.g., gold, copper, aluminum, platinum, etc.), or any other suitable electrically conductive material (e.g., a heavily doped semiconductor).

[0049] The outer electrodes 305 can be configured to shape the electric field 306 in the amplification region 320 of the electrodes 304 corresponding to the outer electrodes 305, and the outer electrodes 305 can not be configured to collect charge carriers. For example, the electric field 306 (e.g., its strength, gradient) can be tuned by introducing a voltage difference between the outer electrodes 305 and their corresponding electrodes 304. In an embodiment, the outer electrodes 305 can have the same voltage as the opposing electrode 301. In an embodiment, the outer electrodes 305 can not necessarily be rings as shown in Figure 3C but can have discrete portions.

[0050] In an embodiment, the opposing electrode 301 can be planar, as shown in Figures 3A to 3C The opposing electrode 301 can include discrete regions.

[0051] Figures 4A to 4D A process of forming the image sensor 300 is shown schematically, in accordance with an embodiment.

[0052] In step 1000, a semiconductor substrate 411 is obtained. The semiconductor substrate 411 can include an intrinsic semiconductor such as silicon. The semiconductor substrate 411 can have a sufficient thickness, thus having a sufficient absorption rate (e.g., greater than 80% or greater than 90%) for the incident radiation particles (e.g., X-ray photons) of interest. The semiconductor substrate 411 can have a thickness of 10 microns or more.

[0053] In steps 1001-1003, the semiconductor substrate 411 can be doped to form a doped region 412 (as shown in steps 1004-1006). The doped region 412 can serve as Figures 3A to 3C a doped region 312 of the mid-radiation absorption layer 311. In Figures 4A to 4D In an example, the doped region 412 to be formed is a continuous layer. In an embodiment, the semiconductor substrate 411 is a silicon substrate, and the desired doped region 412 is lightly doped and has a non-zero dopant concentration gradient extending from the surface into an inner region of the semiconductor substrate 411. The dopant concentration can gradually decrease from the surface to the inner region of the semiconductor substrate 411.

[0054] In step 1001, a mask layer 402 is formed on the surface of the semiconductor substrate 411. The mask layer 402 can act as a shielding layer configured to retard the entry of dopants into the semiconductor substrate 411 in the doping step 1002. The mask layer 402 can include a material such as silicon dioxide. The thickness of the mask layer 402 can be determined according to the doping conditions in step 1002 and the desired doping profile of the doped region 412 (as shown in steps 1004-1006) to be formed. The mask layer 402 can be formed on the surface by various techniques such as thermal oxidation, vapor deposition, spin coating, sputtering, or any other suitable process.

[0055] In step 1002, the surface of the semiconductor substrate 411 is lightly doped using a suitable dopant 10 by a doping technique such as dopant diffusion and ion implantation. The rate of entry of the dopant into the semiconductor substrate 411 can be controlled by the mask layer 402, the dosage of the dopant being doped, and the doping details such as the energy of the dopant during ion implantation.

[0056] In step 1003, the doped semiconductor substrate 411 is annealed to drive the dopant into an inner region of the semiconductor substrate 411. The dopant diffuses into the inner region at a high temperature (e.g., about 900 °C). The annealing duration can be extended to facilitate the diffusion of the dopant into the inner region. The high temperature environment of the annealing can also help the annealing to remove defects of the semiconductor substrate 411.

[0057] In addition to controlling the doping and annealing conditions, the doping (step 1002) and annealing (step 1003) can be repeated multiple times to form the doped region 412 with a desired doping profile.

[0058] In an embodiment, the doped region 412 can include discrete regions. The mask layer 402 can have a pattern with regions of different thicknesses. A portion of the dopant can penetrate the thinner regions of the mask layer and form the discrete regions of the doped region 412, while the thicker regions of the mask layer prevent the dopant from entering the semiconductor substrate 411.

[0059] In step 1004, the mask layer 402 can be removed by wet etching, chemical mechanical polishing, or some other suitable technique.

