Patent Description:
An image sensor or imaging sensor is a sensor that can detect a spatial intensity distribution of a radiation. An image sensor usually represents the detected image by electrical signals. Image sensors based on semiconductor devices may be classified into several types, including semiconductor charge-coupled devices (CCD), complementary metal-oxide-semiconductor (CMOS), N-type metal-oxide-semiconductor (NMOS). A CMOS image sensor is a type of active pixel sensor made using the CMOS semiconductor process. Light incident on a pixel in the CMOS image sensor is converted into an electric voltage. The electric voltage is digitized into a discrete value that represents the intensity of the light incident on that 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 incidents on the pixel, the light generates electrical charges and the charges are stored on the capacitor. The stored charges are converted to an electric voltage and the electrical voltage is digitized into a discrete value that represents the intensity of the light incident on that pixel.

<CIT> discloses a cross-strip radiation detector comprising a semiconductor having a radiation side and an opposing signal collecting side. A plurality of parallel radiation-side electrode strips is formed on the radiation side of the semiconductor and multiple signal-collecting-side electrodes are formed on a signal collecting side.

The described embodiments seek to provide a significant reduction in low-energy tailing in a semiconductor detector by provision of a novel arrangement of electrodes that share.

<CIT> discloses a radiation detector having intermediate electrodes formed between a photoelectric conversion layer and an avalanche layer. The electrodes are formed in a lattice defined by the arrangement of electric charge collection electrodes.

<CIT> discloses an imaging device comprising a photoelectric conversion layer, a multiplication and accumulation layer utilising avalanche electron multiplication and a wiring substrate for reading out signal charges. The photoelectric conversion layer includes a first conductive impurity layer, an electron acceleration layer and a second conductive impurity layer to which a voltage is applied. An insulating layer is disposed between the electron acceleration layer and the multiplication and accumulation layer.

<CIT> discloses a semiconductor photodetector configured to provide charge carrier avalanche multiplication at high field regions of a semiconductor material layer. A semiconductor current amplifier may provide current amplification by impact ionization near a high field region. A plurality of metal electrodes is formed on a surface of a semiconductor material layer and electrically biased to produce a non-uniform high electric field in which the high electric field strength accelerates avalanche electron-hole pair generation.

According to a first aspect of the present invention, as defined in independent claim <NUM>, there is provided an apparatus comprising: a radiation absorption layer configured to generate charge carriers therein from a radiation particle absorbed by the radiation absorption layer, the radiation absorption layer comprising semiconductor material having a doped region; a first electrode on a surface of the radiation absorption layer in electrical contact with the doped region; a second electrode on an opposite surface of the radiation absorption layer from the first electrode; wherein the first electrode comprises a tip which is tapered or has the shape of a cone, frustum, prism, pyramid, cuboid or cylinder to define a geometry configured to generate an electric field between the first and second electrodes in an amplification region of the radiation absorption layer, the electric field having a field strength sufficient to cause an avalanche of the charge carriers in the amplification region.

Preferred embodiments of the apparatus of the invention are defined in dependent claims <NUM> to <NUM>.

According to a second aspect of the present invention, as defined in independent claim <NUM>, there is provided a method comprising: forming a doped region of a semiconductor substrate by doping a surface of the semiconductor substrate with dopants; driving the dopants into the semiconductor substrate by annealing the semiconductor substrate; controlling doping profile of the doped region by repeating doping and annealing the semiconductor substrate; forming a first electrode on a surface of the semiconductor substrate, wherein the first electrode is in electrical contact with the doped region and comprises a tip which is tapered or has the shape of a cone, frustum, prism, pyramid, cuboid or cylinder; forming a second electrode on an opposite surface of the radiation absorption layer from the first electrode; and forming an outer electrode arranged around the first electrode, wherein the outer electrode is electrically insulated from the first electrode.

Preferred embodiments of the method of the invention are defined in dependent claims <NUM> to <NUM>.

Charge carrier avalanche is a process where free charge carriers in a material are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the material, thereby ionizing them (impact ionization) and releasing additional charge carriers which accelerate and collide with further atoms, releasing more charge carriers-a chain reaction. Impact ionization is a process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound electron out of its bound state (in the valence band) and promote it to a state in the conduction band, creating an electron-hole pair. One example of an electronic device using the charge carrier avalanche is an avalanche photodiode (APD), which uses charge carrier avalanche to generate an electric current upon exposure to light. An APD will be used as an example to describe the charge carrier avalanche but the description may be applicable to other electronic devices that use the charge carrier avalanche.

