HK1228138B - Sensing system for flight time and sensor for flight time - Google Patents

Description

Time-of-flight sensing systems and time-of-flight sensors

Technical Field

The present invention relates generally to imaging systems and, in particular, to time-of-flight imaging systems.

Background Art

As the popularity of three-dimensional (3D) applications continues to grow in applications such as imaging, movies, games, computers, and user interfaces, interest in 3D cameras has also increased. A typical passive method for generating 3D images is to use multiple cameras to capture a stereoscopic image or multiple images. Using stereoscopic images, objects in the image can be triangulated to generate a 3D image. One disadvantage of this triangulation technique is that it is difficult to use small devices to generate 3D images because a minimum separation distance must exist between each camera to generate a three-dimensional image. In addition, this technique is complex and therefore requires a large amount of computer processing power to generate 3D images in real time.

For applications that require real-time acquisition of 3D images, active depth imaging systems based on optical time-of-flight measurements are sometimes utilized. A time-of-flight system typically employs a light source that directs light at an object and a sensor that detects light reflected from the object. The time between emission from the light source and detection of the reflected light by the sensor indicates the distance of the object relative to the sensor. An electronic signal is sent within the time-of-flight system to coordinate the emission of light by the light source and the detection of the reflected light by the sensor. However, delays in the electronic signal can compromise the time-of-flight calculation that indicates the distance of the object from the sensor. Previous approaches to reducing signal delays have included reducing the resistance and capacitance of metal interconnects, but design rules limit the width of the metal interconnects and therefore limit the minimum achievable resistance (and therefore the propagation delay of the electronic signal). The use of digital algorithms for calibrating the resulting delayed signal has also been used previously. However, this increases the processing requirements of the time-of-flight system.

Summary of the Invention

In one aspect, the present invention describes a time-of-flight sensing system comprising: a light source that emits light pulses toward an object; a control circuit coupled to send a synchronization signal when one of the light pulses is emitted by the light source; and a time-of-flight pixel array having a plurality of time-of-flight pixel cells, wherein each of the time-of-flight pixel cells comprises: a photosensor configured to generate an image signal in response to receiving photons from the light pulse reflected from the object; a delay circuit coupled to generate a delayed synchronization signal in response to the synchronization signal, wherein the delay circuit includes a delay transistor, and wherein the time-of-flight pixel array includes a transistor gradient, wherein the transistor gate length of the delay transistor changes as the time-of-flight pixel cell approaches closer to the center of the time-of-flight pixel array so that each of the time-of-flight pixel cells receives its corresponding delayed synchronization signal simultaneously; and a signal circuit coupled to generate a distance signal in response to receiving the delayed synchronization signal and the image signal, wherein the distance signal represents the distance from the photosensor to the object.

On the other hand, the present invention describes a time-of-flight sensor, comprising: a control circuit coupled to synchronously send a synchronization signal and an emission signal to a light source, wherein the emission signal activates the light source to emit a light pulse; and a time-of-flight pixel array having a plurality of time-of-flight pixel units, wherein each of the time-of-flight pixel units comprises: a photosensor configured to generate an image signal in response to receiving photons from the light pulse reflected from an object; a delay circuit coupled to generate a delayed synchronization signal in response to the synchronization signal, wherein the delay circuit includes a delay transistor, and wherein the time-of-flight pixel array includes a transistor gradient, wherein the transistor gate length of the delay transistor changes so that each of the time-of-flight pixel units simultaneously receives its corresponding delayed synchronization signal; and a signal circuit coupled to generate a distance signal in response to receiving the delayed synchronization signal and the image signal, wherein the distance signal represents the distance from the photosensor to the object.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

1 is a block diagram of an example time-of-flight sensing system, according to an embodiment of the invention.

2A illustrates a schematic block diagram of an example pixel cell of a time-of-flight sensor, according to an embodiment of the disclosure.

2B illustrates an example delay circuit including transistors having different transistor gate lengths, according to an embodiment of the invention.