[0060] In step 1005, electrodes 404 can be formed on the semiconductor substrate 411. The electrodes 404 can be used as the electrodes 304 of the image sensor 300. The electrodes 404 can be in electrical contact with the doped region 412. In the example of step 1005, the electrodes 404 each include a tapered tip that extends into the semiconductor substrate 411. Forming the electrodes 404 can involve forming a mask with openings on the surface of the semiconductor substrate 411 by a suitable technique such as photolithography. The shape and location of the openings correspond to the shape and location of the footprint of the electrodes 404 to be formed. By etching portions of the substrate 411 that are not covered by the mask, recesses of the desired shape and size are formed in the surface of the semiconductor substrate 411. The etching process can be performed by a technique such as dry etching (e.g., deep reactive ion etching), wet etching (e.g., anisotropic wet etching), or a combination thereof. A conductive material such as a metal (e.g., gold, copper, aluminum, platinum, etc.) can be deposited into the recesses by a suitable technique such as physical vapor deposition, chemical vapor deposition, spin coating, sputtering, etc. to form the electrodes 404. The mask can be left in place and used as a passivation layer for the surface of the substrate 411. In an embodiment, the mask can be removed and a passivation material 403 can be applied to passivate the surface of the substrate 411.

[0061] In optional step 1006, an outer electrode 405 can be formed around the electrodes 404. The electrode 405 can be used as the outer electrode 305 of the image sensor 300. Figure 3C Forming the outer electrode 405 can involve mask formation and metal deposition processes similar to step 1005.

[0062] In step 1007, a sandwich layer 413 can be bonded on the other surface of the substrate 411. The sandwich layer 413 can include one or more sandwich-type structures formed by InGaAs layers 212 sandwiched between InP layers 211. Opposing electrodes 401 can be formed on the surface of the sandwich layer 413. The opposing electrodes 401 can serve as the opposing electrodes 301 of the image sensor 300. In the example of step 1007, the opposing electrodes 401 are planar, and can be formed by depositing a conductive material such as metal on the other surface of the semiconductor substrate 411 by a suitable technique such as vapor deposition, sputtering, etc.

[0063] Forming the image sensor 300 can include Figures 4A to 4D some intermediate steps not shown in the flowchart, such as surface cleaning, polishing, surface passivation. The order of the steps shown in the flowchart can be changed to accommodate different formation needs. Figures 4A to 4D

[0064] Figure 5 A system including an image sensor 503 as an embodiment of the image sensor 300 described herein is schematically shown. The system includes an X-ray source 501. X-rays emitted from the X-ray source 501 pass through an object 510 (e.g., a diamond, a tissue sample, a human body part such as a breast), are attenuated to different degrees by the internal structure of the object 510, and are projected onto the image sensor 503. The image sensor 503 forms an image by detecting the intensity distribution of the X-rays. The system can be used for medical imaging such as chest radiography, abdominal radiography, dental radiography, mammography, etc. The system can be used for industrial CT, e.g., diamond defect detection, scanning trees to visualize years and cellular structure, scanning concrete-like building materials after being loaded, etc.

[0065] Figure 6 An X-ray computed tomography (X-ray CT) system is schematically shown. An X-ray CT system uses computer-processed X-rays to produce tomographic images (virtual "slices") of a particular region of an object being scanned. The tomographic images can be used for diagnostic and therapeutic purposes in various medical disciplines, or for defect detection, failure analysis, metrology, assembly analysis, and reverse engineering. The X-ray CT system includes an image sensor 603 as an embodiment of the image sensor 300 described herein and an X-ray source 601. The image sensor 603 and the X-ray source 601 can be configured to rotate in synchronization along one or more circular or helical paths.

[0066] Figure 7 ​An X-ray microscope or X-ray micro-CT 700 is schematically illustrated. The X-ray microscope or X-ray micro-CT 700 can include an X-ray source 701, focusing optics 704, and an image sensor 703 that is an embodiment of the image sensor 300 described herein to detect X-ray images of a sample 702.