An APD may work in the Geiger mode or the linear mode. When the APD works in the Geiger mode, it may be called a single-photon avalanche diode (SPAD) (also known as a Geiger-mode APD or G-APD). A SPAD is an APD working under a reverse bias above the breakdown voltage. Here the word "above" means that absolute value of the reverse bias is greater than the absolute value of the breakdown voltage. A SPAD may be used to detect low intensity light (e.g., down to a single photon) and to signal the arrival times of the photons with a jitter of a few tens of picoseconds. A SPAD may be in a form of a p-n junction under a reverse bias (i.e., the p-type region of the p-n junction is biased at a lower electric potential than the n-type region) above the breakdown voltage of the p-n junction. The breakdown voltage of a p-n junction is a reverse bias, above which exponential increase in the electric current in the p-n junction occurs. An APD working at a reverse bias below the breakdown voltage is operating in the linear mode because the electric current in the APD is proportional to the intensity of the light incident on the APD.

<FIG> schematically shows the electric current in an APD as a function <NUM> of the intensity of light incident on the APD when the APD is in the linear mode, and a function <NUM> of the intensity of light incident on the APD when the APD is in the Geiger mode (i.e., when the APD is a SPAD). In the Geiger mode, the current shows a very sharp increase with the intensity of the light and then saturation. In the linear mode, the current is essentially proportional to the intensity of the light.

<FIG> schematically show the operation of an APD, according to an embodiment. <FIG> shows that when a radiation particle (e.g., an X-ray photon) is absorbed by an absorption region <NUM>, one or more (<NUM> to <NUM> for an X-ray photon) electron-hole pairs maybe generated. The absorption region <NUM> has a sufficient thickness and thus a sufficient absorptance (e.g., ><NUM>% or ><NUM>%) for the incident photon. For soft X-ray photons, the absorption region <NUM> may be a silicon layer with a thickness of <NUM> microns or above. The electric field in the absorption region <NUM> is not high enough to cause avalanche effect in the absorption region <NUM>. <FIG> shows that the electrons and hole drift in opposite directions in the absorption region <NUM>. <FIG> shows that avalanche effect occurs in an amplification region <NUM> when the electrons (or the holes) enter that amplification region <NUM>, thereby generating more electrons and holes. The electric field in the amplification region <NUM> is high enough to cause an avalanche of charge carriers entering the amplification region <NUM> but not too high to make the avalanche effect self-sustaining. A self-sustaining avalanche is an avalanche that persists after the external triggers disappear, such as radiation particles incident on the APD or charge carriers drifted into the APD. The electric field in the amplification region <NUM> may be a result of a doping profile in the amplification region <NUM>, or the structure of the amplification region <NUM>. For example, the amplification region <NUM> may include a p-n junction or a heterojunction that has an electric field in its depletion zone. The threshold electric field for the avalanche effect (i.e., the electric field above which the avalanche effect occurs and below which the avalanche effect does not occur) is a property of the material of the amplification region <NUM>. The amplification region <NUM> may be on one or two opposite sides of the absorption region <NUM>.

<FIG> schematically shows a cross-sectional view of an apparatus <NUM> suitable for radiation detection, according to an embodiment. The apparatus <NUM> may comprise a radiation absorption layer <NUM> and one or more electrodes <NUM> on the radiation absorption layer <NUM>. The radiation absorption layer <NUM> may be configured to generate charge carriers therein from a radiation particle absorbed by the radiation absorption layer <NUM>. The one or more electrodes <NUM> may be configured to generate an electric field <NUM> in the radiation absorption layer <NUM>. Each of the one or more electrodes <NUM> may have a geometry (e.g., a small tapered tip) shaping the electric field <NUM> so that the electric field <NUM> in one or more portions (i.e., one or more amplification regions <NUM>) of the radiation absorption layer <NUM> has a field strength sufficient to cause an avalanche of the charge carriers (e.g., electrons or holes) in the one or more amplification regions <NUM>. The_charger carriers, either generated by the avalanche or directly from the radiation particles, drift to and are collected by the one or more electrodes <NUM> or a different electrode. The apparatus <NUM> may further include a passivation material <NUM> configured to passivate a surface of the radiation absorption layer <NUM> to reduce recombination of charge carriers at the surface. The apparatus <NUM> may_further comprise a counter electrode <NUM> on the radiation absorption layer <NUM>, the counter electrode <NUM> being opposite the one or more electrodes <NUM>. The counter electrode <NUM> may be configured to collect charge carriers in the radiation absorption layer <NUM>.

