# Patent

## Title
High sensitivity, high resolution detection of signals

## Abstract

A system and method providing for the detection of an input signal by distributing the input signal into independent signal components that are independently amplified. Detection of an input signal comprises generating from the input signal a plurality of spatially separate elementary charge components, each having a respective known number of elementary charges, the number of the plurality of spatially separate elementary charge components being a known monotonic function of the magnitude of said input signal; and independently amplifying each of the plurality of spatially divided elementary charge components to provide a respective plurality of signal charge packets, each signal charge packet having a second number of elementary charges greater than the respective known number by a respective amplification factor.

## Background
This is a continuation of application Ser. No. 10/208,530, filed Jul. 30, 2002 now U.S. Pat. No. 6,885,827.


TECHNICAL FIELD
The present invention relates generally to signal detection and, more particularly, to detecting a weak or low level signal, such as a weak optical or electrical signal, and preferably to detecting such a weak signal with high resolution of the signal magnitude.
BACKGROUND OF THE INVENTION
Recording and measuring a weak signal represented by, for example, one to several dozen elementary charge carriers presents challenging and acute problems for the designers of modern sensors and transducers for myriad applications in diverse fields of science and technology. In these sensors and transducers, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes, or ions, depending on the specific types and versions of the devices being developed for this purpose. Signal charge packets of such elementary charge carriers are amplified and converted to a signal (e.g., to a voltage signal) that is fed into a recording or analyzing device itself and/or as a feedback signal into a controller of the mechanisms or processes that are monitored with the sensors or transducers.
In many applications, such as those relating to laser information and metering devices, recording and image transfer systems, and radiation or particle detecting systems in the physics and nuclear engineering fields, high-speed sensor devices with critical threshold parameters are in an acute demand. Such applications demand sensors capable of detecting and recording of electrical signals that are not only weak (e.g., as few as one or several elementary charge carriers), but also short in duration and/or rapidly varying (i.e., have a large bandwidth). Accordingly, these applications require a sensor capable of amplifying such electrical signals over a wide bandwidth and with a low noise level. It is well known that the first stage of signal amplification primarily determines the basic parameters and characteristics of the sensor device, such as its threshold sensitivity, signal resolution, and response speed (e.g., bandwidth), and presently, generally two paradigms or approaches are being followed in developing sensors having signal amplification characteristics suited for detecting and recording weak electrical signals.
One widespread approach consists in “perfecting” or optimizing traditional analog amplifiers, in which all sets of the input charge carriers are amplified simultaneously. Noise reduction with simultaneous improvement of the threshold sensitivity is achieved primarily by decreasing the geometric size of the amplifier stage of the device. By applying such a scaling approach to charge-coupled device (CCD) video amplifiers, for example, it is possible to attain a threshold sensitivity of several dozen electrons. Such a scaling approach, however, does not solve the problem of recording signals consisting of several electrons.
Another approach to sensing weak electrical signals is using avalanche amplification (multiplication) of signal carriers, which generally is the most sensitive and high-speed method of amplification known in the art. As is well known, avalanche amplification is based on impact ionization arising in a strong electric field, wherein the signal carriers accelerating in an electric field ionize the atoms of the working medium of the amplifier, thus resulting in multiplication (e.g., duplication) of the signal carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal (excessive) noise level and the response time grow rapidly with increasing multiplication factor. Due to these problems associated with using a large multiplication factor, traditional avalanche photodiodes use a rather low multiplication factor, M, typically less than 103, that does not allow for detecting and recording signals consisting of several electrons in a wide band.
Avalanche multiplication has also been applied to recording individual ionizing particles using a Geiger-Muller counter. A particle entering such a device initiates an avalanche-like process of multiplication of the signal carriers up to a necessary recording level. More recently, this principle has been successfully used for recording single charge carriers in semiconductor avalanche-type photodiodes. This Geiger-Muller principle of amplification, however, does not allow for distinguishing between signals of one and several input charge carriers (i.e., it does not provide high resolution of the number of charge carriers).
It may be appreciated, therefore, that there remains a need for further advancements and improvements in detecting weak signals, and particularly in providing a system and method for high sensitivity and high resolution detection of signals, as well as for such high resolution detection of weak signals with a high bandwidth.
SUMMARY OF THE INVENTION
The present invention provides such advancements and overcomes the above mentioned problems and other limitations of the background and prior art, by providing a system and method providing for the detection of an input signal by distributing the input signal into independent signal components that are independently amplified. In accordance with an aspect of the present invention, a system and method providing for the detection of an input signal comprises generating from the input signal a plurality of spatially separate elementary charge components, each having a respective known number of elementary charges, the number of the plurality of spatially separate elementary charge components being a known monotonic function of the magnitude of said input signal; and independently amplifying each of the plurality of spatially divided elementary charge components to provide a respective plurality of signal charge packets, each signal charge packet having a second number of elementary charges greater than the respective known number by a respective amplification factor.
In accordance with another aspect of the invention, the number of the plurality of spatially separate elementary charge components is proportional to the magnitude of said input signal. Also, each of the plurality of signal charge packets has a sufficient second number of elementary charges to provide for detection thereof.
In accordance with yet another aspect of the invention, the independent amplification of the plurality of spatially divided elementary charge components provides each of said plurality of signal charge packets with substantially the same said second number of elementary charges.
In accordance with still another aspect of the invention, the plurality of spatially separate elementary charge components each have substantially a same first number of elementary charges, the respective known numbers thereby each being substantially equal to said first number. This first number may be substantially equal to one to within a statistically significant metric.
In accordance with the present invention, the input signal may be optical or electrical. If the input signal is an optical signal, the spatially separate elementary charge components may be provided by splitting the optical signal into a plurality of photon signal packets. If the input signal is an electrical signal, the spatially divided elementary charge components are provided by splitting the electrical signal into each of the spatially divided elementary charge components.
In accordance with a further aspect of the present invention, the independent amplification of each of the plurality of spatially divided elementary charge components is provided by avalanche multiplication, and this may include multi-stage avalanche multiplication.