3 illustrates a cross-section of a time-of-flight sensor included in a time-of-flight imaging system, according to an embodiment of the invention.

4 illustrates a time-of-flight sensor including a transistor gradient corresponding to an example delay layer of the time-of-flight sensor, in accordance with an embodiment of the invention.

5 illustrates a timing diagram showing the voltage on a capacitor with respect to emission by a light source and reception of reflected light by a time-of-flight sensor, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of time-of-flight image sensors and time-of-flight imaging systems are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. However, one skilled in the relevant art will recognize that the techniques described herein can be practiced without one or more of the specific details or with other methods, components, materials, and the like. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the various places in which the phrases "in one embodiment" or "in an embodiment" appear throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIG1 is a block diagram of an example time-of-flight sensing system 100 according to an embodiment of the present invention. As shown, the illustrated example time-of-flight sensing system 100 includes a time-of-flight pixel array 112, readout circuitry 107, function logic 105, control circuitry 116, and a light source 102 for sensing the round-trip distance to an object 106. In the example illustrated in FIG1 , the pixel array 112 is a two-dimensional (2D) array of time-of-flight pixel cells 118. In one embodiment, the 2D array is 1920 pixel cells 118 wide and 1080 pixel cells 118 high. As illustrated, each pixel cell 118 is arranged in rows and columns to acquire time-of-flight information for an image object 106 focused on the pixel array 112. Therefore, the time-of-flight information can then be used to determine the distance or depth information to the object 106.

In one example, control circuitry 116 controls light source 102 and synchronizes light source 102 with transmit signal 127 to transmit light pulse 104 toward object 106. Reflected light pulse 108 then reflects back to pixel array 112 as shown. Pixel cells 118 in pixel array 112 sense photons from reflected light pulse 108 and generate image signals in response to the incident photons. Readout circuitry 107 then reads out the image signals via bit lines 103 as shown. In one example, readout circuitry 107 may include an analog-to-digital converter and an amplifier to convert and amplify the image signals received via bit lines 103. The image signals read out by readout circuitry 107 may then be passed to digital circuitry included in function logic 105. In one example, function logic 105 may determine time-of-flight and distance information for each pixel cell 118. In one example, function logic 105 may also store the time-of-flight information and/or even manipulate the time-of-flight information (e.g., crop, rotate, and/or adjust for background noise). In one example, readout circuitry 107 reads out an entire row of time-of-flight information at a time along bit line 103 (illustrated), or in another example, the time-of-flight information may be read out using various other techniques (not illustrated), such as serial readout or fully parallel readout of all pixels simultaneously. In the illustrated example, control circuitry 116 is further coupled to pixel array 112 to control the operation of pixel array 112 and to synchronize the operation of pixel array 112 with light source 102. For example, control circuitry 116 may initiate a global synchronization signal to each of the pixel cells that is synchronized with emission signal 127 to initiate time-of-flight image capture.

In one embodiment, light source 102 is a narrow-band infrared light emitting diode (LED). Pixel array 112 can be covered by a bandpass optical filter that passes infrared light of the same wavelength as emitted by light source 102 and passes light of other wavelengths so that pixel array 112 receives only light having the wavelength of the narrow-band infrared LED. This arrangement reduces the likelihood that ambient light will bias the measurements of time-of-flight sensing system 100.

Time-of-flight sensing system 100 can be implemented using a stacked chip approach. For example, pixel array 112 can be included in a pixel die, while readout circuitry 101, function logic 105, and control circuitry 116 can be included in a separate ASIC die. In this example, the pixel die and ASIC die are stacked and coupled together during manufacturing to implement time-of-flight sensing system 100.