The radiation absorption layer <NUM> may comprise a semiconductor material such as silicon. The radiation absorption layer <NUM> may have a sufficient thickness and thus a sufficient absorbance (e.g., ><NUM>% or ><NUM>%) for incident radiation particles of interest (e.g., X-ray photons). The radiation absorption layer <NUM> may have a thickness of <NUM> microns or above.

In an embodiment, the radiation absorption layer <NUM> may be an intrinsic semiconductor. In an embodiment, the radiation absorption layer <NUM> may comprise a doped region <NUM> that is lightly doped with a dopant. A semiconductor is considered to be lightly doped when the semiconductor contains a proportion of dopant to semiconductor atom being small enough so that the electronic states of the dopants at the Fermi level are localized (i.e., the band of the dopant may not overlap with the conduction or valence band of the semiconductor). For instance, lightly doped silicon may have a ratio of dopants to silicon atoms on the order of <NUM>/<NUM><NUM>. The doped region <NUM> may extend a few microns from a surface into the interior region of the radiation absorption layer <NUM>, and may have a non-zero concentration gradient of the dopant. In the example of <FIG>, the concentration of the dopant gradually decreases from the surface to the interior region of the radiation absorption layer <NUM>. The doped region <NUM> may be in electrical contact with the electrodes <NUM>. In an embodiment, the doped region <NUM> may comprise discrete regions, each of which is around one of the electrodes <NUM>.

The one or more electrodes <NUM> may comprise a conducting material such as a metal (e.g., gold, copper, aluminum, platinum, etc.), or any other suitable conducting materials (e.g., a heavily doped semiconductor). The one or more electrodes <NUM> may have small dimensions or a suitable shape so that the electric field <NUM> near the one or more electrodes <NUM> is concentrated. For example, the one or more electrodes <NUM> may comprise a tip with a shape of cone, frustum, prism, pyramid, cuboid, or cylinder, etc. In the example of <FIG>, the tip is flat and cylindrical. The flat tips of the electrodes <NUM> in <FIG> each have a contact area with the radiation absorption layer <NUM> small enough to have the electric field <NUM> near the tips become strong enough to cause avalanche of charge carriers near the tips. In other words, the strength of the electrical field <NUM> increases when approaching the electrodes <NUM>, and the amplification regions <NUM> in <FIG> are regions around the tips of the electrodes <NUM> where the electrical field <NUM> is strong enough to cause avalanche of charge carriers. In an embodiment, the one or more amplification regions <NUM> correspond to the one or more electrodes <NUM> respectively. An amplification region <NUM> corresponding to one electrode <NUM> may not be joined with another amplification region <NUM> corresponding to another electrode <NUM>. In an embodiment, the electrical field <NUM> is not strong enough to cause self-sustaining avalanche; namely, the electric field <NUM> in the amplification regions <NUM> should cause avalanche when there are incident radiation particles in the radiation absorption layer <NUM> but the avalanche should cease without further incident radiation particles in the radiation absorption layer <NUM>.

When the radiation hits the radiation absorption layer <NUM>, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of the radiation may generate <NUM> to <NUM> charge carriers. One type (electrons or holes) of the charge carriers drift toward the amplification regions <NUM>. The charge carriers may drift in directions such that substantially all (more than <NUM>%, more than <NUM>%, more than <NUM>% or more than <NUM>% of) charge carriers generated by a radiation particle incident around the footprint <NUM> of one of the electrodes <NUM> flow to the amplification region <NUM> corresponding to the electrode <NUM>. Namely, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow beyond the amplification region <NUM> corresponding to the electrode <NUM>. When the charge carriers enter the amplification region <NUM>, the avalanche effect occurs and causes amplification of the charge carriers. The amplified charge carriers can be collected through the corresponding electrodes <NUM>, as an electric current. In the linear mode, the electric current is proportional to the number of incident radiation particles around the footprint <NUM> of the electrode <NUM> per unit time (i.e., proportional to the radiation intensity). The electric currents at the electrodes <NUM> may be compiled to represent a spatial intensity distribution of radiation, i.e., an image.

<FIG> shows a variant of the apparatus <NUM>, where the electrodes <NUM> may extended into the radiation absorption layer <NUM>, according to an embodiment. The portion of each of the electrodes <NUM> extending into radiation absorption layer <NUM> may have small dimensions or a suitable shape so that the electric field <NUM> near the portion is concentrated. For example, the portion may comprise a tip with a shape of cone, frustum, prism, pyramid, cuboid, or cylinder, etc. In the example of <FIG>, the tip is tapered, and the electric field <NUM> near the tapered tips become strong enough to cause avalanche of charge carriers near the tips. In other words, the strength of the electrical field <NUM> increases when approaching the portions of the electrodes <NUM>, and the amplification regions <NUM> in <FIG> are regions around the portions where the electrical field <NUM> is strong enough to cause avalanche of charge carriers.