BRIEF DESCRIPTION OF THE DRAWINGS
Additional aspects, features, and advantages of the invention will be understood and will become more readily apparent when the invention is considered in the light of the following description made in conjunction with the accompanying drawings, wherein:
FIG. 1 shows an illustrative functional block diagram of a discrete amplifier in accordance with an embodiment of the present invention;
FIGS. 2A–2E depict functional components of two channels of the discrete amplifier during various operational conditions of a discrete amplifier, in accordance with an embodiment of the present invention;
FIG. 3 depicts shown a schematic cross-sectional view of a portion of a discrete amplifier implemented as a semiconductor device, in accordance with an illustrative embodiment of the present invention;
FIG. 4 illustrates two-stage cascade amplification in accordance with an alternative embodiment of the invention;
FIG. 5A shows a sequence of material layers corresponding to the structure of FIG. 3;
FIGS. 5B–5D depict energy band diagrams corresponding to the material layer structure depicted in FIG. 5A during various operational conditions of a discrete amplifier, in accordance with an embodiment of the present invention;
FIGS. 6A and 6B schematically depict a plan view and cross-sectional view, respectively, of a charge-coupled device (CCD) charge splitter to demonstrate an illustrative way of splitting charge to effect the distributor function in accordance with various embodiments of the present invention;
FIG. 7A and FIG. 7B respectively show a plan view and a cross-sectional view of a discrete amplifier based on CCD technology, in accordance with an embodiment of the present invention;
FIG. 8 illustrates a discrete amplifier that employs distribution in the optical domain, in accordance with an embodiment of the present invention;
FIG. 9 illustrates an optical splitter made in the form of a fiber optic plate, in accordance with an embodiment of the present invention;
FIG. 10 is cross-sectional view of the discrete amplifier of FIG. 8, but magnified to focus on the area of two adjacent channels, in accordance with an embodiment of the present invention;
FIGS. 11A and 11B illustrate an embodiment of a discrete amplifier that employs both optical splitting and charge domain splitting, in accordance with an embodiment of the present invention;
FIG. 12 illustrates an embodiment of an optical splitter made as a fiber-optic linear array dividing the light signal, projected on the narrow input part of the splitter onto the separate channels of the storage area associated with electrodes of a charge coupled splitter, in accordance with the embodiment of the invention depicted in FIGS. 11A and 11B; and
FIGS. 13A–D depict an illustrative embodiment of a charge distributor that provides different known sized signal components at each of its outputs, in accordance with various alternative embodiments of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an illustrative functional block diagram of a discrete amplifier 10 in accordance with an embodiment of the present invention. As depicted, discrete amplifier 10 is a multichannel amplifier, with each channel (referenced by subscript i, i=1, 2, 3, 4 . . . n) comprising an amplifier 16i responsive to a quantifier 20i and coupled to an integrator 18i, which is coupled to quantifier 20i and to governor 22i. A distributor 12 is responsive to an input signal 11 and provides an output coupled to the input of each channel amplifier 16i. A reader 24 is coupled to each integrator 18 and provides an amplified output signal 25 representative of input signal 11. Thus, discrete amplifier 10 may be viewed as a multichannel amplifier having a distributor 12 coupled to an amplifier stage 16, which is responsive to a quantifier stage 20 and is coupled to an integrator stage 18, which in turn is coupled to quantifier stage 20, governor stage 22 and reader 24. As will be further understood, the lines/arrows schematically depicted as interconnecting functional components indicate a functional coupling or relationship, and do not necessarily indicate that the interconnected functional components, or structural components corresponding to the functional components, are conductively coupled or directly connected structurally. Additionally, discrete amplifier 10 may be implemented in various technologies; for example, it may be a solid state, semiconductor, or vacuum device, depending on the application.
Input signal 11 may be any optical or electrical signal of interest, and may directly or indirectly represent a primary signal of interest. More specifically, for example, input signal 11 may be the primary optical or electrical signal to be measured or may be the output of a sensor that transduces, converts or transforms a primary signal (e.g., an acoustic, pressure, or other mechanical signal; an uncharged particle/species; a charged particle/species or other electrical signal; a magnetic signal; an optical or other electromagnetic signal (e.g., x-ray)). Accordingly, an input signal, as used herein, is not limited to any particular application or sensor, but is an optical or electrical signal that directly or indirectly represents any signal or particle/species to be detected or measured.
Distributor 12 spatially distributes (e.g., splits, divides, spreads out, or couples) input signal 11 into a number of independent signal components (photon or charge components) of known respective magnitudes (e.g., substantially equal magnitudes) that excite or stimulate independent amplifying channels. The illustration in FIG. 1 of distributor 12 being coupled to each of the n amplifier channels input schematically depicts that the distributor is capable of providing a signal component to the input of each channel; however, under intended operating conditions, all amplifier channel inputs do not invariably receive such a signal component. Instead, the number of amplifier channel inputs that receive a signal component varies with the magnitude of input signal 11. More specifically, the known respective magnitudes (e.g., substantially equal magnitudes) of the signal components (the “size” of each of the signal components) generated by distributor 12 is substantially independent of the input signal 11 magnitude, and concomitantly, the number of signal components (and hence the number of excited or stimulated independent amplifying channels) represents the input signal magnitude. In an embodiment of the invention, the signal component magnitude corresponds to one fundamental unit (e.g., one photon or one electronic charge species) of the input signal 11, although in alternative implementations the signal component magnitude can be established to correspond to a multiple of the fundamental unit of the input signal. Additionally, distributor 12 may be designed such that the magnitude of each signal component itself can be one unit (e.g., one photon or one electronic charge species) or more than one unit, provided it is substantially independent of the input signal 11 magnitude. While the present invention may be practiced by having distributor 12 provide any known sized output signal components (but not necessarily equal nor necessarily unit sized), for ease and clarity of exposition the ensuing description is set forth according to illustrative implementations having distributor 12 provide equal sized output signal components typically of one fundamental unit in size (e.g., one photon or one electron per output channel of the distributor).
In accordance with the present invention, such distribution of the input signal 11 may be provided in the optical and/or electrical domain. If the input signal is an optical signal, then an optical splitter may be implemented with a sufficient number of output channels such that the input signal is divided into a number of spatially independent photon packets and each packet is assured (e.g., within a statistical metric) of having substantially the same number of photons. These photon packets may be provided as the respective signal components output by distributor 12, incident onto the corresponding inputs of amplifier stage 16 to generate a corresponding electrical charge that is amplified therein. Alternatively, within distributor 12 these photon packets may be converted to substantially equal magnitude electrical charge packets, which undergo splitting in the charge domain into electrical signal components that are coupled to corresponding inputs of amplifier stage 16. In another embodiment, an optical input signal 11 may be converted first in its entirety into an electrical signal, which is then split in the charge domain into electrical signal components that are coupled to corresponding inputs of amplifier stage 16. For an electrical (e.g., charge) input signal 11, distributor 12 may split the input signal in the electrical (e.g., charge) domain to provide substantially equal sized charge packets (their size independent of the input signal magnitude) as the electrical signal components that are coupled to corresponding inputs of amplifier stage 16. As may be appreciated, splitting an optical signal into photon packets may be implemented in one optical splitting stage or in a series of optical splitting stages. Similarly, splitting any electrical signal (or charge packets) into charge packets (or smaller charge packets) may be implemented in one or a series of charge splitting stages. Typically, however, splitting in a single stage is advantageous with respect to speed and/or efficiency or signal loss (e.g., losses due to charge transfer inefficiency in a CCD splitter).
The number n of channels over which distributor 12 is capable of providing a signal component is selected based on the dynamic range of the input signal 11 to be sensed. The desired resolution or discrimination (or quantization error) of the input signal is another parameter affecting the distributor 12 design. For example, assuming that it is desired to detect over an acquisition period an input signal 11 having from 1 to 100 photons, with a resolution of one photon, then distributor 12 (and, generally, discrete amplifier 10) should be designed with a sufficient number of channels n to ensure within a statistical metric that the maximum input signal (i.e., 100 photons) is split into independent photon packets that each contain only one photon. By way of example, if the desired resolution (i.e., discrimination of the input signal magnitude) were to within about 2 photons, then, similarly, distributor 12 should be designed with a sufficient number of channels n to ensure within a statistical metric that the maximum input signal (i.e., 100 photons) is split into independent photon packets that each contain two photons.
Accordingly, by operation of distributor 12, an input signal is directly distributed (i.e., distributed prior to any physical conversion or transformation; e.g., an optical input signal split into independent photon packets, or an electrical input signal split into independent charge packets) or indirectly distributed (i.e., distributed after a physical conversion or transformation; e.g., an optical input signal converted to an electrical signal that is split into independent charge packets) or both directly and indirectly distributed (e.g., an optical input signal distributed into spatially separate and substantially equal sized photon packets, which are converted into respective electrical signal packets that are each further distributed into independent and substantially equal sized charge packets) among a number of independent amplifying channels to ensure, within some designed or determinable statistical tolerance or metric, that a substantially equal number (e.g. one) of signal components (e.g., a charge component or a photon component) are input to each independent amplifying channel. In this way, the number of stimulated amplifier channels 16i is a monotonic function (e.g., proportional, and including any smooth or step-like monotonic function) of the input signal magnitude; hence, as further explained below, the input signal may be quantified or measured according to the number of amplifier channel outputs that provide an amplified signal indicating that the amplifier channel was excited or stimulated by a distributed component of the input signal.
