Method and apparatus for generating charge from a light pulse

A method and apparatus for generating charge from a light pulse. In one example, a light sensor includes an active region for generating an electric charge in response to a light pulse. A drift region is formed within a substrate and receives the electric charge from the light sensor. A spatial charge distribution is produced within the drift region in response to an electric field. The drift region includes an outer edge and an inner edge. The volume of the drift region decreases from the outer edge to the inner edge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to sensing, receiving, and processing light signals and, more particularly, to generating charge from a light pulse.

2. Description of the Related Art

In general, three-dimensional imaging systems employing active sources, such as laser detection and ranging (LADAR) systems, suffer from one primary problem: sensors designed to obtain two-dimensional amplitude images are not adept at rendering an image in three-dimensions. While there have been many attempts at adopting such two-dimensional sensors to three-dimensional imaging, such systems have always been found to be lacking, particularly in range resolution and sensitivity.

For example, one type of known three-dimensional imaging approach uses very high pixel sampling rates in various forms to determine time of flight for the laser pulse to travel from the laser to a target and on to a detector. The time of flight of an illuminating pulse is very difficult to measure since one nanosecond of time resolution is required to achieve one foot of depth resolution. As such, these systems typically employ high-speed counting and high-speed clocking circuits for operation. In cases where a depth resolution of inches is necessary (i.e., sub-nanosecond time differences must be resolved), the required operating speed of these counting and clocking circuits is difficult to achieve. Other known systems measure phase shifts between the illuminating signal and the signal returned from the target. These systems are susceptible to noise and provide inadequate sensitivity when the signal reflected from the target is very weak.

Therefore, there exists a need in the art for a method and apparatus for accurately resolving sub-nanosecond differences between times-of-arrival of light pulses.

SUMMARY OF THE INVENTION

The present invention is a device for resolving relative times-of-arrival of a plurality of light pulses comprising a plurality of drift-field detectors. Each drift-field detector comprises a light sensor and a semiconductor drift region. Each light sensor generates an electrical charge from at least one of the plurality of light pulses. Each semiconductor drift region receives the electrical charge from its respective light sensor and, pursuant to an electric field therein, produces a spatial charge distribution. The spatial charge distribution for each of the semiconductor drift regions is stored in an analog storage device associated therewith. In one embodiment of the invention, the analog storage devices comprise charge-coupled device (CCD) registers. The relative positions of the charge distributions in the semiconductor drift regions can be used to calculate the relative times-of-arrival of the light pulses. The present invention can be used in three-dimensional imaging applications, where the relative times-of-arrival of reflected light pulses are used to calculate the depth of the scene.

Another aspect of the invention relates to a method and apparatus for generating charge from a light pulse. In one embodiment of the invention, a light sensor includes an active region for generating an electric charge in response to a light pulse. A drift region is formed within a substrate and receives the electric charge from the light sensor. A spatial charge distribution is produced within the drift region in response to an electric field. The drift region includes an outer edge and an inner edge. The volume of the drift region decreases from the outer edge to the inner edge.

To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an apparatus for resolving relative times-of-arrival of light pulses without relying upon high-speed counting and clocking circuitry. As described in detail below, the present invention comprises a plurality of drift-field detectors generally formed in an array. Each drift-field detector comprises a semiconductor drift region coupled to a light sensor. The present invention resolves relative times-of-arrival of light pulses by measuring the distance a photo-generated charge packet moves through an electric field in the drift region for each drift-field detector. The apparatus of the present invention can be used in three-dimensional imaging applications, where a drift-field detector is used at each pixel of a three-dimensional image sensor and the time-of-arrival of a reflected light pulse incident on each pixel in the imaging array is used to produce a three-dimensional image. By eliminating clocking limitations, the present invention can resolve sub-nanosecond time-of-arrival differentials, advantageously providing depth information in an imaged scene to an accuracy of a centimeter or better. Those skilled in the art will appreciate that the present invention is useful in any application that requires resolving relative times-of-arrival of light pulses with high accuracy.