<FIG> shows a variant of the apparatus <NUM>, where the apparatus <NUM> may further comprise one or more outer electrodes <NUM>, according to an embodiment. The one or more outer electrodes <NUM> correspond to and locate around the one or more electrodes <NUM> respectively. The outer electrodes <NUM> are electrically insulated from the electrodes <NUM>. For example, an insulation region (e.g., a portion of the passivation material <NUM>) may exist in between an outer electrode <NUM> and its corresponding electrode <NUM>.

In the example of <FIG>, the outer electrode <NUM> and its corresponding electrode <NUM> are coaxial. The outer electrode <NUM> may comprise a conducting material such as a metal (e.g., gold, copper, aluminum, platinum, etc.), or any other suitable conducting materials (e.g., a heavily doped semiconductor).

The outer electrode <NUM> may be configured to shape the electric field <NUM> in the amplification region <NUM> of the electrode <NUM> corresponding to the outer electrode <NUM>, and the outer electrode <NUM> may not be configured to collect charge carriers. For example, the electric field <NUM> (e.g., its strength, gradient) may be tuned by introducing a voltage difference between the outer electrode <NUM> and its corresponding electrode <NUM>. In an embodiment, the outer electrode <NUM> may have a same voltage with the counter electrode <NUM>. In an embodiment, the outer electrode <NUM> may not necessarily be a ring as shown in <FIG>, but can have discrete portions.

In an embodiment, the counter electrode <NUM> may be planar, as shown in <FIG>. The counter electrode <NUM> may comprise discrete regions.

In an embodiment, as shown in <FIG>, the counter electrode <NUM> each may have geometry similar to one of the electrodes <NUM> and may extend into the radiation absorption layer <NUM>. Avalanche of charge carriers may also take place in regions near the discrete regions of the counter electrode <NUM>. The counter electrode <NUM> may comprise a conducting material such as a metal (e.g., gold, copper, aluminum, platinum, etc.), or any other suitable conducting materials (e.g., a heavily doped semiconductor).

In the example of <FIG>, the electrodes <NUM> and the counter electrode <NUM> have structures extending into the radiation absorption layer <NUM>. For example, the structures may be holes are drilled into the radiation absorption layer <NUM>(e.g., by deep reactive-ion etching (DRIE) or laser and filled with a metal. The structures may form an Ohmic contact or a Schottky contact with the materials of the radiation absorption layer <NUM>. The structures of the electrodes <NUM> and the structures of the counter electrode <NUM> may form an interdigitate pattern but should not be electrically short. Each of the structures of the electrodes <NUM> may be spaced by a short distance (e.g., <NUM>, <NUM> or <NUM>) from the nearest one of the structures of the counter electrode <NUM> to have the electric field <NUM> in between these structures become strong enough to cause avalanche of charge carrier in between these structures. In other words, the amplification regions <NUM> in <FIG> are regions in between these structure or near the tips of the structures where the electrical field <NUM> is strong enough to cause avalanche of charge carriers. These structures may help collecting charge carriers generated from a radiation particle or by the avalanche. The charge carriers only need to drift to one of these structures rather than the surfaces of the radiation absorption layer <NUM>, thereby reducing the chance of recombination or trapping. The time for the charge carriers to be collected by these structures may be on the order of <NUM>-<NUM> ns.

<FIG> schematically illustrates a process of forming the apparatus <NUM>, according to an embodiment.

In step <NUM>, a semiconductor substrate <NUM> is obtained. The semiconductor substrate <NUM> may comprise an intrinsic semiconductor such as silicon. The semiconductor substrate <NUM> may have a sufficient thickness and thus a sufficient absorbance (e.g., ><NUM>% or ><NUM>%) for incident radiation particles of interest (e.g., X-ray photons). The semiconductor substrate <NUM> may have a thickness of <NUM> microns or above.

In step <NUM>-step <NUM>, the semiconductor substrate <NUM> may be doped to form a doped region <NUM> (shown in step <NUM>-step <NUM>). The doped region <NUM> may function as the doped region <NUM> of the radiation absorption layer <NUM> in <FIG>. In the example of <FIG>, the doped region <NUM> to be formed is a continuous layer. In an embodiment, the semiconductor substrate <NUM> is a silicon substrate, the desired doped region <NUM> is lightly doped and have a non-zero concentration gradient of the dopant extending a few microns from the surface into the interior region of the semiconductor substrate <NUM>. The concentration of the dopant may gradually decrease from the surface to the interior region of the semiconductor substrate <NUM>.