It is appreciated, therefore, that such distribution of the input signal would not be achieved, for example, by conventional imaging with a one or two-dimensional detector array (e.g, a CMOS active pixel sensor) because such a conventional imaging device does not distribute the image over the pixels in a manner that ensures, for an arbitrary image signal, that substantially the same number of photons impinge on each excited pixel over a given acquisition or integration time (e.g., that the average or nominal photon flux of a stimulated pixel is substantially independent of the input signal magnitude as well as time; the number of stimulated pixels is a monotonic function of the input signal magnitude). More specifically, in such an imaging array example, wherein to provide for two-dimensional imaging each array element receives a photon flux corresponding to the image intensity in the region of that element, there is no mechanism or operation that uniformly distributes the overall image intensity over the array elements in components having a common magnitude that is independent of the overall image intensity. While it is hypothetically possible that for random or arbitrary time intervals an image might excite a number of array elements with approximately the same number of photons while not exciting any other array elements, and the number of such excited array elements may vary or fluctuate, a conventional imaging array and methodology for detecting such an image nevertheless does not distribute the image inasmuch as it does not include any mechanism or operation to ensure such distribution within a statistically significant metric.
In accordance with an embodiment of the present invention, each amplifier 16i has two states, referred to for convenience as ON and OFF, that are established or controlled based on the output voltage of quantifier 20i. According to a feature of an embodiment of the invention, the two-state operation of amplifier 16i under control from quantifier 20i together with the operation of integrator 18i ensures that the magnitude of the signal coupled to reader 24 from a stimulated channel is (1) essentially independent of fluctuations in the size of the signal component input to the amplifier 16i of that channel and (2) substantially the same magnitude as for all other stimulated channels. In the ON state, amplifier 16i waits for any signal component coupled to its input from distributor 12, the appearance of such a signal at its input initiating or stimulating amplifier 16i with a given probability Pamp to commence amplifying the input signal component. This amplification process is stopped only after amplifier 16i switches to the OFF state in response to the output voltage of quantifier 20i. Accordingly, amplifier 16i may be characterized by Pamp and by two voltages Uon and Uoff that correspond to the quantifier output voltage that causes amplifier 16i to switch ON and OFF, respectively. As further described below, amplifier 16i may be an avalanche multiplication amplifier (e.g., an avalanche diode), which advantageously may be implemented as a threshold amplifier employing self-limited gain. Also, as may be appreciated, since depending on the implementation, the signal components output by distributor 12 may either be charge components or photon components, amplifier 16i is appropriately designed for efficient coupling of the signal component. For example, if the signal component output by the distributor is optical, then amplifier 16i will advantageously include or be integrated with a photon absorption/photoconversion region to efficiently absorb the incident photon packet and generate a corresponding charge signal that is amplified.
For clarity, it is noted that photoconversion of photon signal components (i.e. components that are not further distributed/split) is a function separate from the distributor function and amplifier function and is generally implemented as an element distinct from the distributor and amplifier. Since, however, in practice the photoconversion element typically may be integrated with the distributor element and/or amplifier element, for convenience and ease of description the photoconversion element may be considered as embodied within the amplifier or the distributor, and has herein been considered integrated with the amplifier (i.e., within the functional block for the amplifier) to highlight the distributing (splitting) function of the distributor. If the photoconversion element were considered as integrated with the distributor, then in the event that the input signal were optical the distributor functional block would only output charge signal components (i.e., as opposed to either charge or photon signal components), and concomitantly amplifier 16i would not be considered as having photoconversion integrated therewith. Again, associating photoconversion of photon signal components with the distributor or amplifier is arbitrary and for convenience, as photoconversion of signal components is not a necessary function of, and is generally functionally and structurally distinct from, the distributor and amplifier, even when a photoconversion element is integrated therewith.
As described, quantifier 20i is an element operative in controlling the state of amplifier 16i in accordance with the voltage established on the integrator. It may therefore be characterized by a voltage transfer factor kq=Uquantifier—output/Uquantifier—input, i.e., the ratio of the quantifier output voltage (i.e., Uquantifier—output) to the quantifier input voltage (i.e., Uquantifier—input), the latter voltage being a function of the integrator 18 potential. The voltage transfer factor is of appropriate polarity to ensure proper switching of amplifier 16i. In a typical implementation, for example, as the amplification process proceeds, the increasing charge generated by amplifier 16i is coupled to the integrator 18 to cause its output voltage (and hence Uquantifier—input) to increase; thus, assuming a negative or low voltage is required to turn off amplifier 16i, Uquantifier—output should decrease with increasing Uquantifier—input.
The charge generated by amplifier 16i is coupled to integrator 18i, causing the integrator 18i output voltage Uint to increase as the generated charge increases. In the illustrative embodiment, this coupling is capacitive, via a capacitance Cint associated with integrator 18i, and as amplifier 16i generates charge through the amplification process, charge Qacc accumulates via capacitive coupling such that Uint=Qacc/Cint. This voltage Uint is coupled or applied simultaneously to both quantifier 20i and governor 22i inputs. At the same time, in accordance with an embodiment of the present invention, charge Qacc accumulated in integrator 18i induces generation of the same amount of charge in Reader 24.(e.g., via a capacitance Cint).
Governor 22 eliminates the accumulated charge Qacc from integrator 18i after integrator 18i achieves an output voltage Uint sufficient to cause amplifier 16i to switch OFF via quantifier 20i. In a simple implementation considered in the illustrative embodiment of FIG. 1, governor 22 eliminates the accumulated charge Qacc by conducting it to ground. In an alternative embodiment, governor 22 may eliminate the accumulated charge by transferring it to reader 24 to, for example, contribute to the generation of output signal 25. Governor 22 may be implemented as having two modes: an open (non-blocking) mode characterized by a resistance Rop, and a closed (blocking) mode characterized by a resistance Rcl, wherein Rcl>>Rop. In accordance with the illustrative embodiment, governor 22 may be characterized by a threshold voltage Ur_on such that upon a voltage input to governor 22 (e.g., coupled from integrator 18i) reaching or exceeding threshold voltage Ur_On, governor 22 opens (i.e., switches to the open mode) with some delay, and upon the voltage input to governor 22 becoming lower than threshold voltage Ur_On, governor 22 switches to the close mode. The delay time (τr) in switching to the open mode after achieving the appropriate threshold Ur_on typically should be greater than the greatest possible time for charge accumulation in integrator 18i, Also, the threshold voltage Ur_On preferably is rather low to provide for nearly full elimination of charge accumulated in the integrator before entering the closed mode.
A reader 24 is coupled to each integrator 18i and provides an amplified output signal 25 representative of input signal 11 based on the number of amplifier channels that provide an output signal (and thus have been stimulated by distributor 12). More specifically, since the number of excited amplifier channels, and thus the number of integrators having accumulated charge packets, corresponds to the input signal, reader 24 is operative in reading the accumulated charge packets to provide an output signal that is a measure of the input signal. It is noted that in an advantageous implementation, where the number of signal components generated by distributor 12 equals the number of elementary units (i.e., photons or charge units) in the input signals, then the number of accumulated charge packets equals the number of elementary units in the input signal, and thus accurate readout of the number of integrated charge packets corresponding to the amplified signal components provides for resolution of single elementary units of the input signal.
Reader 24 may be implemented to function in a “discrete” mode or in an “analog” mode. More specifically, in a “discrete” mode, reader 24 essentially individually counts, by individually sensing, the amplification channels that provide output charge packets. In an “analog” mode, instead of individually counting or sensing the accumulated charge packets, reader 24 essentially combines (e.g, sums) the individual accumulated charge packets to form a combined charge packet that is measured or sensed (e.g, by conversion to a current or voltage signal).
For a “discrete” mode readout, reader 24 may be implemented, for example, as a multiplexer circuit that individually and sequentially selects each amplifier channel output to provide individual output signals (e.g., pulses) that are subsequently processed (e.g., counted). As another example, each amplifier channel output may be coupled to a separate comparator (e.g., charge comparator), and a decoder or summing circuit receives the outputs from all the comparators to generate an output signal proportional to the number of accumulated charges. As yet a further example, reader 24 may be iimplemented as a CCD shift register to readout the accumulated charge packets (e.g., the accumulated charge packets may be in parallel dumped directly into separate CCD potential wells, or may be used in parallel to generate corresponding charge packets in separate CCD potential wells). As may be appreciated, a feature of individually sensing the accumulated charge packets is that this approach is not sensitive to variations in the magnitude of the amplified charge packet, provided that the amplifiying channels provide sufficient amplification to detect each accumulated charge packets.
To effect an “analog” readout, reader 24 may be implemented, for example, as a capacitance coupled capacitively with each integrator 18i such that charging of a given integrator 18i induces charging of equal magnitude in the reader 24 output capacitance and all the induced charges corresponding to the integrators are additive, thus providing a combined charge packet. The magnitude of the combined charge packet (and hence the magnitude of the corresponding current or voltage signal) is proportional to the number of accumulated charge packets, and thus, to the number of elementary or fundamental units (i.e., charges or photons) in the input signal. It is appreciated, however, that in such an analog readout implementation wherein, for example, reader 24 sums the outputs of each channel, preferably the charge induced in reader 24 by each excited channel is substantially the same to provide for high sensitivity and high resolution detection of the input signal. That is, the amplification noise is preferably reduced to a minimum level required for detecting the number of fundamental units (charges or photons) in the input signal (preferably with single charge/photon resolution), by means of equalization of the charge packets.
For this purpose, the formation of each accumulated charge packet that contributes to the combined charge packet is carried out by dosed accumulation of a given amount (dose) of elementary charges in this packet by operation of integrator 18i together with quantifier 20i. These components cooperate to terminate the amplification process in amplifier 16i upon generating a given amount (dose) of elementary charges in the charge packets accumulated by integrator 18i, Alternatively, or in addition (e.g, if the amplification process cannot be terminated as rapidly and/or as accurately as desired for equalization), such dosed accumulation may be effected by eliminating any excessive charge (i.e., charge exceeding the desired dose) generated by the amplifier. Such elimination may be effected upon accumulating the desired dose to avoid accumulation of any charge beyond the desired dose (e.g, by providing shunting excess charge upon accumulating the desired dose), or may be effected after accumulating charge in excess of the desired dose.