FIG2A illustrates a schematic block diagram of an example time-of-flight pixel cell 218 of a time-of-flight sensor according to an embodiment of the present invention. Pixel cell 218 is an example of pixel cell 118 shown in FIG1 . As shown in the depicted example, pixel cell 218 includes a photosensor 220 and pixel support circuitry 249. Pixel support circuitry 249 includes charge control logic 222, a programmable current source 226, a capacitor C _TOF 232, a reset circuit 234, and an output circuit 238. Photosensor 220 senses photons of reflected light 208 reflected from object 106. In one example, photosensor 220 may include a single-photon avalanche diode (SPAD).

The photosensor 220 may include a Geiger mode photon detection unit, such as a single photon avalanche diode (SPAD) fabricated on a silicon substrate. A Geiger mode APD generates pulses of equal amplitude when struck by a photon. A Geiger mode APD has a p-n junction that is biased above its breakdown voltage so that each electron-hole pair triggers an avalanche multiplication process, which causes the current at the output of the photon detection unit to quickly reach its final value. This avalanche current continues until the avalanche process is suppressed using a suppressor element. The SPAD's sensitivity to even a single photon makes it a suitable choice for time-of-flight applications.

Delay circuit 273 is coupled to generate a delayed synchronization signal 216 in response to receiving a synchronization signal 215 from control circuit 116. Control circuit 116 is coupled to send synchronization signal 215 to all pixel cells simultaneously when it initiates light pulse emission from light source 102. As previously discussed, ideally, a global synchronization signal arrives at each pixel simultaneously to obtain an optimal calculation of the time of flight for each pixel cell 218. However, the signal path from synchronization signal 215 to each pixel cell has a different length due to the physical arrangement of each pixel cell 218. Therefore, the propagation delay from the different signal path lengths contributes to a less accurate time of flight calculation. For example, in FIG4 , a network of drivers 420 of control circuit 116 delivers synchronization signal 215 to the pixel cells. However, the network of drivers 420 is distributed around the edges of pixel array 412. Therefore, without delay compensation, pixel cells in the middle of pixel array 412 will receive synchronization signal 215 at a later time than pixel cells along the edges of pixel array 412 because pixel cells near the edges are closer to driver 420. However, strategically inserting different delay circuits in different delay layers of the time-of-flight sensor 400 will allow the synchronization signal to arrive at each pixel cell substantially simultaneously.

FIG2B illustrates example delay circuits 273A and 273B including transistors having different transistor gate lengths according to an embodiment of the present invention. Delay circuits 273A and 273B are examples of delay circuit 273. Delay circuits 273A and 273B are inverters. Therefore, when synchronization signal 215 is logic low, delayed synchronization signals 216A and 216B will be logic high. In delay circuit 273A, a P-channel metal-oxide-semiconductor (PMOS) 276A is coupled to supply voltage VDD and an NMOS 278A. The gate of PMOS 276A is coupled to receive synchronization signal 215. Similarly, in delay circuit 273A, an N-channel metal-oxide-semiconductor (NMOS) 278A is coupled to a reference (e.g., ground), and the gate of NMOS 278A is also coupled to receive synchronization signal 215.

Delay circuit 278 is similar to delay circuit 273A, differing only in that gate length 277B of PMOS 276B is longer than gate length 277A of PMOS 276A. Furthermore, gate length 279A of NMOS 278A is longer than gate length 279B of NMOS 278B. In both delay circuits 273A and 273B, the longer the gate length 277, the longer the delay that will result in delayed synchronization signal 216. Therefore, when delay circuit 273A receives a logic low as synchronization signal 215, generating a corresponding logic high for delayed synchronization signal 216A will occur in a shorter amount of time than when delay circuit 273B generates a logic high for delayed synchronization signal 216B in response to the same synchronization signal 215. Therefore, delay circuit 273A may be used in pixel cells 118 located near the center of pixel array 112 (and away from driver 420), while delay circuit 273B may be used in pixel cells 118 located closer to driver 420. And further, the gate lengths of the PMOS transistors in the other delay circuits can be more finely tuned to produce a delay somewhere between the delays produced by delay circuits 273A and 273B.