In step <NUM>, a mask layer <NUM> is formed on a surface of the semiconductor substrate <NUM>. The mask layer <NUM> may serve as a screening layer configured to retard entry of dopants into the semiconductor substrate <NUM> in the step <NUM> of doping. The mask layer <NUM> may comprise a material such as silicon dioxide. The thickness of the mask layer <NUM> may be determined according to doping conditions in step <NUM> and desired doping profile of the doped region <NUM> (shown in step <NUM>-step <NUM>) to be formed. The mask layer <NUM> may be formed onto the surface by various techniques, such as thermal oxidation, vapor deposition, spin coating, sputtering or any other suitable processes.

In step <NUM>, a surface of the semiconductor substrate <NUM> is light doped with a suitable dopant <NUM> by a doping technique such as dopant diffusion and ion implantation. The rate of dopant entering into the semiconductor substrate <NUM> may be controlled by the mask layer <NUM>, the dose of dopants doped, and doping details such as the energy of the dopants during an ion implantation.

In step <NUM>, the semiconductor substrate <NUM> being doped is annealed to drive the dopants into the interior region of the semiconductor substrate <NUM>. The dopants diffuse into the interior region at elevated temperatures (e.g., around <NUM>). The annealing duration may be prolonged to promote diffusion of the dopants into the interior region. The high-temperature environment of the annealing may also help anneal out defects of the semiconductor substrate <NUM>.

Besides controlling the doping and annealing conditions, the doping (step <NUM>) and annealing (step <NUM>) may be carried out in a repeating manner for a number of times to form the doped region <NUM> with a desired doping profile.

In an embodiment, the doped region <NUM> may comprise discrete regions. The mask layer <NUM> may have a pattern with areas of different thicknesses. A portion of dopants can penetrate through the thinner areas of the mask layer and form discrete regions of the doped region <NUM>, while the thicker areas of the mask layer prevent the dopants entering into the semiconductor substrate <NUM>.

In step <NUM>, the mask layer <NUM> may be removed by wet etching, chemical mechanical polishing or some other suitable techniques.

In step <NUM>, electrodes <NUM> may be formed onto the semiconductor substrate <NUM>. The electrodes <NUM> may function as the electrodes <NUM> of the apparatus <NUM>. The electrodes <NUM> may be in electrical contact with the doped region <NUM>. In the example of step <NUM>, the electrodes <NUM> each comprise a tapered tip extending into the semiconductor substrate <NUM>. Forming the electrode <NUM> may involve forming a mask with openings on the surface of the semiconductor substrate <NUM> by suitable techniques such as lithography. Shapes and locations of the openings correspond to the footprint shapes and locations of the electrodes <NUM> to be formed. Recesses of desired shape and dimensions are formed into the surface of the semiconductor substrate <NUM> by etching portions of the substrate <NUM> uncovered by the mask. The etching process may be carried out by a technique such as dry etching (e.g., deep reactive-ion etching), wet etching (e.g., anisotropic wet etching), or a combination thereof. Conducing materials such as metal (e.g., gold, copper, aluminum, platinum, etc.) may be deposited into the recesses to form the electrodes <NUM> by a suitable technique such as physical vapor deposition, chemical vapor deposition, spin coating, sputtering, etc. The mask may be kept and server as a passivation layer of the surface of the substrate <NUM>. In an embodiment, the mask may be removed and a passivation material <NUM> may be applied to passivate the surface of the substrate <NUM>.

In optional step <NUM>, outer electrodes <NUM> may be formed around the electrodes <NUM>. The electrodes <NUM> may function as the outer electrodes <NUM> in <FIG>. Forming the outer electrodes <NUM> may involve mask forming and metal deposition processes similar to the step <NUM>.

In step <NUM>, a counter electrode <NUM> may be formed on another surface of the semiconductor substrate <NUM>. The counter electrode <NUM> may function as the counter electrode <NUM> of the apparatus <NUM>. In the example of step <NUM>, the counter electrode <NUM> is planar and may be formed by depositing conducting materials such as metals onto the other surface of the semiconductor substrate <NUM> by a suitable technique such as vapor deposition, sputtering, etc..

Forming the apparatus <NUM> may comprise some intermediate steps such as surface cleaning, polishing, surface passivation, which are not shown in <FIG>. The order of the steps shown in <FIG> may be changed to suit different formation needs.