The operation of the discrete amplifier embodiment of FIG. 1 is now described with reference to FIGS. 2A–2E, which for clarity of exposition depicts functional components of only two channels of the discrete amplifier.
FIG. 2A depicts an initial condition at an instant wherein input signal 11 has provided to distributor 12 a stimulus (e.g., electrons or photons), which distributor 12 distributes or splits into individual, channelized signal components 30i and 30i+1 that are provided to stimulate respective inputs of amplifiers 16i and 16i+1 with input signal components 32i and 32i+1. Insufficient time has elapsed such that amplifiers 16i and 16i+1 have not yet begun amplifying input signal components 32i and 32i+1, while sufficient time has elapsed since the arrival of a previous stimulus from input signal 11 such that the other functional elements of amplifier 10 are in their quiescent state. More specifically, there is no charge in integrators 18i and 18i+1 and hence each respective integrator output voltage Uint equals zero, quantifiers 20i and 20i+1 each support a voltage on their respective amplifier, so amplifier is ON and waits for a signal charge (i.e., UAmp>Uon), an entrance voltage on a governer is 0, the governer is in the closed mode with no current, the charge on the adder is absent.
In FIG. 2A, these amplifier input signal components 32i and 32i+1 are referenced distinctly from signal components 30i and 30i+1 to indicate that these former and latter components are not necessarily physically the same components (though they can be the same components). For example, signal components 32i and 32i+1 may be charged species (e.g., electrons) that are physically distinct from and generated by non-conductive coupling (e.g., capacitive) to charged (e.g., electrons) signal components 30i and 30i+1. Alternatively, charged signal components 32i and 32i+1 may be the same physical charged components as signal components 30i and 30i+1, but represent these components after they are transported (e.g., injected or conducted) to amplifiers 16i and 16i+1. In yet another implementation, signal components 30i and 30i+1 may be individual photons, whereas input signal components 32i and 32i+1 may be photoelectrons corresponding thereto. As described, in practice, such photoelectron conversion may be integrated with the amplifier input or with the distributor 12, or may be separately provided in an intervening element; in any event, such photoelectron conversion is not a necessary function of distributor 12 or amplifier 16 and may be considered a separate function, regardless of whether the element or structure that provides this photoconversion is integrated with the distributor 12 or amplifier 16.
As described, each channelized signal component (each of components 30i and 30i+1 and, similarly, each of components 32i and 32i+1) has substantially the same magnitude (though components 32i and 32i+1 do not necessarily have the same magnitude as components 30i and 30i+1), which magnitude is independent of the input signal magnitude, and may be any multiple of the fundamental unit or species (e.g., photon or charge unit) comprising input signal 11. In various implementations, however, the signal component magnitude is advantageously implemented as equal to one unit of input signal 11 (e.g., one photon for optical input signals or one charged species (e.g., one electron) for charge input signals). Further, in the event that input signal 11 is an optical signal that is converted to charge units (e.g., electrons) corresponding to components 30i and 30i+1 and/or components 32i and 32i+1, in various applications each photon may generate more than one charge, though typically only one charge is generated. In somewhat general terms, then, assuming no losses or inefficiencies in the distributor, it may be understood that assuming input signal 11 provides some integer number N of fundamental units or species to the input of distributor 12, then Γ=αN species will be distributed into an integer number K of signal components, each signal component having an equal number β of fundamental species or units, and hence N is proportional to K. While α typically equals one, it may be greater than one; for example, where each photon of the input optical signal generates two electrons upon photoconversion. Also, as noted, in a typical implementation, β equals one, and hence K=N, indicating that the number of stimulated amplifier channels equals the number of fundamental species in the input signal.
Referring now to FIG. 2B, amplifiers 16i and 16i+1 have begun amplifying input signal components 32i and 32i+1 to generate charge that accumulates in integrators 18i and 18i+1 as charge packets 34i and 34i+1, which causes the voltage on the integrators to increase. Each of the accumulated charge packets 34i and 34i+1 and its associated increasing integrator output voltage induces (e.g., by capacitive coupling) charging in reader 24, which in this illustrative operational embodiment is implemented to read or sense the accumulated charge in an “analog” mode. Thus, as schematically depicted, the charge 36 in reader 24 equals the sum of accumulated charge packets 34i and 34i+1.
As illustrated in FIG. 2C, amplifiers 16i and 16i+1 continue amplifying, thus increasing the accumulated charge packets 34i and 34i+1 and their associated integrator output voltages to a level such that via quantifiers 20i and 20i+1 (which provide any necessary voltage transformation) the supply voltage of amplifiers 16i and 16i+1 is reduced to switch amplifiers 16i and 16i+1 to their respective OFF states. The coupling of the accumulated charge packets 34i and 34i+1 and their associated integrator output voltages to governors 22i and 22i+1 causes the input voltages to these governors to exceed the threshold voltage needed for the governors to switch to the open mode; however, because of the built-in opening delay time, governors 22i and 22i+1 remain in the closed mode, thus allowing only a small leakage current to pass through them. During this time interval, charge 36 in reader 24, which equals the sum of accumulated charge packets 34i and 34i+1, reaches its maximum value.
Over the time interval represented by FIG. 2D, governors 22i and 22i+1 open and the current through each of them grows sharply, thus discharging accumulated charge packets 34i and 34i+1 and decreasing their associated integrator output voltages.
As shown in FIG. 2E, after governors 22i and 22i+1 completely eliminate (i.e., completely discharge) all accumulated charge in charge packets 34i and 34i+1, all elements of the discrete amplifier are in an initial state corresponding to FIG. 2A, but before input signal 11 provides a stimulus to distributor 12. Absent any accumulated charge, integrators 18i and 18i+1 provide a zero voltage output signal that via quantifiers 20i and 20i+1 causes a sufficient supply voltage to be applied to amplifiers 16i and 16i+1 such that these amplifiers are switched to their respective ON states, enabling each amplifiers 16i and 16i+1 to amplify any subsequent signals that may be provided to their respective inputs. Also, there is no charge in reader 24, as there is no accumulated charge in integrators 18i and 18i+1.
Referring now to FIG. 3, there is shown a schematic cross-sectional view of a portion of a discrete amplifier implemented as a semiconductor device, in accordance with an illustrative embodiment of the present invention. More specifically, FIG. 3 shows only two adjacent amplifier channels (and associated reader structure) from among a total of N such channels, and does not show a distributor structure. As described, the number N of such channels that are implemented in practice depends on the maximum number of species (e.g., electrons) in the input signal.
As shown, the discrete amplifier includes a p-type Si (p-Si) layer 100 (e.g., a substrate or epitaxial layer) in which is formed n+ (i.e., heavily doped n-type) Si conductive regions 102i and 102i+1 (using the notation, as above, wherein the subscript denotes the associated amplifier channel), thus forming a p-n junction. As is well known, n+Si conductive regions 102i and 102i+1 may be formed by, for example, diffusion or ion implantation, and in an alternative embodiment a metal or silicide may instead be used for such regions. Conductive channel-electrodes 103i and 103i+1 (e.g., heavily-doped (n+) polysilicon) are conductively connected to respective n+Si regions 102i and 102i+1 by conductive plugs 108i and 108i+1 (e.g., metal or heavily doped (n+) polysilicon) which are formed through insulator 104 (e.g., silicon dioxide). Insulator 104 also separates electrodes 103i and 103i+1 from a common (i.e., shared) conductive electrode 106 (e.g., metal), which is also respectively coupled to electrodes 103i and 103i+1 by semiconductor plugs 105i and 105i+1 (e.g., SiC), which have a wider band gap than the bandgap associated with channel-electrodes 103i and 103i+1.
As may be appreciated, while FIG. 3 schematically depicts device elements and their structural relationship and electrical properties to illustrate components and elements that may be used to implement a discrete charge amplifier in accordance with an embodiment of the present invention, there are many ways of implementing these elements as well as the device design, including a variety of semiconductor fabrication technologies and processes for forming such a device structure and its component layers. For example, while FIG. 3 shows a continuous insulator 104 substantially surrounding channel-electrodes 103i and 103i+1, in practice, insulator 104 may be formed as more than one insulating layer, possibly using different oxide formation processes (e.g., thermal oxidation of Si, chemical vapor deposition (CVD) techniques), and/or different insulating materials (e.g., silicon dioxide, silicon nitride, etc.). Similarly, channel-electrodes 103i and 103i+1 may be patterned according to well known techniques, such as by wet or dry etching, or by damascene or dual-damascene (by which conductive plugs 108i and 108i+1 would also be formed).
As noted, FIG. 3 shows only a portion of the discrete amplifier; for example, it does not show a distributor, nor a second electrode that would be disposed and used to apply a potential across p-Si layer 100 relative to electrode 106, to appropriately bias the discrete amplifier channels during operation. When such bias is applied, amplifiers are provided by avalanche regions 101 and 101i+1, which are regions of maximal electric field intensity in the p-Si layer. In this embodiment of the invention, avalanche multiplication of charge carriers is used as the amplification mechanism, and it is presumed that the distributor design provides a single charge carrier per channel to be excited. A basic characteristic of the amplifier is the probability that it operates (i.e., initiates avalanche multiplication) in response to a single charge carrier. To increase this probability, the applied voltage is set such that the potential difference across the avalanche regions 101 and 101i+1 exceeds the breakdown value by at least on one volt. This avalanche breakdown initiation probability may be further enhanced by also implementing the amplifiers (i.e, avalanche regions 101i and 101i+1) as multi-cascade structures, as described further below in connection with FIG. 4.
In the embodiment of FIG. 3, the quantifier is implemented from conductive regions 102i and 102i+1 forming with p-Si layer 101 a blocking contact (in this embodiment, a p-n+ junction) for the majority carriers (i.e., holes in this implementation) of the amplifier semiconductor material. In this embodiment, the heavy doping of the n+ conductive regions 102i and 102i+1 provides for a low minority carrier (hole) concentration in these regions to minimize the dark current due to holes from these regions being swept (e.g., holes within a diffusion length of the junction) into the amplification regions of p-Si layer 101. A basic function provided by the quantifier is switching OFF the amplifiers provided by avalanche regions 101i and 101i+1 upon accumulation of a required charge package by the integrators provided by channel-electrodes 103i and 103i+1, with the minimal error (e.g., with minimal variance in the amount of accumulated charge upon effecting turnoff). Accordingly, to more accurately achieve this switching control, as shown in the illustrative embodiment, conductive regions 102i and 102i+1 are provided as small-area, high radius-of-curvature regions such that they each control small avalanche amplification regions. Additionally, implementing a small area quantifier mitigates the hole current from channel-electrodes 103i and 103i+1 relative to the collection of electrons from the amplifiers. Further, because of their small areas, to efficiently capture signal carriers each channel may implement more than one quantifier (e.g,. two as shown in FIG. 3, each controlling a separate amplification region) coupled to the integrator for that channel.