FIG. 1depicts a block diagram showing an exemplary three-dimensional imaging system100incorporating the apparatus of the present invention. The system100comprises a light source102, a drift-field detector array104, a processor106, control circuitry107, and a display108. The light source102produces light pulses124to illuminate a target118. The light pulses124reflect from the target118and are focused onto the drift-field detector array104by an optical lens116. The detected light signals are processed by processor106, under control of control circuitry107, for display as an image on display108.

More specifically, the light source102comprises a light emitting diode (LED) or laser source capable of emitting a pulse of light124of a particular wavelength. The wavelength of the light pulse124depends upon the particular application of the imager100, and is generally in the range between ultraviolet and infrared wavelengths. As shown, the light pulse124passes through optical lens114before traveling to a target118. Alternatively, the light source102can transmit the light pulse124to the target118without the aid of the optical lens114if the light source102is sufficiently powerful.

Axis128represents the distance between the target118and the system100, with the origin at the system100. The target118comprises a first portion130that is a distance Z1from the system100, a second portion122that is a distance Z2from the system100, and a third portion120that is a distance Z3from the system100. The light pulse124illuminates the target118, causing at least some of the light to be reflected back toward the system100in the form of reflected light126. The reflected light comprises a multiplicity of scattered light pulses. The reflected light126passes through optical lens116, which focuses the reflected light126onto the drift-field detector array104. The drift-field detector array104comprises a plurality of drift-field detectors1101through110N(collectively110) and respective analog storage devices1121through112N(collectively112). A 4×4 array of drift-field detectors110is shown for simplicity, but the present invention can have an M×N array of drift-field detectors110, where M and N are integers having a value of 1 or more. The optical lens116operates such that a reflected light pulse from a point on the surface of the target118will only fall upon the ithdrift-field detector110iin the array104that is focused upon such point. That is, each of the drift-field detectors110has a field of view (FOV) that dictates which light pulses in the reflected light126will be detected by a given drift-field detector110i.

FIG. 2depicts a schematic diagram showing an individual drift-field detector110iin accordance with the present invention. The drift-field detector110icomprises a light sensor204and a semiconductor drift region202. The light sensor204comprises a light sensitive detector, such as a silicon photodetector (e.g., a PIN photogate detector). The choice of light sensitive detector for the light sensor204is dictated by the wavelength of operation. For example, if the light source102of the system100transmits an illuminating pulse in the ultraviolet or visible spectrum, then the light sensor204can comprise a silicon photodetector. If the light source102of the system100transmits an illuminating pulse in the short-wave infrared light (SWIR) spectrum, the light sensor204can comprise a platinum silicide detector, or a III-IV detector and appropriate readout circuitry (e.g., control circuitry107). In this manner, the present invention can provide for an “eye-safe” imaging system. In any case, all that is required is for the light sensor204to generate a charge proportional to the amount of incoming photon energy incident upon it, and that this charge be injected into the drift region202in the form of electrons or holes, as described below.

In one embodiment, the drift region202comprises an N-buried channel formed in silicon having a known length. Alternatively, the drift region202can be formed of P-type silicon, wherein holes are injected into the drift region202from the light sensor204. In either case, the drift region202is electrically coupled to the light sensor204such that charge (be it electrons or holes) is injected into the drift region202from the light sensor204when light is detected. In one embodiment, the light sensor204and the drift region202are formed monolithically on a silicon substrate. This allows for production of the drift-field detector110iin standard silicon foundries using standard design rules for cost-effective fabrication. In addition, the appropriate detector readout circuitry (e.g., control circuitry107) can be incorporated into the same silicon substrate as the drift region. Alternatively, the light sensor204can be fabricated apart from the drift region202and then be bump bonded thereto.