FIG4 illustrates a time-of-flight sensor 410 including transistor gradients for example delay layers corresponding to a pixel array 412 included in the time-of-flight sensor 410, in accordance with an embodiment of the present invention. FIG4 includes example delay layers 451, 452, 453, 454, and 455 within the pixel array 412, which includes a plurality of pixel cells 218. The pixel cells 218 within delay layer 455 have delay circuits 273 including delay transistors (e.g., PMOS 276) that impart a longer delay to the synchronization signal 215 than the delay circuits 273 in delay layer 454. Configuring the gate lengths of the delay transistors in the delay circuits 273 differently in different delay layers allows the synchronization signal 216 to arrive at the signal circuits 248 in the different delay layers (substantially) at the same time. Similarly, the delay circuits 273 in delay layer 454 will impart a longer delay to the synchronization signal 215 than the delay circuits 273 in delay layer 453, the delay circuits 273 in delay layer 453 will impart a longer delay to the synchronization signal 215 than the delay circuits 273 in delay layer 452, and the delay circuits 273 in delay layer 452 will impart a longer delay to the synchronization signal 215 than the delay circuits 273 in delay layer 451. The delay circuits 273 in delay layer 451 can be designed to have no delay or the lowest possible amount of delay. Generally speaking, the delay of the synchronization signal 215 is longer at the edges of the pixel array 412 and becomes shorter as the pixel cells 218 get closer to the center of the pixel array 412 because the propagation from the driver 420 becomes longer as the pixel cells 218 move away from the driver 420. This creates a transistor gradient, where the transistor gate lengths of the delay transistors change (becoming progressively larger or smaller) to allow each of the pixel cells to receive its corresponding delayed synchronization signal 216 (substantially) at the same time.

It should be understood that the delay layers 451-455 are exemplary and that the actual delay layers may vary depending on the physical location of the driver 420. Furthermore, the gate length of the delay transistor may, in some examples, be modified on a pixel-by-pixel basis, such that the gate length of the delay transistor is continuously (grayscale) adjusted as the pixel cell is further away from the driver 420. Furthermore, while the illustrated example of the delay circuit 273 includes increasing the gate length 277 of the PMOS 276 to increase the delay of the synchronization signal 215, those skilled in the art will appreciate that, in different circuit configurations, modifying the gate length of the NMOS transistor may achieve the disclosed delay of the synchronization signal 215.

Referring again to FIG. 2A , signal circuitry 248 is coupled to generate a distance signal 231 in response to receiving delayed synchronization signal 216 and an image signal from photosensor 220. Distance signal 231 represents the distance from object 106 to a particular pixel cell 218. Charge control logic 222 of signal circuitry 248 is coupled to photosensor 220 to detect when photosensor 220 senses a photon of reflected light 208 reflected from object 106. Charge control logic 222 is further coupled to receive delayed synchronization signal 216 from delay circuitry 273. Signal circuitry 248 is configured to generate a time-of-flight (TOF) signal 230 in response to receiving the image signal and delayed synchronization signal 216 from photosensor 220. TOF signal 230 represents the time between the emission of one of the light pulses from light source 102 and the receipt of a photon from the light pulse at pixel cell 218.

Programmable current source 226 is coupled to receive TOF signal 230. Programmable current source 226 is coupled to reference current source 213, which sinks ( FIG. 2A ) or supplies reference current I _REF 217 to current source 226 in response to TOF signal 230. In one embodiment, reference current source 213 is coupled to provide reference current I _REF 217 to all pixel cells 218 included in pixel array 112. In the illustrated embodiment, current source 226 is coupled to supply a constant current I _H 228 to charge capacitor C _TOF 232 in response to TOF signal 230.