As noted, in the embodiment of FIG. 3, the integrator is provided by channel-electrodes 103i and 103i+1. The integrator is thus connected directly to the quantifier, so that the quantifier may control switching of the amplifier region directly in response to the electrostatic potential of the integrator. As may be appreciated, the integrator capacitance (Cint), and hence the integrator area, is a parameter related to the designed gain M (i.e., the number of electrons that accumulate in the integrator before the amplifier region is switched OFF via the quantifier) determined by capacity of the integrator.
The governor in this illustrative embodiment is provided by the heterojunction formed between wide-band semiconductor plugs 105i and 105i+1 and channel-electrodes 103i and 103i+1 (i.e., which provide the integrator). As described, the governor appropriately discharges the accumulated charge and provides for returning the amplifier regions to their ON conditions after they have been turned OFF by action of the quantifier in response to the accumulated charge reaching the desired gain. As also described, such switching from the Close mode to the Open mode occurs with a delay after the instant that the electric field intensity in amplification area falls below the critical (i.e., breakdown) value, to ensure that the amplifier has sufficient time to switch OFF, thus preventing an uncontrollable microplasma mode that would occur if the amplifier were not switched OFF. In absence of a charge in the integrator, the governor of the illustrative embodiment advantageously facilitates amplifier operation (i.e., maintaining sufficient electric field intensity for avalanche breakdown in the amplifier regions) by allowing dark (i.e., non-signal) minority carriers (i.e., electrons) generated in the space charge region of the p-Si 100 of the amplifier semiconductor to flow as dark current to electrode 106. Note, that in such a mode (Closed), the governor conductivity is sufficiently low such that upon the occurrence of an avalanche current, the minority carriers (i.e., electrons in this embodiment) are primarily accumulated by integrator rather than conducting through the governor to electrode 106.
As may be appreciated, electrode 106 is capacitively coupled via insulator 104 to channel-electrodes 103i and 103i+1 (i.e., channel integrators), and thus provides for a reader in an “analog” mode.
As noted above, multicascade amplification may be used to enhance the avalanche multiplication probability in response to a single charge carrier stimulus. As an example of such multi-cascade amplification, FIG. 4 illustrates two-stage cascade amplification in accordance with an alternative embodiment of the invention. As shown, embedded within p-Si layer 100 are clusters 126 of a material having a dielectric permittivity less than the dielectric permeability of the surrounding p-Si layer 100 semiconductor material. Accordingly, region 127 between clusters 126 has an enhanced electric field intensity sufficient for avalanche multiplication of an electron entering this region, thus creating an initial charge package, which upon drifting to within region 101 is subject to further avalanche multiplication, i.e., amplifying the initial charge package a second time, to provide a resulting charge package that is accumulated in the integrator provided by channel-electrodes 103. As may be appreciated, therefore, region 127 and region 101 correspond to a first and second amplification stage, respectively. While such a multi-cascade amplifier may be implemented for each of the pairs of amplifier regions of each channel in FIG. 3, alternatively, as shown in FIG. 4, each amplifier channel may employ only one amplifying region, hence having only one associated quantifier n+ Si region 102 which may be directly overlaid (i.e., without an intervening insulator layer and contact plug) by channel electrode 103.
Referring now to FIGS. 5A–5D, the operation of an illustrative discrete charge amplifier such as that of FIG. 3 is described with reference to the energy band diagram for this structure under various operating conditions. More specifically, FIG. 5A shows a sequence of material layers corresponding to the structure of FIG. 3, and also further shows a second electrode 107 that is typically coupled to p-Si layer 100 a p+ layer (not shown), though other intervening layers may be used depending on the implementation. A supply voltage Usup is applied between electrodes 106 and 107, with the polarity as shown to reverse bias p-n+ junction formed between p-Si layer 100 and conductive region 1021. FIGS. 5B–5D each depict an energy band diagram along the sequence of material layers shown in FIG. 5A, with the depicted position and displacement along the layers being the same throughout FIGS. 5A–5D for clarity of illustration in associating energy band characteristics with material layers.
Referring now to FIG. 5B, shown is the energy band diagram when the discrete amplifier is in an initial condition corresponding to that described above for FIG. 2E. As is understood, Ec identifies the conduction band energy, Ev identifies the valence band energy, Ef identifies the Fermi level energy, ΔEc denotes the conduction band offset, ΔE1 denotes the energy difference between the valence band of semiconductor plug 105i and the Fermi level in electrode 106 (i.e., ΔE1 is the energy barrier height for holes in electrode 106). The applied voltage Usup is shown distributed primarily between the drop Uamp across p-Si layer 100 (which includes depleted amplifier region 101i which supports most of Uamp) and the drop Ur across semiconductor plug 105i (which provides for the governor), since electrode 103i (i.e., integrator) and conductive (n+) region 102i (i.e., quantifier) have high conductivity and thus the voltage drop on these elements can be considered as negligible. In this initial condition, for convenience Uamp is designated as equal to Uon, and Ur is designated as equal to Ur0.
The governor is in the CLOSED state and the potential difference Ur between electrode 103i (i.e., integrator) an electrode 106 is supported at an approximately constant level due to an approximately constant leakage current through the governor. This voltage drop Ur is also generally small (e.g, about zero) since the leakage current flowing into integrator electrode 103i is very small, as it primarily represents dark current from p-Si layer 100 (which includes amplifier region 101). Thus, the positive potential applied to electrode 106 is transferred to conductive (n+) region 102i (i.e., quantifier) nearly unchanged, and quantifier thus provides for a high electric field across amplifier region 101; such that the amplifier is in its ON state, awaiting an avalanche initiating event.
Leakage current through wide bandgap semiconductor plug 105i (which provides for the governor) is due to different physical mechanisms such as electron thermionic emission from integrator electrode 103i into the wide bandgap semiconductor plug 105i (depicted by electron 60 surmounting ΔEc), as well as electron/hole thermogeneration in wide bandgap semiconductor plug 105i. These mechanisms have a weak, non-ohmic dependence on voltage Ur, and thus provide an upper-limit on the leakage current through the governor. As described, this upper limit is sufficient to sink the weak thermogenerated dark current from p-Si layer 100, but any significant increase of current flowing into integrator electrode 103i leads to charge accumulation in the integrator. Accordingly, it is appreciated that in the illustrative embodiment, heterobarrier ΔEc and barrier ΔE1 are important design parameters for implementing the governor.
Referring now to FIG. 5C, the energy band diagram is shown during a time period in which an initial charge is being amplified by avalanche multiplication in amplifier region 101, corresponding to the condition described in connection with FIGS. 2B and 2C. This avalanche process results in the generation of electron-hole pairs, with the electrons 62 drifting toward conductive (n+) region 102i (i.e., quantifier) and accumulating in integrator electrode 103i (e.g., electron 66), and with the holes 64 drifting toward electrode 107. More specifically, electrons accumulate in integrator electrode 103i since the current (leakage) supported by the governor is much less than the current flowing into the integrator.
The integrator charging causes an increase in the voltage Ur up to the a threshold value (Ur_on) sufficient for the governor to switch to its OPEN mode after a delay. Although the increase in the voltage drop Ur corresponds to an equal (i.e., neglecting other voltage drops) decrease in the amplifier voltage Uamp, the amplifier voltage nevertheless remains above a critical value such that the amplifier continues to generate charge. It is noted that a large value of Uon is generally advantageous as it reduces the average avalanche time required to provide a given multiplication M. By way of example, in practice for a device design based on that shown in FIG. 3, Uon may be selected to exceed the critical voltage by about 5 to 10 volts.
The increase in the voltage drop Ur to Ur_on results in hole generation in the region of semiconductor plug 105i that adjoins electrode 106 (within an electron tunneling distance from electrode 106). More specifically, hole generation is caused by field-assisted (i.e, the field narrows the barrier width) tunneling of electrons from the valence band of wide bandgap semiconductor plug 105i into electrode 106. Equivalently, this process may be understood as field-assisted tunnel injection of holes from the electrode 106 to the valence band of wide bandgap semiconductor plug 105i through the energy barrier ΔE1. Since the heterojunction barrier ΔEc is greater than ΔE1, outflow of electrons accumulated in integrator electrode 103i by thermionic emission over the barrier ΔEc (or by field assisted emission) is negligible, and charging of Integrator by electrons from the amplifier continues.
The delay between the beginning of hole injection in the semiconductor plug 105i (which provides for the governor) region and the beginning of Integrator discharging is provided by the time for these holes (e.g., hole 68) to drift through semiconductor plug 105i. As a result of this delay, integrator charging continues until Uamp is reduced to a value Uoff. This delay, which allows for the amplifier regions 101 to turn OFF before the governor enters its OPEN state, is important too ensure that substantially equal sized charge packets are accumulated at each channel.
As may be appreciated, the height of barrier ΔEc should be high enough to prevent field-induced electron injection (i.e, thermionic field emission or Fowler-Nordheim tunneling) through it up to the maximum voltage on the governor when the amplifier is switched OFF (i.e., up to Ur=Ur0+Uon−Uoff), thus providing for accumulation of electrons in the Integrators at any governor voltage during operation. Energy barrier ΔE1 is advantageously selected sufficiently small so that Ur_on is small. Also, it may be understood that it is important that the hole injection process in the region of semiconductor plug 105i (governor) begins before the amplifier region 101 exits from its threshold mode (i.e., Ur_on <Uon−Ucr, where Ucr is the critical voltage).
Since the integrators and electrode 106 are capacitively coupled via insulator 104 (see, e.g, FIG. 3), the electrons accumulated in the Integrators result in a positive “image” charge on electrode 106, indicating that electrons flow from electrode 106 to an external circuit, thus providing output current Iout.
FIG. 5D illustrates the energy band diagram upon commencing the discharging of the accumulated charge to return the discrete amplifier to its initial condition, and corresponds to conditions described in connection with FIGS. 2C and 2D. More specifically, Uamp has been reduced to a value Uoff during the described delay time, which time is expired as the injected holes have traversed the governor and reached integrator electrode 103i, thus initiating discharging by recombination with the accumulated charge, corresponding to the governor being in the OPEN mode. During the time interval between the amplifier being switched to its OFF state and just prior to discharging commencing, the voltage across the semiconductor plug 105i (governor) remains substantially constant at a value Ur=Ur0+Uon−Uoff. The rate of Integrator discharging depends on hole injection current, which is determined by the energy barrier height ΔE1.