<FIG> schematically shows a system comprising an imaging sensor <NUM> being an embodiment of the apparatus <NUM> described herein. The system comprises an X-ray source <NUM>. X-ray emitted from the X-ray source <NUM> penetrates an object <NUM> (e.g., diamonds, tissue samples, a human body part such as breast), is attenuated by different degrees by the internal structures of the object <NUM>, and is projected to the image sensor <NUM>. The image sensor <NUM> forms an image by detecting the intensity distribution of the X-ray. The system may be used for medical imaging such as chest X-ray radiography, abdominal X-ray radiography, dental X-ray radiography, mammography, etc. The system may be used for industrial CT, such as diamond defect detection, scanning a tree to visualize year periodicity and cell structure, scanning building material like concrete after loading, etc..

<FIG> schematically shows an X-ray computed tomography (X-ray CT) system. The X-ray CT system uses computer-processed X-rays to produce tomographic images (virtual "slices") of specific areas of a scanned object. The tomographic images may be used for diagnostic and therapeutic purposes in various medical disciplines, or for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering. The X-ray CT system comprises the image sensor <NUM> being an embodiment of the apparatus <NUM> described herein and an X-ray source <NUM>. The image sensor <NUM> and the X-ray source <NUM> may be configured to rotate synchronously along one or more circular or spiral paths.

<FIG> schematically shows an X-ray microscope or X-ray micro CT <NUM>. The X-ray microscope or X-ray micro CT <NUM> may include an X-ray source <NUM>, focusing optics <NUM>, and the image sensor <NUM> being an embodiment of the apparatus <NUM> described herein, for detecting an X-ray image of a sample <NUM>.

<FIG> schematically shows a system <NUM> suitable for laser scanning, according to an embodiment. The system <NUM> comprises a laser source <NUM> and a detector <NUM> being an embodiment of the apparatus <NUM> described herein. The laser source <NUM> may be configured to generate a scanning laser beam. The scanning laser beam may be infrared. In an embodiment, the laser source <NUM> may perform two-dimensional laser scanning without moving part. The detector <NUM> may be configured to collect return laser signals after the scanning laser beam bounces off an object, building or landscape and generate electrical signals. The system <NUM> may further comprise a signal processing system configured to process and analyze the electrical signals generated by the detector <NUM>. In one embodiment, the distance and shape of the object, building or landscape may be obtained. The system <NUM> may be a Lidar system (e.g., an on-vehicle Lidar).

<FIG> schematically shows a radiation detector <NUM>, as an example. The radiation detector <NUM> has an array of pixels <NUM>. The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel <NUM> is configured to detect radiation from a radiation source incident thereon and may be configured measure a characteristic (e.g., the energy of the particles, the intensity distribution) of the radiation. Each pixel <NUM> may 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 to digitize an analog signal representing the total energy of a plurality of incident radiation particles into a digital signal. The pixels <NUM> may be configured to operate in parallel. For example, when one pixel <NUM> measures an incident radiation particle, another pixel <NUM> may be waiting for a radiation particle to arrive. The pixels <NUM> may not have to be individually addressable.

<FIG> schematically shows a cross-sectional view of the radiation detector <NUM>, according to an embodiment. The radiation detector <NUM> may comprise a radiation absorption layer <NUM> being an embodiment of the apparatus <NUM> described herein, and an electronics layer <NUM> (e.g., an ASIC) for processing or analyzing electrical signals generated by incident radiation or charge carrier avalanche within the radiation absorption layer <NUM>.

The electronics layer <NUM> may include an electronic system <NUM> suitable for processing or interpreting the electrical signals. The electronic system <NUM> may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessors, and memory. The electronic system <NUM> may include one or more ADCs. The electronic system <NUM> may include components shared by the pixels or components dedicated to a single pixel. For example, the electronic system <NUM> may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. The electronic system <NUM> may be electrically connected to the pixels by vias <NUM>. Space among the vias may be filled with a filler material <NUM>, which may increase the mechanical stability of the connection of the electronics layer <NUM> to the radiation absorption layer <NUM>. Other bonding techniques are possible to connect the electronic system <NUM> to the pixels without using vias.

<FIG> and <FIG> each show a component diagram of the electronic system <NUM>, according to an embodiment. The electronic system <NUM> may include a first voltage comparator <NUM>, a second voltage comparator <NUM>, a counter <NUM>, a switch <NUM>, a voltmeter <NUM> and a controller <NUM>.