A variable voltage source206is coupled on one end to the light sensor204, and on the other end to the drift region202. The variable voltage source206generates an electric field in the drift region202. The voltage of voltage source206is controlled by processor106through control circuitry107. In the embodiment shown, the variable voltage source206is coupled using ohmic connections. Alternatively, the variable voltage source206can be coupled to the light sensor204and drift region202via a plurality of gates (not shown) disposed thereon for generating the electric field. In any case, the variable voltage source206is controlled via switch208. Switches208for the drift-field detectors110are controlled via control circuitry107. In one embodiment, control circuitry107comprises a CMOS multiplexer capable of selectively controlling each switch208in the array104, as well as the voltage applied by respective variable voltage source206. In this manner, the processor106can control the electric field for specific ones of the drift-field detectors110. In such an embodiment, the CMOS multiplexer can be formed monolithically with the light sensors204and/or the drift regions202.

In addition, the drift region202is associated with an analog storage device112i. The analog storage device112ican comprise a charge-coupled device (CCD) register having a plurality of bins214formed therein. In such an embodiment, the analog storage device112ican be formed monolithically with the light sensor204and/or the drift region202. CCD transfer gate210acts as the interface between the drift region202and the analog storage device112ifor the transfer of charge therebetween. Each CCD transfer gate210is controlled by control circuitry107. In one embodiment, control circuitry107comprises a second CMOS multiplexer capable of selectively controlling each CCD transfer gate210in the array104. In this manner, the processor106can control the charge transfer between specific ones of the drift-field detectors110and their respective analog storage device112. Again, the second CMOS multiplexer can be formed monolithically with the other components of the array104.

In operation, a light pulse strikes the surface of the light sensor204and photon energy is converted into electric charge. The charge integration time for the light sensor204can be gated using control gates and a charge dump drain (not shown). The electric charge is injected into the drift region202. The variable voltage source206supplies a voltage differential across the drift region202such that an electric field is produced therein. This electric field is enabled and disabled by switch208. When the electric field is applied, the charge injected into the drift region202moves through the semiconductor material at a rate determined by the electric field combined with thermal diffusion. This rate also depends upon other factors, such as the type and temperature of the semiconductor material. Thus, a charge distribution will form in the drift region202having a certain shape and position. When the electric field is removed (by opening switch208), the charge distribution will remain fixed within the drift region202, but the shape will continue to disperse due to thermal diffusion. The velocity of electrons due to thermal diffusion, however, can be adjusted to be much less than the velocity of electrons where the electric field is applied to the drift region202.

In order to retain the position and shape of the charge distribution in the drift region202, the charge distribution is transferred to the analog storage device112i. The position of the charge distribution in the drift region202essentially “freezes” for a time long enough to move the charge from the drift region202to the analog storage device112i. In the present embodiment, the analog storage device112iis a CCD register having a plurality of bins214capable of storing charge. Specifically, once the electric field is removed from the drift region, CCD transfer gate210operates to transfer the charge distribution from the drift region202to the plurality of bins214. The number of bins214depends on the desired resolution of the charge distribution. That is, more bins214in the CCD register results in the storing of more detail of the shape and position of the charge distribution in the drift region202. In one embodiment, the transfer time from the drift region202to the bins214is in the range of 5 to 20 ns at room temperature to keep the thermally induced dispersion in the drift region within desirable limits. The operation of the analog storage devices112is described in more detail below with respect to FIG.3.

Returning toFIG. 1, since portion120of the target118is farther away from the system100than portion122, light reflected from portion120will take longer to reach the system100than light reflected from portion122. Thus, different light pulses in the reflected light126will arrive at the system100at different times. The difference between times-of-arrival of light pulses can be used to determine the depth of the scene. The present invention can resolve the relative times-of-arrival of light pulses incident on an array of drift field detectors104using the charge distribution in each of the drift field detectors110.

Specifically, each of the drift-field detectors110is activated (i.e., the switch208is closed and the electric field applied in each drift region202via control circuitry107) at some time tstartafter the illuminating pulse124has been transmitted. This time can coincide with the arrival of the first light pulse reflected from the target118, but this does not necessarily have to be the case. The time tstartcan coincide with the arrival of the first light pulse of interest that is reflected from the target118. As described more fully below, the time difference between when the illuminating pulse124is transmitted and when the drift-field detectors110are activated controls the range of the system100.