The voltage V _TOF 233 accumulated on capacitor C _TOF 232 represents the round-trip distance from light source 102 to object 106 and back to pixel cell 218. In FIG2B , reset circuit 234 is coupled to capacitor C _TOF 232 to reset the accumulated voltage V _TOF 233 on capacitor C TOF 232 in response to a reset signal (not illustrated) received from control circuit 116 after the _accumulated voltage V _TOF 233 is read out from capacitor C _TOF 232 via output circuit 238. As shown in the example, signal circuit 248 also includes output circuit 238 to read out V _TOF accumulated on capacitor C _TOF . Output circuit 238 may include an output switch, an amplifier, and a row select switch to selectively couple capacitor C _TOF to bit line 240 for readout purposes. Reset circuit 234 may include a transistor that resets voltage 233 to a reference voltage (e.g., ground). In one example, the charge control logic 222 includes a latch coupled to receive the delayed synchronization signal 216 and the image signal from the photosensor 220, and the reset circuit 234 resets the latch in the charge control logic 222 after reading the voltage V _TOF 233. The reset circuit 234 can also reset the circuits in the charge control logic 222 when the reset circuit 234 is reset in response to a reset signal from the control circuit 116.

To further illustrate the functionality of the example pixel support circuit 249, FIG5 illustrates an example timing diagram 500 showing an example voltage V _TOF 533 on capacitor 232 in relation to the emission of light by the light source and the reception of reflected light by the time-of-flight sensor, in accordance with an embodiment of the present invention. At time t0, the light source 102 is activated to emit a light pulse illustrated by the light source emission waveform 502. Also at time t0, the synchronization signal 215 transitions to a logic high and is distributed to the pixel cells 218 in the pixel array 112, such that each pixel cell 218 can begin its time-of-flight measurement at the same time that the light source 102 emits the light pulse 104. At time t2, the photosensor 220 receives the reflected light pulse 108, causing the photosensor 220 to generate a logic high signal, as shown in waveform 520. The time-of-flight 596 of the light pulse emitted by the light source 102 is calculated by measuring the time between the emission of the light pulse 104 and the reception of the reflected light pulse 108. Once the time-of-flight (TOF) is known, the distance L from the light source 102 to the object 106 can be determined using the following relationship in Equations 1 and 2 below:

Where c is the speed of light (approximately equal to 3 x 10 ⁸ m/s), and TOF is the amount of time it takes a pulse of light to travel to and from an object as shown in FIG. 1 .

At time t1, the signal circuit 248 of pixel cell 218 receives the corresponding delayed synchronization signal 216 (not shown in FIG5 ). The time between time t0 and time t1 will be the same for each of pixel cells 218, but times t0 and t1 will be shared between the inherent propagation delay of synchronization signal 215 and the designed delay introduced by delay circuit 273. The inherent propagation and designed delays integrated into the delay circuit 273 of each pixel cell will be separated so that all pixel cells receive their delayed synchronization signal 216 simultaneously. Between time t1 and time t2, current source 226 charges capacitor 232 with a constant current I _H 228, which increases the voltage across capacitor 232 at a known rate as shown. When TOF signal 230 indicates a photon incident on photosensor 220, current source 226 stops supplying current I _H 228 to capacitor 232, and the voltage level of capacitor 232 remains at a stable voltage. Thus, the higher the voltage 233, the longer the time-of-flight 596 of the transmitted pulse, and therefore the longer the distance to the object 106 for a particular pixel cell 218. At time t3, the voltage 233 on the capacitor 232 can be read out as a distance signal 231 representing the distance from the light source 102 to the object 106. Because the time between times t0 and t1 is known and designed into the time-of-flight sensor, the distance signal 231 can be adjusted or calibrated to account for the known delay between times t0 and t1 in the distance calculations described above.

As discussed above, ideally, signal circuitry 248 receives delayed synchronization signal 216 (substantially) simultaneously so that each pixel cell 218 begins its time-of-flight measurement at the same time, regardless of whether the pixel cell is proximate to driver 420. Light source emission 502 illustrates that light pulse 104 is emitted from light source 102 in response to emission signal 127. Synchronization signal 215 is sent to pixel cells 218 in pixel array 112 so that each pixel cell can begin its time-of-flight measurement at the same time as light pulse 104 is emitted.