[0067] Figure 8 A system 800 suitable for laser scanning according to an embodiment is schematically illustrated. The system 800 includes a laser source 810 and a detector 820 that is an embodiment of the image sensor 300 described herein. The laser source 810 can be configured to produce a scanning laser beam. The scanning laser beam can be infrared. In an embodiment, the laser source 810 can perform two-dimensional laser scanning without moving parts. The detector 820 can be configured to collect returned laser signals after the scanning laser beam bounces off an object, building, or landscape and produce electrical signals. The system 800 can also include a signal processing system configured to process and analyze the electrical signals produced by the detector 820. In one embodiment, the distance and shape of the object, building, or landscape can be obtained. The system 800 can be a lidar system (e.g., a vehicle-mounted lidar).

[0068] Figure 9A A top view of an image sensor 900 having an array of pixels 950 according to an embodiment is schematically illustrated. The array can be a rectangular array, a honeycomb array, a hexagonal array, or any other suitable array. Each pixel 950 is configured to detect radiation incident thereon from a radiation source and can be configured to measure a characteristic of the radiation (e.g., energy of a particle, intensity distribution). Each pixel 950 can have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident radiation particle into a digital signal or digitize an analog signal representing the total energy of multiple incident radiation particles into a digital signal. The pixels 950 can be configured to operate in parallel. For example, while one pixel 950 is measuring an incident radiation particle, another pixel 950 can be waiting for a radiation particle to arrive. The pixels 950 can not necessarily be individually addressable.

[0069] Figure 9B A cross-sectional view of an image sensor 900 according to an embodiment is schematically illustrated. The image sensor 900 can include a radiation-absorbing layer 910 that is an embodiment of the image sensor 300 described herein, and electronics 920 (e.g., an ASIC) for processing or analyzing electrical signals produced by incident radiation or an avalanche of charge carriers within the radiation-absorbing layer 910.

[0070] The electronics 920 can include an electronic system 921 suitable for processing or interpreting electrical signals. The electronic system 921 can include analog circuits such as filter networks, amplifiers, integrators, and comparators or digital circuits such as microprocessors and memories. The electronic system 921 can include one or more ADCs. The electronic system 921 can include components that are common to multiple pixels or components that are dedicated to a single pixel. For example, the electronic system 921 can include an amplifier dedicated to each pixel and a microprocessor that is common among all pixels. The electronic system 921 can be electrically connected to the pixels through vias 931. The space between the vias can be filled with a filler material 930, which can increase the mechanical stability of the connection of the electronics layer 920 to the radiation-absorbing layer 910. Other bonding techniques can connect the electronic system 921 to the pixels 150 without using vias.

[0071] Figure 10A and Figure 10B each shows a component diagram of the electronic system 921 according to an embodiment. The electronic system 921 can include a first voltage comparator 1901, a second voltage comparator 1902, a counter 1920, a switch 1905, a voltmeter 1906, and a controller 1910.