Assume that one particular drift-field detector1101within the array104is focused upon portion130of target118. The associated drift-field detector1101will detect the reflected light pulse and generate a charge packet in response to the incoming photon energy. This charge is injected into the associated drift region202and begins to drift in response to the electric field. At some later time, a reflected light pulse will arrive at optical lens116from portion122of target118and will be detected by another drift-field detector1102. Again, the charge will be injected into the drift region202of this second drift-field detector1102and will begin to drift. Hitherto the charge in the drift region202of the first drift-field detector1101has continued to drift. In a similar fashion, another drift-field detector1103will detect a reflected light pulse from portion120of target118at yet a later time. This charge is injected into the drift region202of this third drift-field detector1103and will begin to drift. Again, hitherto the charge in both drift regions202of the first and second drift-field detectors1101and1102has continued to drift. Finally, at some time tstopthe electric fields in the drift regions202of the drift-field detector array104will be turned off, and all drifting of charge will cease (with the exception of thermal diffusion, as described above).

As described above, the charge distributions in the drift-field detectors110are transferred to analog storage devices112at some time after tstop. The processor106can then read the charge from the analog storage devices112via control circuitry107. Once read out, the processor106uses the relative positions of the charge distributions in the drift regions202to calculate the relative times-of-arrival of the light pulses. Given the relative times-of-arrival of the light pulses, the processor106can compute a three-dimensional image that can be shown on display108.

FIGS. 4A through 4Care graphs showing charge distributions in drift regions of the three drift-field detectors1101,1102, and1103.FIGS. 4A through 4Cshare common axes. Axis402represents the carrier density in the drift region202having units of electrons per μm2. Axis404represents position in the drift region202having units of μm. Assume each drift region has a length of approximately 100 μm and an electric field of approximately 10 V/100 μm. Assume also that time tstartis time t=0, and time tstopis time t=10 ns. Finally, assume that the first light pulse arrives at time t=0, the second light pulse arrives at time t=8 ns, and the third light pulse arrives at time t=9 ns.

FIG. 4Ashows the charge distributions right after the electric fields are removed from the three drift regions at time t=10 ns. Curve410represents the charge distribution in the drift region202of the first drift-field detector1101, curve408represents the charge distribution in the drift region202of the second drift-field detector1102, and curve406represents the charge distribution in the drift region202of the third drift-field detector1103. After 10 ns of the applied electric field, the centroid of the charge distribution410has drifted to a position of 100 μm. After 2 ns of the applied electric field, the centroid of the charge distribution408has drifted to a position of 20 μm. Finally, after 1 ns of the applied electric field, the centroid of the charge distribution406has drifted to a position of 10 μm. The shape of each charge distribution spreads due to thermal diffusion as it drifts due to the electric field. The effects of thermal diffusion are most apparent in the drift region of the first drift-field detector1101, where the charge has been drifting for 10 ns (i.e., curve410).

As can be seen fromFIG. 4A, sub-nanosecond differences between times-of-arrival of light pulses can be easily discerned using centroid detection. Using known diffusion characteristics, it is possible to find the centriod of a charge distribution with high accuracy (e.g., better than a tenth of a nanosecond). Given the start time of the electric field, the position of the centriod of the charge distribution, and the rate of drift in the semiconductor material, the time-of-arrival of the light pulse that gave rise to the injected charge can be determined. Thus, each drift-field detector110iin the array104can collect information to determine the relative time-of-arrival of a light pulse striking its light sensor204.

FIGS. 4B and 4Cshow the effects of thermal diffusion on the charge distributions in drift-field detectors1101,1102, and1103.FIG. 4Bshows the charge distributions 10 ns after the electric field has been removed. As the charge thermally diffuses, the peak amplitude of the distribution decreases. The centroid, however, remains in a fixed position.FIG. 4Cshows the normalized charge distributions 100 ns after the electric field has been removed. As illustrated, the charge distributions almost completely overlap, and thus make it difficult to distinguish among their positions to determine the times-of arrival. In one embodiment, the charge distribution in each drift region202is transferred into its respective analog storage device112iwithin 5 to 20 ns after the electric field is removed.