FIG2A illustrates that a time-of-flight pixel cell 218 can be implemented in a stacked chip approach. For example, as shown in the example, light sensor 220 can be included in pixel die 247, while pixel support circuitry 249 of pixel cell 218 illustrated in FIG2 can be included in a separate ASIC die 250. FIG3 illustrates a cross-section of a time-of-flight sensor 301 included in a time-of-flight sensing system 300, according to an embodiment of the present invention. The time-of-flight sensing system 300 is implemented such that pixel die 348 is coupled to ASIC die 350 in the stacked chip approach of FIG3. As shown, the time-of-flight sensing system 300 includes a light source 302 that emits a light pulse 304 directed at an object 306. The emitted light pulse 304 reflects back from the object 306, which is shown as a reflected light pulse 308.

In one example, the time-of-flight sensing system 300 also includes a pixel die 348 that includes a plurality of pixel cells (including pixel cell 318) arranged in a time-of-flight pixel array. In the example, each pixel cell 318 includes a photosensor 320, which in the illustrated example includes a SPAD that is optically coupled to receive reflected light pulses 308 from the object 306 through a corresponding microlens 310 as shown. In another example, the microlens 310 can be omitted. Each photosensor 320 of each pixel cell 318 is coupled to corresponding pixel support circuitry 349, which is disposed in the ASIC die 350 as shown.

As shown in the depicted example, the pixel support circuitry 349 of each pixel cell 318 is also coupled to a single reference current source 317 included in the ASIC die 350. Reference current source 317 is coupled to provide a reference current I _REF 313 for use by each pixel support circuitry 349 to program an internal programmable current source included in each pixel support circuitry 349. In the illustrated example, control circuitry 116 is also included in the ASIC die 350 and is coupled to synchronously provide a transmit signal 327 to the light source 302 and the pixel cells 318. Emitted light pulses 304 are emitted from the light source 302 in response to the transmit signal 327, and a synchronization signal 315 is sent to the pixel cells 318 so that each pixel cell 318 can begin its time-of-flight measurement synchronously with the emission of the light pulses 304.

The above description of the illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Although specific embodiments and examples of the present invention are described herein for illustrative purposes, as those skilled in the relevant art will recognize, various modifications are possible within the scope of the present invention.

These modifications can be made to the present invention in light of the above detailed description. The terms used in the appended claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the present invention is to be determined entirely by the appended claims, which are to be construed in accordance with established rules of claim interpretation.

Claims (20)