The first voltage comparator <NUM> is configured to compare the voltage of an electrode (e.g., one of the electrodes <NUM> in <FIG>) to a first threshold. The first voltage comparator <NUM> may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the electrode over a period of time. The first voltage comparator <NUM> may be controllably activated or deactivated by the controller <NUM>. The first voltage comparator <NUM> may be a continuous comparator. Namely, the first voltage comparator <NUM> may be configured to be activated continuously, and monitor the voltage continuously. The first voltage comparator <NUM> configured as a continuous comparator reduces the chance that the system <NUM> misses signals generated directly by an incident radiation particle or by charge carrier avalanche. The first voltage comparator <NUM> configured as a continuous comparator is especially suitable when the incident radiation intensity is relatively high. The first voltage comparator <NUM> may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator <NUM> configured as a clocked comparator may cause the system <NUM> to miss signals generated directly by some incident radiation particles or by 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 successive radiation particles is relatively long. Therefore, the first voltage comparator <NUM> configured as a clocked comparator is especially suitable when the incident radiation intensity is relatively low. The first threshold may be <NUM>-<NUM>%, <NUM>%-<NUM>%, <NUM>-<NUM>%, <NUM>-<NUM>% or <NUM>-<NUM>% of the maximum voltage one incident radiation particle may generate directly in the radiation absorption layer or after being amplified by avalanche in the radiation absorption layer. The maximum voltage may depend on the energy of the incident radiation particle (i.e., the wavelength of the incident radiation), the material of the radiation absorption layer <NUM>, magnitude of charge carrier avalanche and other factors. For example, the first threshold may be <NUM> mV, <NUM> mV, <NUM> mV, or <NUM> mV.

The second voltage comparator <NUM> is configured to compare the voltage to a second threshold. The second voltage comparator <NUM> may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the electrode over a period of time. The second voltage comparator <NUM> may be a continuous comparator. The second voltage comparator <NUM> may be controllably activated or deacivated by the controller <NUM>. When the second voltage comparator <NUM> is deactivated, the power consumption of the second voltage comparator <NUM> may be less than <NUM>%, less than <NUM>%, less than <NUM>% or less than <NUM>% of the power consumption when the second voltage comparator <NUM> is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term "absolute value" or "modulus" |x| of a real number x is the non-negative value of x without regard to its sign. Namely, |x| = <MAT>. The second threshold may be <NUM>%-<NUM>% of the first threshold. The second threshold may be at least <NUM>% of the maximum voltage one incident radiation particle may generate directly in the radiation absorption layer or after being amplified in the radiation absorption layer. For example, the second threshold may be <NUM> mV, <NUM> mV, <NUM> mV, <NUM> mV or <NUM> mV. The second voltage comparator <NUM> and the first voltage comparator <NUM> may be the same component. Namely, the system <NUM> may have one voltage comparator that can compare a voltage with two different thresholds at different times.

The first voltage comparator <NUM> or the second voltage comparator <NUM> may include one or more op-amps or any other suitable circuitry. The first voltage comparator <NUM> or the second voltage comparator <NUM> may have a high speed to allow the system <NUM> to operate under a high flux of incident radiation particle. However, having a high speed is often at the cost of power consumption.

The counter <NUM> is configured to register a number of radiation particles reaching the radiation absorption layer. The counter <NUM> may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a <NUM> IC and a <NUM> IC).

The controller <NUM> may be a hardware component such as a microcontroller and a microprocessor. The controller <NUM> is configured to start a time delay from a time at which the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on which electrode is used. The controller <NUM> may be configured to keep deactivated the second voltage comparator <NUM>, the counter <NUM> and any other circuits the operation of the first voltage comparator <NUM> does not require, before the time at which the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase "the rate of change of the voltage is substantially zero" means that temporal change of the voltage is less than <NUM>%/ns. The phase "the rate of change of the voltage is substantially non-zero" means that temporal change of the voltage is at least <NUM>%/ns.

The controller <NUM> may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller <NUM> is configured to activate the second voltage comparator at the beginning of the time delay. The term "activate" means causing the component to enter 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 causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cut off power, etc.). The operational state may have higher power consumption (e.g., <NUM> times higher, <NUM> times higher, <NUM> times higher) than the non-operational state. The controller <NUM> itself may be deactivated until the output of the first voltage comparator <NUM> activates the controller <NUM> when the absolute value of the voltage equals or exceeds the absolute value of the first threshold.

The controller <NUM> may be configured to cause the number registered by the counter <NUM> to increase by one, if, during the time delay, the second voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold.

The controller <NUM> may be configured to cause the voltmeter <NUM> to measure the voltage upon expiration of the time delay. The controller <NUM> may be configured to connect the electrode to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electrode is connected to an electrical ground for a finite reset time period. The controller <NUM> may connect the electrode to the electrical ground by controlling the switch <NUM>. The switch <NUM> may be a transistor such as a field-effect transistor (FET).