The length of the drift region202and the magnitude of the electric field dictate the time tstop. In the above example, the drift region202of each the drift-field detectors110was 100 μm and the electric field was 10 V/100 μm. In that example, each of the drift-field detectors110could only be activated for 10 ns after the first signal of interest arrived. If they were activated for any longer, charge would begin to drift out of the drift region202, and time-of-arrival data for the first incoming reflected light pulses would be lost. In that example, the drift-field detector array104can resolve centimeters of resolution with a total range of about 30 meters. Thus, the length of the drift field202and the magnitude of the electric field dictate the maximum depth range of the system100. The time tstartcontrols where the range begins. That is, the range is a window that can be moved forward and away from the system100by controlling when the drift-field detectors110are activated relative to the emission of light pulses124. The resolution and depth range can be zoomed by varying the magnitude of the electric field (by varying the voltage of variable voltage supply206). For example, the electric field can be set such that the drift-field detector array104can resolve millimeters of resolution with a total range of about 3 meters. Additionally, particular groups of drift-field detectors110can have a higher or lower magnitude electric field than other groups by employing selective control via control circuitry107. In one embodiment, selective control is implemented via CMOS multiplexers as described above. In such an embodiment, the drift-field detector array104would allow the system100to zoom in on particular portions of the target118.

The above discussion assumed that three light pulses differing in times-of-arrival struck three different drift-field detectors1101,1102, and1103.FIG. 5illustrates a case where multiple light pulses that differ in times-of-arrival strike a single drift-field detector. As shown, light source102transmits an illuminating pulse502towards the target508. Drift fields5041,5042, and5043for three drift-field detectors are shown, having fields of view5061,5062, and5063, respectively. The field of view5062for the second drift field5042covers portions of the target508having two different depths Z1and Z2. Thus, drift field5042will contain two discernible charge distributions. This result is inherent in the design of the present invention. Thus, the present invention can advantageously discern multiple distances within a single drift-field detector using a signal illuminating pulse. Resolving multiple distances within a single drift-field detector significantly enhances the processing of three-dimensional data.

FIG. 3depicts a block diagram showing one embodiment of analog storage devices112. The analog storage devices112comprise M vertical CCD registers3021through302M(collectively302), a horizontal CCD register304, and an electrometer308. Each of the vertical CCD registers302comprises a multiplicity of bins310for storing charge. The horizontal CCD register304also comprises a multiplicity of bins312. An M×N array of drift-field detectors110is shown. Each column of drift-field detectors110is associated with one of the vertical CCD registers302. Each of the vertical CCD registers302is further coupled to the horizontal CCD register304. Operation is in accordance with what is known in the art as interline transfer. The charge distribution in each drift region is first transferred to bins310substantially as described above with respect to FIG.2. Then, for each of the vertical CCD registers302, the charge in a first set of bins310associated with the first drift-field detector110in the column is transferred to bins312in the horizontal CCD register304. The horizontal CCD register304comprises at least enough bins312to hold charge data from a detector in each of the vertical CCD registers302. All the charge in each of the vertical CCD registers302is then moved down in charge-transfer fashion to fill the empty bins.

Once this first set of charge is in the horizontal CCD register304, this charge is transferred using standard CCD practice to be detected by electrometer308. The electrometer308can comprise a floating diffusion electrometer stage known in the art. The electrometer308converts charge to voltage, which then can be read out by the processor106through control circuitry107of FIG.1. The processor106then displays the information on display108. This process repeats until all of the charge is read out from the analog storage devices112.

