The pinned photodiode is the primary photodetector structure used in most CCD and CMOS image sensors. This paper reviews the development, physics, and technology of the pinned photodiode.

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A Review of the Pinned Photodiode for CCD and CMOS Image Sensors

This paper presents a CMOS time-of-flight (ToF) range image sensor using high-speed lock-in pixels with background light canceling capability. The proposed lock-in pixel uses MOS gate-induced lateral electric field control of depleted potential of pinned photodiode for implementing a multiple-tap charge modulator while achieving a high-speed charge transfer for high-time resolution. A TOF image sensor with 320 x 240 effective pixels is implemented using a 0.11- $\mu \text{m}$ CMOS image sensor process. The TOF sensor has a range resolution of less than 12 mm without background light and 20 mm under background line for the range from 0.8 to 1.8 m and integration time of 50 ms. The effectiveness of in-pixel background light canceling with a three-tap output pixel is demonstrated.

A Time-of-Flight Range Image Sensor With Background Canceling Lock-in Pixels Based on Lateral Electric Field Charge Modulation

We propose a CMOS image sensor with time-division multiplexing pixel architecture using standard pinned-photodiode for capturing 2-D color image as well as extracting 3-D depth information of a target object. The proposed pixel can alternately provide both color and depth images in each frame. Two split photodiode and four transfer gates in each pixel improve the transfer speed of generated electrons to be capable of demodulating a high-frequency time-of-flight signal. In addition, four-shared pixel architecture acquires a color image with high spatial resolution and generates a reliable depth map by inherent binning operation in charge domain. A 712 496 pixel array has been fabricated using a 0.11μm standard CMOS imaging process and fully characterized. A 6m pixel with 34.5% aperture ratio can be operated at 10-MHz modulation frequency with 70% demodulation contrast. We have successfully captured both images of exactly same scene from the fabricated test chip. It shows a depth uncertainty of less than 60 mm and a linearity error of about 2% between 1 and 3 m distance with 50-ms integration time. Moreover, high-gain readout operation enables to improve the performance, achieving about 43-mm depth uncertainty at 3 m.

A CMOS Image Sensor Based on Unified Pixel Architecture With Time-Division Multiplexing Scheme for Color and Depth Image Acquisition

The invention pertains to a system for determining a distance to an object comprising: a solid-state light source for projecting a pattern of discrete spots of laser light towards the object in a sequence of pulses; a detector comprising a plurality of picture elements, for detecting light representing the pattern of discrete spots as reflected by the object in synchronization with said pulses; and processing means to calculate the distance to the object as a function of exposure values generated by said picture elements. The picture elements are configured to generate exposure values by accumulating, for each pulse of said sequence, a first amount of electrical charge representative of a first amount of light reflected by said object during a first time window and a second electrical charge representative of a second amount of light reflected by said object during a second time window, said second time window occurring after said first time window.

System and method for determining a distance to an object

A range image sensor 1 is provided with a semiconductor substrate 1A having a light incident surface 1BK and a surface 1FT opposite to the light incident surface 1BK, a photogate electrode PG, first and second gate electrodes TX1, TX2, first and second semiconductor regions FD1, FD2, and a third semiconductor region SR1. The photogate electrode PG is provided on the surface 1FT. The first and second gate electrodes TX1, TX2 are provided next to the photogate electrode PG. The first and second semiconductor regions FD1, FD2 accumulate respective charges flowing into regions immediately below the respective gate electrodes TX1, TX2. The third semiconductor region SR1 is located away from the first and second semiconductor regions FD1, FD2 and on the light incident surface 1BK side and has the conductivity type opposite to that of the first and second semiconductor regions FD1, FD2.

Range sensor and range image sensor

The present invention relates to a concept for optical distance measurement, wherein a radiation pulse is emitted in the direction of an object of measurement. At least two different transfer gates which couple a photoactive region to at least two different evaluating capacities are driven during different drive intervals so that charge carriers generated during the drive intervals by a radiation pulse reflected from the object of measurement and/or by ambient radiation can be transported from the photoactive region to the evaluating capacities each coupled to the at least two transfer gates. Another transfer gate is driven during a time outside the drive intervals of the at least two transfer gates to connect the photoactive region to a reference potential terminal acting as a charge carrier sink during the time outside the drive intervals of the at least two transfer gates.