During the discharging process, the voltage Ur decreases and upon reaching about Ur_on, hole injection stops; however, holes already injected into semiconductor plug 105i (governor) continue to drift to the integrator, thus rapidly and completely discharging the accumulated charge and concomitantly returning voltage Ur to Ur0 and, via conductive region 102i (quantifier), restoring Uamp to Uon (i.e., returning the amplifier to its ON state), which is greater than the critical voltage by a designed amount. Accordingly, the discrete charge amplifier is restored to its initial state, as described in connection with the band diagram of FIG. 5B and with the operational description of FIG. 2E.
It is further noted that in the illustrative embodiment it is important for the electric field strength in amplifier region 101 to return to a level sufficient for avalanche multiplication before the integrator is fully discharged. If, however, a free carrier appears in amplifier region 101 before the channel is fully recovered (i.e., before the integrator is fully discharged and the voltage Uon is across the amplifier region 101), then this free carrier will (according to the relevant avalanche probability) be amplified, but with an avalanche multiplication coefficient less than designed. Such pre-full-recovery amplification represents an amplification noise source. Accordingly, to diminish this effect, the integrator discharging process should minimize the time for Ur to reach to Ur0 (i.e., there should be a fast transition of the amplifier region 101 from its OFF state to its designed ON state). As may be appreciated, the mechanism provided to delay the governor switching from the OPEN mode the CLOSED mode enables such rapid discharging.
As may be appreciated, depending on the application, the illustrative portion of a discrete charge amplifier shown in FIG. 3 may be implemented with a variety of material layer structures between electrode 107 and p-Si layer 100 and/or a variety of distributor structures to provide an electron stimulus to a given number K of amplifier channels. Accordingly, its description has been set forth without specifically showing a distributor, and has focussed on describing the charge amplification, accumulation, readout (e.g., summation), and discharging/resetting operations. Additionally, however, those skilled in the art recognize that there are myriad alternatives semiconductor device designs for implementing even the functional portion of the discrete charge amplifier depicted in FIG. 3. The ensuing description provides illustrative embodiments that show, by way of example, distributing charge and optical input signals, coupling the distributed signals into amplifier channels, as well as alternative device designs for implementing a discrete charge amplifier.
Initially, with reference to FIGS. 6A (plan view) and 6B (cross-sectional view), there is shown a charge-coupled device (CCD) charge splitter to demonstrate an illustrative way of splitting charge to effect the distributor function in accordance with various embodiments of the present invention. This illustrative CCD charge splitter may be described as follows, which for clarity does not explicitly specify materials, any doping regions or layers, inter-electrode isolation or gaps, etc. As schematically depicted, electrodes 80, 82, and multistage electrode 84 are formed on an insulator 86, which overlies a semiconductor material 88. Multistage electrode 84 is comprised of multiple independent, commonly driven gates or, similarly, multiple electrostatically isolated channels (e.g., isolated by interelectrode gap potential barriers). Alternatively, multistage electrode 84 may be a single, common electrode overlying independent amplifier channels such that charge transferred into a common potential well formed by electrode 84 may be channelized by operation of the amplifier channels.
Initially, an input charge signal is localized in a potential well under an electrode 80. By appropriately clocking (driving) the gates, this charge is first transferred to be localized in the semiconductor material in a potential well disposed below electrode 82, as schematically depicted in FIG. 6B by charge 90 and potential well 92. The charge spreads out and distributes itself evenly in this potential well over the entire areal extent of the electrode 82. Then, upon a further clocking operation, this charge is transferred into independent potential wells located under each of the channel electrodes comprising multichannel electrode 84, thus effecting even splitting of the input charge signal. If there are a sufficient number of independent channels associated with electrode 84, then it is possible to ensure, within a statistical metric, that for a given maximum sized input charge packet that the splitting results in K channels of individual electrons. Accordingly, as previously noted, the required number of splitter/amplifier channels is defined by required dynamic range and noise level of registration (i.e, discrimination level) of a signal.
It is understood that the charge-coupled splitter may be applied to myriad discrete charge amplifier designs and, for example, may be integrated with the discrete charge amplifier structure depicted in FIG. 3 to provide electron input signals to a number K of the amplifier channels, the number K being proportional to the input signal magnitude. Those skilled in the art understand that there are various ways of implementing electrode 107 of FIG. 5A in conjunction with the charge-coupled splitter design. For instance, the charge-coupled splitter may transfer charge in a light or moderately doped p-layer which overlies a p+ layer that is contacted at the periphery of charge-coupled splitter to provide for electrode 107.
While the charge-coupled splitter is well suited for splitting charge in a variety of semiconductor based designs, the underlying principle of coupling (e.g, by charge transport, or by capacitive coupling) an input charge signal from a smaller area to a larger area, thus spatially distributing the charge, which can then be directed into individual amplifier channels, may be implemented in a variety of ways in diverse implementations, including vacuum and solid state embodiments of a discrete amplifier according to the present invention.
FIG. 7A and FIG. 7B respectively show a plan view and a cross-sectional view of a discrete amplifier based on CCD technology. As shown in the cross-sectional view of an area of one amplifier channel (i.e, FIG. 7B), this illustrative embodiment of a discrete amplifier is designed to provide for a generally lateral charge transport during avalanche multiplication in the amplifier region 101 of Si p-layer 100.
This illustrative embodiment generally includes three charge-coupled regions: a charge storage area associated with storage electrode 115, a charge splitting area associated with splitting electrode 116, and a charge amplification area associated with amplification electrode 117. The amplification area associated with amplification electrode 117 provides for discrete amplification and includes N channels located under the common amplification electrode 117. The discrete amplifier is based on a heavily doped p+ silicon (Si) substrate 110, which is used as one of the discrete charge amplifier electrodes. Overlying this Si substrate 110 is an epitaxial, low doped (p−) layer 109 (e.g., doped to a concentration of about 1013–1014 cm−3 and having a thickness of about 5–10 micron, which prevents parasitic avalanche breakdown outside the amplification area 101). In the storage and splitting areas, and in part of the amplification area, low-doped (p−) layer 109 is overlaid by a p-type layer 100 (e.g., doped at a concentration of about 1015–1016 cm−3 and having a thickness of about 2–3 microns). Materials and/or structures corresponding to those in the discrete amplifier embodiment of FIG. 3 are referenced by the same numbers. More specifically, the discrete charge amplifier of FIG. 7A and 7B includes an avalanche amplification area 101, a quantifier 102, an integrator 103, and a governor 105, corresponding to the equivalently referenced structures/materials in FIG. 3. In FIG. 7A, amplification electrode 117 is depicted as partially removed (torn away) to expose underlying integrators 103 for clarity of illustration.
As described for the charge-coupled splitter of FIGS. 6A and 6B, a charge signal to be detected (amplified and recorded) is stored in a potential well formed under an storage electrode 115. As may be understood, this signal may originate in many different ways. For example, it may represent (directly or indirectly) charge generated by other electrical circuitry on the same semiconductor substrate, or may alternatively result from photoconversion of an optical signal (e.g., directly detected in the potential well below electrode 115). By way of example, as shown in FIG. 7B, an optical signal 123 is incident directly on and projected through electrode 115 (which is hence made as a transparent electrode; alternatively backside illumination may be used), thus generating a charge packet in the potential well controlled by storage electrode 115.
The charge packet thus stored under electrode 115 is first transferred by charge coupling to a potential well formed under splitting electrode 116, thus spatially distributing the charge. Next, the charge packet is transferred and split into individual electrons by charge coupling into individual amplifying channels disposed beneath amplifier electrode 117. In effecting this transfer with amplifier electrode 117, the operating avalanche voltage is applied via electrode 117, and hence the individual, channelized electrons undergo avalanche amplification in amplifier region 101, resulting in charge accumulation in the integrators 103, which can be read out using electrode 117, which is capacitively coupled in common to the integrators of all channels. As described for the above embodiments, governor 105 is operative in discharging the integrators 103.
While the above embodiments illustrate distributing an optical or electrical input signal in the charge domain, alternative embodiments of the present invention may also provide distribution of optical signals entirely, or at least in part, in the optical domain. In accordance with the present invention, FIG. 8 illustrates an illustrative embodiment of a discrete amplifier that employs distribution in the optical domain.
A heavily doped silicon (p+) substrate 210 is used as one of the electrodes, and an epitaxial Si layer is disposed over substrate 210 to provide for the discrete amplifier channels 113. Since in this embodiment the input optical signal is incident from the heavily doped silicon (p+) substrate 210, to reduce the optical losses, the semiconductor substrate in the channel area is advantageously etched to a width not exceeding about one or several microns. Conductive (e.g., metal) electrodes 107 are formed on the thick edge portions of heavily doped substrate 210. The second electrode contact is provided by electrode 106 which is formed on insulator 104. A light signal is incident on the discrete amplifier through optical splitter (distributor) 112, which adjoins heavily doped Si substrate 210 via an intervening anti-reflective coating 111 provided on the substrate.
FIG. 9 illustrates an embodiment of a splitter 112 made in the form of a fiber optic plate allocating the light signal projected onto a narrow input part of the splitter among individual discrete amplifier (DA) channels 113. More specifically, as shown, a respective optical waveguide (e.g., respective optical fibers, shown as splitter channels 114) of splitter 112 is coupled to each discrete amplifier channel 113, and the output of a single optical fiber that guides the input optical (e.g., light) signal is distributed in parallel among these respective optical waveguides comprising splitter channels 114. As schematically depicted, in this illustrative embodiment, the splitter channels provide single photons 125.
FIG. 10 is cross-sectional view of the discrete amplifier of FIG. 8, but magnified to focus on the area of two adjacent channels. As shown, adjoining the heavy doped Si substrate 210 is a lightly doped p-Si epitaxial layer 209, which functions as a photoconverter (i.e., generates electrons by absorbing incident photons). By way of example, layer 209 is doped at a concentration of about 1013–1014 cm−3, and has a width selected based on the absorption depth for the long-wave boundary of the spectral range of the optical signal to be detected, and is typically not less than 10 microns. Silicon layer 208 functions as a transporter (i.e., transporting photogenerated electrons to the amplifying region), and in this embodiment has the same doping level and thickness as photoconverter layer 209, thus providing for high quantum efficiency.