The embodiment shown inFIG. 3for the analog storage devices112allows the present invention to bin multiple fields from the drift regions. Specifically, a first illuminating pulse illuminates the target as described above with respect to FIG.1. The drift-field detectors110detect the reflected light pulses, and the analog storage devices112store the charge distributions. At this point, however, the vertical CCD registers302are not read into the horizontal CCD register304. A second illuminating pulse illuminates the target and the process is repeated. After each reflected pulse, the charge in each drift region drifts to give time resolution, the field is removed, and the charge pattern is loaded into the vertical CCD register304. The summing of charge, or “charge binning”, in the analog storage devices112is substantially noiseless. In this embodiment, the time separation of the illuminating pulses must be greater than the maximum drift time plus the transfer time from the drift regions to the analog storage devices112. Charge binning allows the present invention to detect reflected light pulses that are very weak thereby increasing system sensitivity.

The use of CCD registers for the analog storage devices112also provides very low readout noise capability. The CCD registers can be cooled using thermoelectric coolers (not shown) so that the binning of charge and readout can be carried over tenths of seconds. It is important to note that the slower the charge is read out from the analog storage locations112, the less noise is introduced into the system. The present invention advantageously allows for very slow readouts when imaging in noisy environments.

In yet another embodiment, the signal-to-noise ratio of the X and Y resolution information provided by array104can be further improved by charge binning the charge distribution after the depth information has been obtained. Specifically, the invention operates as described above to obtain a three-dimensional image. That is, the charge distribution from each of the drift-field detectors110is stored in the analog storage devices112. Charge binning can be used to increase system sensitivity. Then, a non-destructive readout of the charge distributions is performed to obtain the information necessary to display the depth of the scene. Then, the charge distribution for each of the drift-field detectors110, spread over multiple bins in the vertical CCD registers302, can be binned into a single CCD stage (e.g., a single CCD stage in horizontal CCD register304) representing a pixel associated with the X and Y position of that particular drift-field detector110. This second stage of charge binning increases the signal-to-noise ratio for a second readout of the two-dimensional information. That is, the charge distributions are summed so as to represent a pixel of the scene without depth information. In another embodiment, only a subset of the drift-field detectors have their charge binned into a single CCD stage. In this embodiment, some of the three-dimensional information is saved for further processing. Again, this selective control can be implemented using control circuitry107comprising a CMOS multiplexer as described above.

FIG. 6is a plan view depicting another exemplary embodiment of a drift field detector600. The drift field detector600may be used as a pixel in a detector array of an imaging system, such as the imaging system100of FIG.1. The drift field detector600comprises a substrate602having a light sensor604, a drift region606, and a readout sensor608. The light sensor604comprises an elliptically shaped active area (e.g., circular active area) defined by an outer edge610and an inner edge612. The drift region606comprises an elliptically shaped semiconductor drift region defined by an outer edge614and an inner edge616. The drift region606is circumscribed by the light sensor604such that the outer edge614of the drift region606is proximate the inner edge612of the light sensor604. A plurality of gates6181through618N(collectively referred to as gates618) are disposed atop the drift region606, where N is an integer greater than one. The gates618are configured as spaced apart concentric ellipses. The readout sensor608is circumscribed by the drift region606and the innermost gate618N, and is proximate the inner edge616of the drift region606. Due to the elliptical geometry, the drift field detector600exhibits a larger fill factor when compared to a drift field detector having a rectangular geometry.

In operation, the light sensor604generates a charge proportional to the amount of photon energy incident on the active area defined by the inner and outer edges610and612. The light sensor604may comprise any type of light sensitive detector known in the art, where the choice of light sensitive detector is dictated by the particular wavelength of light to be detected. The generated charge is injected into the drift region606. A voltage differential is established across the gates618to produce an electric field within the drift region606.

The injected charge moves inward within the drift region606under the influence of the electric field at a rate determined by the magnitude of the electric field combined with the carrier mobility. A charge distribution is thus formed within the drift region606having a certain shape and position determined by both the electric field and thermal diffusion. The electric field is then removed from the drift region606in response to various trigger events. For example, the electric field may be established for a pre-defined period of time. Alternatively, the electric field may be deactivated in response to detection of charge at the readout sensor608. If the drift field detector600is part of an array, then the electric field may be deactivated in response to detection of charge at a readout sensor of another one of the drift field detectors in the array.