In an embodiment, the system <NUM> has no analog filter network (e.g., a RC network). In an embodiment, the system <NUM> has no analog circuitry.

The voltmeter <NUM> may feed the voltage it measures to the controller <NUM> as an analog or digital signal.

The system <NUM> may include a capacitor module <NUM> electrically connected to the electrode, wherein the capacitor module is configured to collect charge carriers from the electrode. The capacitor module can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electrode accumulate on the capacitor over a period of time ("integration period") (e.g., as shown in <FIG>, between t<NUM> to t<NUM>, or t<NUM>-t<NUM>). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The capacitor module can include a capacitor directly connected to the electrode.

<FIG> schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by an incident radiation particle or charge carrier avalanche in the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve). The voltage may be an integral of the electric current with respect to time. At time t<NUM>, the radiation particle hits the radiation absorption layer, charge carriers start being generated and being amplified in the radiation absorption layer, electric current starts to flow through the electrode, and the absolute value of the voltage of the electrode starts to increase. At time t<NUM>, the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1, and the controller <NUM> starts the time delay TD1 and the controller <NUM> may deactivate the first voltage comparator <NUM> at the beginning of TD1. If the controller <NUM> is deactivated before t<NUM>, the controller <NUM> is activated at t<NUM>. During TD1, the controller <NUM> activates the second voltage comparator <NUM>. The term "during" a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller <NUM> may activate the second voltage comparator <NUM> at the expiration of TD1. If during TD1, the second voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t<NUM>, the controller <NUM> causes the number registered by the counter <NUM> to increase by one. At time te, all charge carriers generated by the radiation particle drift out of the radiation absorption layer <NUM>. At time ts, the time delay TD1 expires. In the example of <FIG>, time ts is after time te; namely TD1 expires after all charge carriers generated by the radiation particle or charge carrier avalanche drift out of the radiation absorption layer <NUM>. The rate of change of the voltage is thus substantially zero at ts. The controller <NUM> may be configured to deactivate the second voltage comparator <NUM> at expiration of TD1 or at t<NUM>, or any time in between.

The controller <NUM> may be configured to cause the voltmeter <NUM> to measure the voltage upon expiration of the time delay TD1. In an embodiment, the controller <NUM> causes the voltmeter <NUM> to measure the voltage after the rate of change of the voltage becomes substantially zero after the expiration of the time delay TD1. The voltage at this moment is proportional to the amount of charge carriers generated by a radiation particle or amplified by the avalanche, which relates to the energy of the radiation particle. The controller <NUM> may be configured to determine the energy of the radiation particle based on voltage the voltmeter <NUM> measures. One way to determine the energy is by binning the voltage. The counter <NUM> may have a sub-counter for each bin. When the controller <NUM> determines that the energy of the radiation particle falls in a bin, the controller <NUM> may cause the number registered in the sub-counter for that bin to increase by one. Therefore, the system <NUM> may be able to detect a radiation image and may be able to resolve radiation particle energies of each radiation particle.

After TD1 expires, the controller <NUM> connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. After RST, the system <NUM> is ready to detect another incident radiation particle. Implicitly, the rate of incident radiation particles the system <NUM> can handle in the example of <FIG> is limited by <NUM>/(TD1+RST). If the first voltage comparator <NUM> has been deactivated, the controller <NUM> can activate it at any time before RST expires. If the controller <NUM> has been deactivated, it may be activated before RST expires.

Although X-ray is used as an example of the radiation herein, the apparatuses and methods disclosed herein may also be suitable for other radiation such as infrared light.

Claim 1:
An apparatus (<NUM>) comprising:
a radiation absorption layer (<NUM>) configured to generate charge carriers therein from a radiation particle absorbed by the radiation absorption layer, the radiation absorption layer comprising semiconductor material having a doped region (<NUM>);
a first electrode (<NUM>) on a surface of the radiation absorption layer in electrical contact with the doped region;
a second electrode (<NUM>);
wherein the first electrode comprises a tip which is tapered or has the shape of a cone, frustum, prism, pyramid, cuboid or cylinder to define a geometry configured to generate an electric field between the first and second electrodes in an amplification region of the radiation absorption layer, the electric field having a field strength sufficient to cause an avalanche of the charge carriers in the amplification region;
said apparatus being characterized in that said second electrode (<NUM>) is on an opposite surface of the radiation absorption layer from the first electrode (<NUM>).