[0072] The first voltage comparator 1901 is configured to compare a voltage at the electrode (e.g., the electrode 150) to a first voltage threshold. The first voltage threshold can be a voltage that is indicative of a state of the pixel 150. For example, the first voltage threshold can be a voltage that is indicative of a state of the pixel 150 in which the pixel 150 is not exposed to radiation. The second voltage comparator 1902 is configured to compare a voltage at the electrode (e.g., the electrode 150) to a second voltage threshold. The second voltage threshold can be a voltage that is indicative of a state of the pixel 150. For example, the second voltage threshold can be a voltage that is indicative of a state of the pixel 150 in which the pixel 150 is exposed to radiation. The first voltage threshold and the second voltage threshold can be different. The first voltage threshold and the second voltage threshold can be the same. Figure 9BThe first voltage comparator 1901 is configured to compare the voltage to a first threshold value. The first voltage comparator 1901 can be configured to directly monitor the voltage, or to calculate the voltage by integrating the current flowing through the electrode over a period of time. The first voltage comparator 1901 can be controllably activated or deactivated by the controller 1910. The first voltage comparator 1901 can be a continuous comparator. That is, the first voltage comparator 1901 can be configured to be continuously activated and continuously monitor the voltage. The first voltage comparator 1901 configured as a continuous comparator reduces the chance that the system 921 misses a signal directly generated by an incident radiation particle or generated by a charge carrier avalanche. The first voltage comparator 1901 configured as a continuous comparator is particularly suitable when the incident radiation intensity is relatively high. The first voltage comparator 1901 can be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator 1901 configured as a clocked comparator can cause the system 921 to miss a signal directly generated by an incident radiation particle or generated by a charge carrier avalanche. When the incident radiation intensity is low, the chance of missing an incident radiation particle is low because the time interval between two consecutive particles is relatively long. Thus, the first voltage comparator 1901 configured as a clocked comparator is particularly suitable when the incident radiation intensity is relatively low. The first threshold value can be 5-10%, 10%-20%, 20-30%, 30-40%, or 40-50% of the maximum voltage that one incident radiation particle can directly generate in the radiation absorption layer or after being avalanche amplified in the radiation absorption layer. The maximum voltage can depend on the energy of the incident radiation particle (i.e., the wavelength of the incident radiation), the material of the radiation absorption layer 910, the charge carrier avalanche size, and other factors. For example, the first threshold value can be 50 mV, 100 mV, 150 mV, or 200 mV.

[0073] The second voltage comparator 1902 is configured to compare the voltage to a second threshold value. The second voltage comparator 1902 can be configured to directly monitor the voltage, or to calculate the voltage by integrating the current flowing through the electrode over a period of time. The second voltage comparator 1902 can be a continuous comparator. The second voltage comparator 1902 can be controllably activated or deactivated by the controller 1910. When the second voltage comparator 1902 is deactivated, the power consumption of the second voltage comparator 1902 can be less than 1%, 5%, 10%, or 20% of the power consumption when the second voltage comparator 1902 is activated. The absolute value of the second threshold value is greater than the absolute value of the first threshold value. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x regardless of its sign. That is, The second threshold value can be 200% to 300% of the first threshold value. The second threshold value can be at least 50% of the maximum voltage that an incident radiation particle can directly generate in the radiation absorption layer or after being amplified in the radiation absorption layer. For example, the second threshold value can be 100 mV, 150 mV, 200 mV, 250 mV, or 300 mV. The second voltage comparator 1902 and the first voltage comparator 1901 can be the same component. That is, the system 921 can have one voltage comparator that can compare the voltage to two different threshold values at different times.

[0074] The first voltage comparator 1901 or the second voltage comparator 1902 can include one or more operational amplifiers or any other suitable circuitry. The first voltage comparator 1901 or the second voltage comparator 1902 can have a high speed to allow the system 921 to operate at a high flux of incident radiation particles. However, having a high speed typically comes at the cost of power consumption.

[0075] The counter 1920 is configured to record the number of radiation particles that reach the radiation absorption layer. The counter 1920 can be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a 4017 IC and a 7490 IC).

[0076] The controller 1910 can be a hardware component, such as a microcontroller and a microprocessor. The controller 1910 is configured to start a time delay from the time when the first voltage comparator 1901 determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold value (e.g., the absolute value of the voltage increases from a value that is below the absolute value of the first threshold value to a value that is equal to or above the absolute value of the first threshold value). The absolute value is used here because the voltage can be negative or positive, depending on which electrode is used. The controller 1910 can be configured to keep any other circuitry that is not needed for the operation of the second voltage comparator 1901, the counter 1920, and the first voltage comparator 1901 deactivated before the time when the first voltage comparator 1902 determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold value. The time delay can expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phrase “the rate of change of the voltage is substantially zero” means that the temporal change of the voltage is less than 0.1% / ns. The phrase “the rate of change of the voltage is substantially not zero” means that the temporal change of the voltage is at least 0.1% / ns.