Concept for optical distance measurement

Die vorliegende Erfindung betrifft ein Konzept zur optischen Abstandsmessung, wobei ein Strahlungspuls (808) in Richtung eines Messobjekts (104) ausgesendet wird. Wahrend unterschiedlicher Ansteuerintervalle (810; 814; 816) werden wenigstens zwei unterschiedliche Transfer-Gates (404; TXn), die einen photoaktiven Bereich (402) mit wenigstens zwei unterschiedlichen Auswertekapazitaten (408; FDn) koppeln, angesteuert, so dass wahrend der Ansteuerintervalle (810; 814; 816) durch einen von dem Messobjekt (104) reflektierten Strahlungspuls und/oder durch Hintergrundstrahlung erzeugte Ladungstrager von dem photoaktiven Bereich (402) zu den jeweils mit den wenigstens zwei Transfer-Gates gekoppelten Auswertekapazitaten transportiert werden konnen. Ein weiteres Transfer-Gates (404-4; TX4) wird wahrend einer Zeit (822; 824; 826) auserhalb der Ansteuerintervalle (810; 814; 816) der wenigstens zwei Transfer-Gates angesteuert, um wahrend der Zeit auserhalb der Ansteuerintervalle der wenigstens zwei Transfer-Gates den photoaktiven Bereich (410) mit einem als Ladungstragersenke wirkenden Referenzpotentialanschluss zu verbinden.

Konzept zur optischen Abstandsmessung

This contribution describes the modeling of CMOS image sensors employed in time-of-flight (ToF) sensor systems for 3D ranging applications. Our model relies on the theoretical description of photo-generation, charge transfer including diffusion, fringing field, and self-induced drift (SID). This method makes it possible to calculate the time-dependent charge carrier generation, transfer, and distribution. The employed approach allows elimination not only of irradiance-dependent charge transfer, but also of undesired reflectance effects, and the influence of ambient light through an in-pixel background measurement. Since the sensor is operated with very short integration times it is crucial to accomplish a fast transfer of the generated charge from the photodetector to the sense node, and speedy conversion into an electrical signal at its output. In our case, we employed a lateral drift field photodetector (LDPD), which is basically a pinned photodiode with a built-in drift field formed by a doping gradient. A novel pixel structure is presented which is optimized for a fast charge transfer by the appliance of the shown model. Numerical calculations predict a two times faster charge collection.

Modeling of CMOS image sensors for time-of-flight applications

This chapter describes the functional principles of single-photon counting in silicon achieved through single-photon counting avalanche diode (SPAD) technology. It explains the different readout modes of single SPADs on the one side, and of arrays of SPADs forming silicon photomultipliers (SiPM) with an analog output, SiPMs with a digital output or SPAD arrays that can be read out in form of frames that form SPAD-imagers, on the other. This silicon based technology is compared to traditionally used photomultiplier tube, multichannel plate, or image intensifier technologies in applications requiring near single-photon counting and time responses in nanosecond or sub-nanosecond regions. This chapter discusses the fabrication issues of SPAD devices as well as figures of merit used for their characterization. Finally, the chapter explains how SPAD arrays are used in time-of-flight based ranging and/or 3D imaging applications.

Silicon based single-photon avalanche diode (SPAD) technology for low-light and high-speed applications

In this article a model is introduced that describes the charge transfer in pixels of an image sensor. The model is suitable for image sensors where lateral drift field photo detectors were implemented and considers the effects of thermal diffusion, drift due to the built-in potential gradient, and self-induced drift. The analytical result is compared with a numerical solution and confirmed by measurements. With this model it is possible to predict the amount of collected charge at the sense node for very short integration times in comparatively long pixel structures. This is particularly important for indirect time-of-flight applications with CMOS image sensors. This approach enables the optimization of the pixel layout as well as an advanced calibration that might possibly enhance the distance precision. The model can also be applied to image sensors featuring pinned photodiodes.

Modeling of the charge transfer in a lateral drift field photo detector

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