In operation, the optical signal to be detected is incident on the narrow front portion of the splitter 112, which distributes the optical signal over the individual splitter output channels 114 in such a manner that not more than one photon is allocated for each DA channel. These photons pass through anti-reflective coating 111 and the heavily doped Si substrate 210 layer, and then are absorbed in photoconverter 209 and in transport Si layer 208, thus creating electron-hole pairs. The electric field separates the electrons and holes, leading the holes to an external circuit through Si substrate 210 and further through the metal electrode 107, and sweeping the electrons through transport layer 208 into the amplification area within p-Si layer 100.
Referring now to FIGS. 11A and 11B, illustrated is an embodiment of a discrete amplifier that employs a CCD type structure for transferring and distributing charge into amplifier channels (similar to the device of FIGS. 7A and 7B) and also employs optical domain splitting using optical splitter 112. More specifically, FIG. 11A shows a plan view of the discrete amplifier with CCD type splitting (not showing optical splitter 112) that is shown in cross-section in FIG. 11B along with optical splitter 112. FIG. 12 illustrates an embodiment of optical splitter 112 made as a fiber-optic linear array dividing the light signal, projected on the narrow input part of the splitter onto the separate channels (i.e., 1151, 1152 . . . 115i . . . 115n) of the storage area associated with electrode 115. Photoelectrons in the separate storage areas are then subject to signal splitting in the charge domain, i.e., two stages of signal splitting are provided.
In this embodiment, since splitting occurs in the optical domain, the storage electrode 115 and splitting electrode 116 and associated underlying regions are multichannel (the individual storage electrode channels indicated by 1151, 1152 . . . 115i . . . 115n, and the individual splitting electrode channels indicated by 1161, 1162 . . . 116i . . . 116n), the number of channels being equal to the number of channels of optical splitter 112. In an illustrative implementation of such a discrete amplifier, the optical signal being recorded is first divided by the splitter 112 into individual photons. As described, the number of channels of the optical splitter is determined based on a required dynamic range of the light signal being detected, i.e. the maximum number of photons in the light pulse. Assuming the photons may have energies allowing for more generating more than one photoelectron per photon, some of the charge storage areas 116 may have more than one electron. Charge domain splitting of each of these separate sets of electrons (i.e., each set associated with an output channel of the optical splitter) distributes the sets of electrons into separate (individual) electrons at the output of the charge splitter (each charge splitter output associated with a separate charge amplifier channel and integrator 103). As indicated by FIG. 11A (which depicts electrode 117 as partially removed (torn away) to expose underlying integrators 103 for clarity of illustration), in this illustrative embodiment, each optical splitter output channel is associated with three charge domain splitter output channels. More specifically, as shown for the nth optical splitter output channel, storage electrode channel 115n is coupled to splitting electrode channel 116n which is charge coupled to split charge among three independent amplifying channels respectively associated with integrators 103n1, 103n2, 103n3 that are collectively referenced as 103n to indicate that they are associated with the nth optical channel. (Similar notation is used in FIG. 11B to reference elements associated with the second (of three) independent amplifier channel associated with the ith optical channel, i.e., integrators 103n1, integrators 103n1, integrators 103n1, integrators 103n1, Since the size of an output signal in this illustrative implementation of a photon sensing discrete amplifier is a function of the photon energy, this embodiment allows for estimating the photon energy based on the photodetector itself without using spectral devices. For example, if the charge splitter output channels associated with a single optical splitter output channel provides a signal indicating that two (of the three) amplifiers was excited, then that optical channel was excited by a photon having an energy greater than twice the bandgap energy. Alternatively, if the charge splitter output channels associated with a single optical splitter output channel provides a signal indicating that one (of the three) amplifiers was excited, then that optical channel was excited by a photon having an energy less than twice the bandgap energy (but at least equal to the bandgap energy).
Thus, by using, for example, a “digital” readout mode, a spectral measurement of the input signal is provided. Also, this implementation also allows for completely excluding the thermal noise effect when single photons having sufficiently high energy at room temperature or even at a higher operating temperature. Alternatively, even in implementations where input photons do not have sufficient energy to generate more than one electron, such splitting in both the optical domain and charge domain may be advantageously used to ensure sufficient overall splitting (based on the dynamic range of the input signal) into, for example, individual electrons that excite the amplifier channels. That is, the optical splitter need not have a sufficient number of channels to ensure (within a statistical metric) splitting into individual photons; however, the charge domain splitting of the photoelectrons ensures splitting into individual electrons (i.e., even where an output channel of the optical splitter provides more than one photon).
In the foregoing description of illustrative embodiments of the present invention, reference is made to the distributor providing output signal components each having a given number of fundamental signal components to within a given (e.g., specified, known, or designed) statistical metric. As also described, the number n of channels over which distributor 12 is capable of providing a signal component is selected based on the dynamic range of the input signal 11 to be sensed. For clarity of exposition, by way of example, consider that an input probability Pn that two or more photons would impinge upon the same cell depends on a splitting factor, λ=k/n, where n is the number of cells (e.g., output channels of the distributor) and k is the number of incident photons. The value of λ, providing some given probability Pn, may be calculated numerically from the expression Pn(λ)=1−eλ(1+λ). Thus, depending on the application, for a particular value of k, the number of cells n would be selected as large as necessary to provide a desired Pn.
As noted above, while for ease of clarity of exposition, above illustrative embodiments of the invention are set forth according to distributor 12 providing equal sized output signal components (typically of one fundamental unit), the present invention may be implemented with distributor 12 providing any known sized output signal components (i.e., not necessarily equal nor necessarily unit sized). For example, each signal component output by distributor 12 may be equal sized but greater than one fundamental unit in size. Alternatively, for example, different outputs of the distributor may provide output signal components of different known sizes (e.g., distributor output channel j outputs 2j, with the sum over all output distributor channels being equal or proportional to the input signal magnitude). Advantageously, where a distributor channel outputs a given sized signal, the corresponding amplifier to which that distributor channel is coupled (i.e., the amplifier channel excited by that distributor channel) is designed as a threshold amplifier having its threshold for initiating amplification equal to the given sized signal output by the corresponding distributor channel.
FIGS. 13A–D depicts an illustrative embodiment of a charge distributor that provides different known sized signal components at each of its outputs. The charge distributor includes a buffer field electrode 130 having an associated potential well containing an initial signal charge package, a field electrode 131 having an associated potential well with a capacity of two electrons, a field electrode 132 having an associated potential well with a capacity of four electrons, and field electrodes 133 and 134 each having an associated potential well with a capacity of eight electrons (each field electrode and its associated potential well are hereafter commonly referenced). The charge distributor also includes resistors R1, R2, R3, R4, which divide the voltage supplied by power supply 137 to provide the required voltage on the field electrodes.
Referring to FIG. 13A, initially power supply 137 is switched off and an initial charge package consisting of sixteen electrons, is in a buffer potential well under buffer electrode 130. After switching to apply the voltage of power supply 137 to the field electrodes, the associated potential wells 131,132 and 133 are filled sequentially according to their capacities (FIGS. 13B, 13C, 13D). Potential well 134 remains empty as the entire initial signal package is redistributed among potential wells 131,132 and 133. Thus, as may be appreciated, a distributor may distribute an input signal into different known sized signal components by distributing or transferring charge among potential wells of different capacity. As may also be appreciated, different potential wells (and possibly all the wells) may be provided with the equal (but not necessarily unit sized) capacities.
In view of the foregoing description of illustrative and preferred embodiments of the present invention, various illustrative variations or modifications thereof, and various background art, it may be appreciated that the present invention has many features, advantages, and attendant advantages. For example, a feature associated with an embodiment of the present invention is that by self-limiting the gain of each of the independent amplifier channels that are excited or stimulated with approximately the same number of input electrical charge, and which contribute equally to the output signal, the noise factor is substantially improved, and concomitantly enhancing immunity to spurious events, such as dark current induced avalanche multiplication.
Additionally, while the foregoing illustrative embodiments, for clarity, describe detecting a single input signal, those skilled in the art understand that the present invention is not limited to single input signal detection, but, for example, may be applied to concurrent and/or simultaneous detection of multi-channel input signals as well as to image detection. For instance, in wavelength division multiplexing applications, the optical signal of each wavelength channel may be detected using a separate discrete charge amplifier in accordance with the present invention. Similarly, to detect a one or two dimensional image, for example, each pixel corresponding to the image may be associated with a separate discrete charge amplifier. In such multi-channel or imaging applications, the photons corresponding to each channel or pixel may be divided in either the photon domain or charge domain among the discrete charge amplifier channels. By way of example, the wavelength channels or image may be coupled into an optical fiber bundle (e.g., a fiber image guide), wherein each optical fiber represents a wavelength channel or pixel and guides the photons corresponding to that channel or pixel to a separate discrete charge amplifier, with the photon signal for a given channel or pixel (i.e., guided by a give optical fiber) being divided in the optical domain (e.g, by using an optical splitter at the end of the optical fiber) and/or the electrical domain (e.g., using a charge-coupled splitter to distribute an electrical signal corresponding to the photoconversion of the entire photon packet guided by the optical fiber).
Although the above description provides many specificities, these enabling details should not be construed as limiting the scope of the invention, and it will be readily understood by those persons skilled in the art that the present invention is susceptible to many modifications, adaptations, and equivalent implementations without departing from this scope and without diminishing its attendant advantages. It is therefore intended that the present invention is not limited to the disclosed embodiments but should be defined in accordance with the claims which follow.