1. A time-of-flight sensing system, comprising: A light source that emits light pulses toward an object; Control circuitry, coupled to transmit a synchronization signal when one of the light pulses is emitted by the light source; and A time-of-flight pixel array having a plurality of time-of-flight pixel units, each of said time-of-flight pixel units comprising: A photoelectric sensor configured to generate an image signal in response to receiving photons from a light pulse reflected from the object; A delay circuit, coupled to generate a delayed synchronization signal in response to the synchronization signal, wherein the delay circuit includes a delay transistor, and wherein the time-of-flight pixel array includes a transistor gradient, wherein the gate length of the delay transistor changes as the time-of-flight pixel cells approach the center of the time-of-flight pixel array, such that each of the time-of-flight pixel cells simultaneously receives its corresponding delayed synchronization signal; and A signal circuit coupled to generate a distance signal in response to receiving the delayed synchronization signal and the image signal, wherein the distance signal represents the distance from the photoelectric sensor to the object. 2. The time-of-flight sensing system of claim 1, wherein the signal circuit includes charging control logic coupled to generate a time-of-flight (TOF) signal in response to receiving the image signal and the delay synchronization signal, the TOF signal representing the time between emitting one of the light pulses and receiving the photon from the light pulse at the corresponding time-of-flight pixel unit. 3. The time-of-flight sensing system of claim 2, wherein the signal circuit further comprises a capacitor and a current source, wherein the current source supplies current to the capacitor to charge the capacitor in response to receiving the TOF signal from the charging control logic, and the voltage on the capacitor represents the distance from the photoelectric sensor to the object. 4. The time-of-flight sensing system of claim 3, wherein the signal circuit further comprises a reset circuit coupled to reset the voltage on the capacitor. 5. The time-of-flight sensing system of claim 3, wherein the signal circuitry further comprises an output circuitry including an amplifier coupled to the capacitor, wherein the amplifier is coupled to amplify the voltage across the capacitor to a readout bit line. 6. The time-of-flight sensing system of claim 3, wherein the current source in each of the time-of-flight pixel units is coupled to the same reference current driver. 7. The time-of-flight sensing system of claim 1, wherein the delay transistor is a P-channel metal-oxide-semiconductor (PMOS) transistor and the gate length of the transistor decreases as the time-of-flight pixel unit gets closer to the center of the time-of-flight pixel array. 8. The time-of-flight sensing system of claim 7, wherein the delay circuit comprises an inverter including the delay transistor. 9. The time-of-flight sensing system of claim 1, further comprising a readout circuit coupled to read out the distance signal from each of the time-of-flight pixel units. 10. The time-of-flight sensing system of claim 1, wherein the photoelectric sensor comprises an avalanche photodiode. 11. The time-of-flight sensing system according to claim 1, wherein the light source is an infrared light-emitting diode (LED). 12. A time-of-flight sensor, comprising: A control circuit, coupled to synchronously send a synchronization signal and a transmission signal to a light source, wherein the transmission signal activates the light source to emit light pulses; and A time-of-flight pixel array having multiple time-of-flight pixel units, each of which includes: A photoelectric sensor configured to generate an image signal in response to receiving photons from a light pulse reflected from an object; A delay circuit coupled to generate a delayed synchronization signal in response to the synchronization signal, wherein the delay circuit includes a delay transistor, and wherein the time-of-flight pixel array includes a transistor gradient, wherein a change in the gate length of the delay transistor causes each of the time-of-flight pixel cells to simultaneously receive its corresponding delayed synchronization signal; and A signal circuit coupled to generate a distance signal in response to receiving the delayed synchronization signal and the image signal, wherein the distance signal represents the distance from the photoelectric sensor to the object. 13. The time-of-flight sensor of claim 12, wherein the signal circuitry includes charging control logic coupled to generate a time-of-flight (TOF) signal in response to receiving the image signal and the delay synchronization signal, the TOF signal representing the time between emitting the light pulse and receiving the photon from the light pulse at the corresponding time-of-flight pixel unit. 14. The time-of-flight sensor of claim 13, wherein the signal circuitry further comprises a capacitor and a current source, wherein the current source supplies current to the capacitor to charge the capacitor in response to receiving the TOF signal from the charging control logic, the voltage on the capacitor representing the distance from the photoelectric sensor to the object. 15. The time-of-flight sensor of claim 14, wherein the signal circuitry further comprises a reset circuit coupled to reset the voltage on the capacitor. 16. The time-of-flight sensor of claim 14, wherein the signal circuitry further comprises an output circuitry including an amplifier coupled to the capacitor, wherein the amplifier is coupled to amplify the voltage across the capacitor to a readout bit line. 17. The time-of-flight sensor of claim 14, wherein the current source in each of the time-of-flight pixel units is coupled to the same reference current driver. 18. The time-of-flight sensor of claim 12, wherein the delay transistor is a P-channel metal-oxide-semiconductor (PMOS) transistor and the gate length of the transistor decreases as the time-of-flight pixel unit gets closer to the center of the time-of-flight pixel array. 19. The time-of-flight sensor of claim 18, wherein the delay circuit comprises an inverter including the delay transistor. 20. The time-of-flight sensor of claim 12, further comprising a readout circuit coupled to read out the distance signal from each of the time-of-flight pixel units.