[0077] The controller 1910 can be configured to activate the second voltage comparator during the time delay, including the start and the expiration. In an embodiment, the controller 1910 is configured to activate the second voltage comparator at the start of the time delay. The term “activate” means to bring a component into an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term “deactivate” means to bring a component into a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cutting off power, etc.). The operational state can have a higher power consumption than the non-operational state (e.g., 10 times, 100 times, 1000 times the power consumption for the non-operational state). The controller 1910 itself can be deactivated until the output of the first voltage comparator 1901 activates the controller 1910 when the absolute value of the voltage equals or exceeds the absolute value of the first threshold.

[0078] The controller 1910 can be configured to cause the number recorded by the counter 1920 to be incremented if, during the time delay, the second voltage comparator 1902 determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold.

[0079] The controller 1910 can be configured to cause the voltmeter 1906 to measure the voltage at the expiration of the time delay. The controller 1910 can be configured to connect the electrode to electrical ground in order to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to electrical ground after the time delay expires. In an embodiment, the electrode is connected to electrical ground for a limited reset time period. The controller 1910 can connect the electrode to electrical ground by controlling the switch 1905. The switch 1905 can be a transistor such as a field effect transistor (FET).

[0080] In an embodiment, the system 921 does not have an analog filter network (e.g., an RC network). In an embodiment, the system 921 does not have analog circuitry.

[0081] The voltmeter 1906 can feed the voltage it measures as an analog or digital signal to the controller 1910.

[0082] The system 921 can include a capacitor module 1909 electrically connected to the electrode, where the capacitor module 1909 is configured to collect charge carriers from the electrode. The capacitor module can include a capacitor in the feedback path of the amplifier. An amplifier so configured is called a capacitance transimpedance amplifier (CTIA). The CTIA has a high dynamic range by preventing the amplifier from saturating, and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. In a period of time (“integration period”), for example, as Figure 11As shown, charge carriers from the electrodes accumulate on the capacitor during the period from t0 to t1 (or between t1 and t2). After the integration period, the capacitor voltage is sampled, and then reset by a reset switch. The capacitor module may include capacitors directly connected to the electrodes.

[0083] Figure 11 The diagram schematically illustrates the time-varying current (upper curve) flowing through the electrode caused by charge carriers generated by an avalanche of charge carriers in the radiation-absorbing layer from incident radiation particles, and the corresponding time-varying voltage of the electrode (lower curve). Voltage can be the integral of current with respect to time. At time t0, radiation particles strike the radiation-absorbing layer, initiating the generation and amplification of charge carriers within the layer. Current begins to flow through the electrode, and the absolute value of the electrode voltage begins to increase. At time t1, a first voltage comparator 1901 determines that the absolute value of the voltage is equal to or exceeds the absolute value of a first threshold V1. Controller 1910 initiates a time delay TD1, and controller 1910 may deactivate the first voltage comparator 1901 at the start of TD1. If controller 1910 is deactivated before t1, it is activated at t1. During TD1, controller 1910 activates a second voltage comparator 1902. As used herein, the term "period" for time delay refers to the start and end (i.e., the end) and any time in between. For example, controller 1910 can activate the second voltage comparator 1902 when TD1 expires. If, during TD1, the second voltage comparator 1902 determines at time t2 that the absolute value of the voltage is equal to or exceeds the absolute value of a second threshold, then controller 1910 increments the count recorded by counter 1920 by 1. e All charge carriers generated by the radiating particles drift out of the radiation-absorbing layer 910. At time t... s The TD1 period expired due to time delay. Figure 11 In the example, time t s At time t e Then; that is, after all charge carriers generated by the avalanche of radiating particles or charge carriers have drifted out of the radiation absorption layer 910, the TD1 period expires. Therefore, the rate of change of voltage in t s The value is essentially zero. Controller 1910 can be configured to activate the second voltage comparator 1902 at the expiration of TD1 or at t2 or any time in between.