## Claims

1. A semiconductor amplifier device that provides a signal representing the magnitude of an input signal, comprising:
a semiconductor region including a plurality of spatially separate charge amplifiers configured such that in response to said input signal being coupled into the semiconductor amplifier device (i) the number of said charge amplifiers excited is a known monotonic function of the input signal magnitude over a predetermined dynamic range of the input signal, and (ii) to within a statistically significant metric each excited charge amplifier is excited by a respective quantity of charge that is known in advance of sensing the input signal and is independent of the input signal magnitude; and
at least one readout electrode coupled to said charge amplifiers to provide an output signal representing the number of charge amplifiers excited by the input signal.

2. The semiconductor amplifier device according to claim 1, wherein the input signal is an optical signal coupled into the semiconductor amplifier device by optical absorption and photogeneration of electrical charge in the semiconductor region.

3. The semiconductor amplifier device according to claim 2, wherein the semiconductor region includes at least one first semiconductor layer for the optical absorption and photogeneration of electrical charge, and at least one other semiconductor layer containing the charge amplifiers.

4. The semiconductor amplifier device according to claim 1, wherein the input signal coupled into the semiconductor amplifier device is an electrical signal.

5. The semiconductor amplifier device according to claim 1, wherein the total number of said charge amplifiers is selected to provide for excited charge amplifiers to be excited, to within a statistically significant metric, by said respective quantity of charge over the predetermined dynamic range of the input signal.

6. The semiconductor amplifier device according to claim 1, wherein the respective quantity of charge that excites each charge amplifier is the same quantity to within a statistically significant metric.

7. The semiconductor amplifier device according to claim 1, wherein the respective quantity of charge that excites each charge amplifier is one electron to within a statistically significant metric.

8. The semiconductor amplifier device according to claim 1, wherein each charge amplifier employs avalanche multiplication.

9. The semiconductor amplifier device according to claim 8, wherein each charge amplifier comprises a plurality of avalanche multiplication regions configured such that in operation (i) the electric field intensity in each of the avalanche multiplication regions is sufficient for avalanche multiplication, and (ii) the electric field intensity between avalanche multiplication regions is insufficient for avalanche multiplication, such that in each excited charge amplifier the respective quantity of charge excites a first one of the avalanche multiplication regions to provide an initial charge packet by avalanche multiplication, and said initial charge packet drifts to, and is subject to avalanche multiplication within, a second one of said plurality of avalanche multiplication regions, each charge amplifier thereby providing for multi-stage avalanche multiplication.

10. The semiconductor amplifier device according to claim 8, wherein the semiconductor amplifier device is biased at a voltage that exceeds the breakdown voltage for avalanche multiplication by the charge amplifiers.

11. The semiconductor amplifier device according to claim 10, wherein the voltage exceeds the breakdown voltage by at least about five volts.

12. The semiconductor amplifier device according to claim 10, wherein each of the excited charge amplifiers generates a respective signal charge packet by avalanche multiplication of the respective quantity of charge by an amplification amount sufficient to detect each said respective signal charge packet.

13. The semiconductor amplifier device according to claim 1, wherein each of the excited charge amplifiers generates a respective signal charge packet by avalanche multiplication of the respective quantity of charge by an amplification amount sufficient to detect each said respective signal charge packet.

14. The semiconductor amplifier device according to claim 1, wherein each of the excited charge amplifiers outputs a respective signal charge packet, and the at least one readout electrode is coupled to the respective charge packets.

15. The semiconductor amplifier device according to claim 14, wherein said at least one readout element is coupled to sum the respective signal charge packets.

16. The semiconductor amplifier device according to claim 14, wherein the at least one readout electrode is implemented as a single readout electrode that contacts each of the plurality of conductive plugs, and that capacitively couples to each of the plurality of conductive integrating electrodes.

17. The semiconductor amplifier device according to claim 16, wherein said semiconductor is silicon and said first conductivity is p-type.

18. The semiconductor amplifier device according to claim 17, wherein said contact plug material is silicon carbide.

19. The semiconductor amplifier device according to claim 14, wherein the readout electrode is coupled to readout each of said plurality of signal charge packets individually.

20. The semiconductor amplifier device according to claim 14, wherein each of said plurality of signal charge packets is provided by a dosed charge accumulation mechanism that results in each of said plurality of signal charge packets having a given quantity of charge.

21. The semiconductor amplifier device according to claim 14, further comprising a plurality of independent integrators each respectively coupled to (i) at least one respective charge amplifier to accumulate the signal charge packets, and (ii) the readout element.

22. The semiconductor amplifier device according to claim 21, further comprising a plurality of quantifier elements, each quantifier element coupled to a respective charge amplifier to gate the operation of the charge amplifier in response to a predetermined quantity of charge accumulated by the integrator.

23. The semiconductor amplifier device according to claim 22, further comprising a plurality of governor elements, each governor element coupled to a respective integrator and operative in eliminating the charge accumulated by said integrator.

24. The semiconductor amplifier device according to claim 23, wherein each of the governors is operative in eliminating the charge accumulated by the respective integrator at time delay after the respective charge amplifier is excited.

25. The semiconductor amplifier device according to claim 24, wherein each of the governors is operative in eliminating the charge accumulated by the respective integrator at time delay after the respective charge amplifier switches from an amplifying state in which the respective charge amplifier generates the charge accumulated by the respective integrator to an off state in which the respective charge amplifier does not amplify charge.

26. The semiconductor amplifier device according to claim 25, wherein in the amplifying state each charge amplifier employs avalanche multiplication at a field that exceeds the breakdown field for avalanche multiplication, and wherein each of the governors is operative in eliminating the charge accumulated by the respective integrator at time delay after the time instant when the electric field in the charge amplifier decreases below the breakdown field for avalanche multiplication.

27. The semiconductor amplifier device according to claim 23, wherein each of the governors is operative in eliminating the charge accumulated by the respective integrator after the respective integrator accumulates the respective signal charge packet.

28. The semiconductor amplifier device according to claim 23, wherein each of the governors is implemented as a heterojunction.

29. The semiconductor amplifier device according to claim 22, wherein the predetermined quantity of charge accumulated by the integrator is predetermined as an amount sufficient for detection.

30. The semiconductor amplifier device according to claim 22, wherein said quantifier provides charge quenching of the amplification process in the charge amplifier upon the predetermined quantity of charge being accumulated by the integrator, such that the gain of the charge amplifier is independent of the statistics of the amplification mechanism.

31. The semiconductor amplifier device according to claim 21, wherein each of said integrators has a capacitance that includes the capacitance of said readout element.

32. The semiconductor amplifier device according to claim 1, wherein the semiconductor amplifier device is a monolithic semiconductor structure comprising:
a region of first conductivity and a plurality of spatially separate regions of opposite conductivity forming separate p-n junctions with the region of first conductivity;
a first conductive electrode conductively coupled to the region of first conductivity to provide a terminal that is common to each of the p-n junctions;
a plurality of conductive integrating electrodes, each conductive integrating electrode conductively coupled to at least a respective one of the spatially separate regions of opposite conductivity;
an insulating layer between the plurality of conductive integrating electrodes and the at least one readout electrode; and
a plurality of conductive plugs, each conductive plug contacting the at least one readout electrode and a respective one of said conductive integrating electrodes, the conductive plugs being made of a material that provides a first energy barrier for a carrier of said opposite conductivity to transport into the conductive plug from the spatially separate regions, and forms a second energy barrier for a carrier of said first conductivity type opposite to transport into the conductive plug from said at least one readout electrode.

33. The semiconductor amplifier device according to claim 32, wherein the charge that excites each excited charge amplifier is amplified by avalanche multiplication in local semiconductor regions of the first conductivity near the p-n junctions.

34. The semiconductor amplifier device according to claim 33, wherein the charge that excites each excited charge amplifier is amplified according to a self-limited gain mechanism.

35. The semiconductor amplifier device according to claim 34, wherein the semiconductor amplifier device is biased at a voltage that exceeds the breakdown voltage for avalanche multiplication by the charge amplifiers.

36. The semiconductor amplifier device according to claim 35, wherein each of the excited charge amplifiers generates a respective signal charge packet by avalanche multiplication of the respective quantity of charge by an amplification amount sufficient to detect each said respective signal charge packet.

37. The semiconductor amplifier device according to claim 1, wherein the semiconductor amplifier device is formed on a substantially planar semiconductor substrate, and each of the charge amplifiers provides for avalanche multiplication of charge carriers transported in a direction substantially normal to the substantially planar surface of the semiconductor substrate.

38. The semiconductor amplifier device according to claim 1, wherein the semiconductor amplifier device is formed on a substantially planar semiconductor substrate, and each of the charge amplifiers provides for avalanche multiplication of charge carriers transported in a lateral direction that is substantially parallel to the substantially planar surface of the semiconductor substrate.

39. The semiconductor amplifier device according to claim 1, wherein the input signal is provided as the output of a particle or x-ray sensor, thereby providing for improved detection in counter-terrorism applications.