In any case, when the electric field is removed, the centroid of the charge distribution will remain fixed within the drift region606, but the shape will continue to disperse due to thermal diffusion. The dispersion is stopped by applying different voltages to the gates over the drift region. In one embodiment of the invention, the charge distribution is binned within the drift region606by establishing an alternating high-low voltage potential across the gates618. The binned charge distribution (e.g., charge histogram) may then be read out of the drift region606using the readout sensor608. Notably, the binned charge may be transported through the drift region606using a particular clocked voltage configuration across the gates618. The charge distribution may be used to determine time-of-arrival of a light pulse, as described above.

FIG. 7is a cross-sectional view of an exemplary embodiment of the drift field detector600taken along the line7—7of FIG.6. Elements ofFIG. 6that are the same or similar to those shown inFIG. 7are designated with identical reference numerals and are described in detail above. In this exemplary embodiment, the substrate602comprises p-type silicon. The light sensor604illustratively comprises a PN photodiode having an exposure gate702, an n+ region704, and a transfer gate706. The n+ region704comprises the active region of the light sensor604. The drift region606comprises an n-type buried-channel708formed within the p-type silicon of the substrate602. The gates618of the drift region606, and the gates702and704of the light sensor604, are separated from the substrate602by a layer of silicon dioxide (SiO2)703, as is known in the art. The readout sensor608illustratively comprises a floating diffusion sense node710defined by an n+ region within the buried-channel708.

In response to the incident light, photo-generated charge is collected in a potential well formed by the PN junction. The exposure gate702controls whether photo-generated charge is collected and acts as an “electronic shutter” for the drift field detector600. After a pre-defined integration period, a bias may be applied to the transfer gate706to inject the collected charge from the light sensor604to the drift region606. As described above, an increasing voltage potential is established across the gates618to generate an electric field within the drift region606. The biasing of the light sensor604and the drift region606may be controlled via bias circuitry712. The injected charge drifts under the influence of the electric field and, upon the occurrence of the desired triggering event, the charge is binned by applying an alternating high-low voltage potential across the gates618to form a charge distribution. The charge distribution is then read out via the sense node710using readout circuitry714. Notably, a clocked voltage potential may be applied to the gates618in order to transfer the charge distribution in a charge-coupled manner to the sense node710.

FIG. 8is a plan view depicting yet another exemplary embodiment of a drift field detector800. The drift field detector800may be used as a pixel in a detector array of an imaging system, such as the imaging system100of FIG.1. The drift field detector800comprises a substrate802having a light sensor804, a drift region806, and a readout sensor808. The light sensor804comprises an active area defined by an outer edge810and an inner edge812. The drift region806comprises a trapezoidal-shaped semiconductor drift region defined by an outer edge814and an inner edge816. The outer edge814of the drift region806is proximate the inner edge812of the light sensor804. A plurality of gates8181through818N(collectively referred to as gates818) are disposed atop the drift region806in spaced apart relation, where N is an integer greater than one. The readout sensor808is proximate the inner edge816of the drift region806. Due to the trapezodial geometry, the drift field detector800exhibits a larger fill factor when compared to a drift field detector having a rectangular geometry. To further increase the fill factor, a microlens (seeFIG. 9) may be disposed atop the active region of the light sensor804to focus the light onto the active region.

In operation, the light sensor804generates a charge proportional to the amount of photon energy incident on the active area defined by the inner and outer edges810and812. The light sensor804may comprise any type of light sensitive detector known in the art, where the choice of light sensitive detector is dictated by the particular wavelength of light to be detected. The generated charge is injected into the drift region806. A voltage differential is established across the gates818to produce an electric field within the drift region806, substantially as described above. The injected charge moves inward within the drift region806under the influence of the established electric field and a charge distribution is formed within the drift region806having a certain shape and position. The electric field is then removed from the drift region806in response to a desired trigger event, similar to the embodiments of drift field detectors described above.