[0084] The controller 1910 can be configured to cause the voltmeter 1906 to measure the voltage when the time delay TD1 expires. In an embodiment, the controller 1910 causes the voltmeter 1906 to measure the voltage after the rate of change of the voltage substantially becomes zero after the time delay TD1 expires. The voltage at this instant is proportional to the amount of charge carriers generated by the radiation particle or amplified by the avalanche, which is related to the energy of the radiation particle. The controller 1910 can be configured to determine the energy of the radiation particle based on the voltage measured by the voltmeter 1906. One method of determining the energy is to bin the voltage. The counter 1920 can have sub-counters for each bin. When the controller 1910 determines that the energy of the radiation particle falls into a bin, the controller 1910 can cause the number recorded in the sub-counter for that bin to be incremented by one. Thus, the system 921 can be able to detect a radiation image and can be able to resolve the radiation particle energies of individual radiation particles.

[0085] After TD1 expires, the controller 1910 connects the electrode to electrical ground for a reset period RST so that the charge carriers accumulated on the electrode can flow to ground and the voltage can be reset. After RST, the system 921 is ready to detect another incident radiation particle. Implicitly, the voltage is measured at the end of RST. Figure 11 In the example of FIG. 19, the rate at which the system 921 can process incident radiation particles is limited to 1 / (TD1+RST). If the first voltage comparator 1901 has been deactivated, the controller 1910 can activate it at any time before RST expires. If the controller 1910 has been deactivated, it can be activated before RST expires.

[0086] Although X-rays are used as an example of radiation herein, the apparatus and methods disclosed herein can also be applicable to other radiation such as infrared light.

[0087] While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. An image sensor, comprising: Multiple avalanche photodiodes (APDs); Each of the APDs includes a radiation absorption layer, which includes an absorption region and an amplification region. The absorption region is configured such that radiation particles absorbed by the radiation absorption layer generate charge carriers in the absorption region. The absorption region includes an InGaAs layer sandwiched between InP layers; The amplified region contains an electric field with a field strength sufficient to cause charge carrier avalanche in the amplified region. The interface between the InGaAs layer and the InP layer is perpendicular to the radiation receiving surface of the radiation absorbing layer.

2. The image sensor according to claim 1, wherein, The absorption region has a thickness of more than 10 micrometers.

3. The image sensor according to claim 1, wherein, The magnified region includes a doped semiconductor that is in electrical contact with the first electrode.

4. The image sensor according to claim 3, wherein, The doped semiconductor has a non-zero dopant concentration gradient.

5. The image sensor according to claim 3, wherein, The geometry of the first electrode is configured to generate the electric field.

6. The image sensor according to claim 3, wherein, The first electrode includes a tip having the shape of a cone, a truncated cone, a prism, a pyramid, a cuboid, or a cylinder.

7. The image sensor according to claim 3, wherein, The first electrode is configured to collect charge carriers generated directly by the radiating particles or by the avalanche.

8. The image sensor according to claim 3, wherein, The first electrode is configured to concentrate the electric field.

9. The image sensor according to claim 3, wherein, The first electrode extends into the radiation-absorbing layer.

10. The image sensor according to claim 1, wherein, At least one of the plurality of APDs includes an electronic device layer.

11. The image sensor of claim 3, further comprising an outer electrode arranged around the first electrode and electrically insulated from the first electrode; wherein, The external electrode is configured to shape the electric field in the amplified region.

12. The image sensor according to claim 11, wherein, The external electrode is configured not to collect charge carriers.

13. The image sensor according to claim 11, wherein, The external electrode comprises discrete regions.

14. The image sensor according to claim 3, further comprising a second electrode on the radiation-absorbing layer, the second electrode being opposite to the first electrode.

15. The image sensor according to claim 14, wherein, The second electrode is configured to collect charge carriers in the radiation-absorbing layer.

16. The image sensor according to claim 14, wherein, The second electrode is planar.

17. The image sensor according to claim 14, wherein, The second electrode includes discrete regions.

18. The image sensor of claim 17, wherein, The discrete region of the second electrode extends into the radiation-absorbing layer.

Images