When the electric field is removed, the charge distribution will remain fixed within the drift region806, but the shape will continue to disperse due to thermal diffusion. The dispersion is stopped by applying different voltages to the gates over the drift region. In one embodiment of the invention, the charge distribution is binned within the drift region806by establishing an alternating high-low voltage potential across the gates818. The charge distribution may then be read out of the drift region806using the readout sensor808. Notably, the charge may be transported through the drift region806using a particular clocked voltage configuration across the gates818. The charge distribution may be used to determine time-of-arrival of a light pulse, as described above.

FIG. 9is a cross-sectional view of an exemplary embodiment of the drift field detector800taken along the line9—9of FIG.8. Elements ofFIG. 8that are the same or similar to those shown inFIG. 9are designated with identical reference numerals and are described in detail above. In this exemplary embodiment, the substrate802comprises p-type silicon. The light sensor804illustratively comprises a PN photodiode having an exposure gate902, an n+ region904, and a transfer gate906. The n+ region904comprises the active region of the light sensor804. In one embodiment, a microlens905may be disposed above the active region of the light sensor804to increase the fill factor of the drift field detector800.

The drift region806comprises an n-type buried-channel908formed within the p-type silicon of the substrate802. The gates818of the drift region806, and the gates902and904of the light sensor804, are separated from the substrate802by a layer of SiO2903, as is known in the art. The readout sensor808illustratively comprises a floating diffusion sense node910defined by an n+ region within the buried-channel908. The light sensor804and the drift region806operate in a manner similar to the light sensor604and drift region606described above with respect toFIGS. 6 and 7. Notably, the biasing of the light sensor804and the drift region806may be controlled via bias circuitry912. The charge distribution may be read out via the sense node910using readout circuitry914. The triangular drift region806occupies less area than a rectangular drift region, allowing the control circuitry (e.g., bias circuitry912and readout circuitry914) to be placed within a saved area850(FIG.8).

For purposes of clarity by example, the drift field detectors600and800have been described as having a p-type silicon substrate. Those skilled in the art will appreciate, however, that the substrate may be formed of n-type silicon. In such an embodiment, the buried-channel is formed of p-type silicon, the light sensor includes a p+ region, and the sense node includes a p+ region. In addition, although the light sensors604and804have been described as a PN photodiodes, those skilled in the art will appreciate that other types of light sensors may be employed, including other types of silicon photodetectors (e.g., PIN photogate), as well as a platinum silicide detectors, a III-IV detectors, and the like along with the appropriate readout circuitry. Furthermore, those skilled in the art will appreciate that other types of readout sensors608and808may be used, including floating gate readouts, current amplifiers, and transimpedance amplifiers. Moreover, although aspects of the invention are described with respect to elliptical and trapezoidal drift regions, those skilled in the art will appreciate that drift regions of other shapes may be employed. In general, a drift field detector of the invention includes a drift region having an outer edge and an inner edge, where the volume of the drift region decreases from the outer edge to the inner edge.

A method and apparatus for generating charge from a light pulse has been described. A drift field detector includes a light sensor and a semiconductor drift field. The light sensor generates electric charge in response to a light pulse and the charge is injected into the drift region. Under influence of an electric field, the photo-generated charge drifts through the drift region. The electric field is deactivated in response to a desired triggering event. A charge distribution within the drift region is either stored in an analog storage device and read out, or is read out directly from the drift region using a sense node. By determining the distance the photo-generated charge drifted within the drift region, the time-of-arrival of the light pulse may be determined with respect to the time the electric field was deactivated with sub-nanosecond accuracy. In one embodiment, the drift field detector is used within an array of detectors. A light pulse illuminates a target and the reflected portions of the illuminating pulse are focused onto the array. The relative times-of-arrival of the different reflected portions may be identified and used to determine the depth and contours of the target.