The present invention relates to a concept for optical distance measurement, as may exemplarily be used in so-called ToF (Time of Flight) three-dimensional imaging. 
There are various applications in which optical radiation is to be detected.  Cheap CMOS (Complementary Metal-Oxide Semiconductor) image sensors and camera systems have become standard elements of everyday life.  In particular in the consumer field, digital and cell phone cameras have developed to become a mass product and it is hard to imagine these not to be there.  In the field of special applications as well, cheap CMOS cameras are becoming more and more interesting compared to high-quality but considerably more expensive CCD (Charge-Coupled Device) image sensors.  In particular realizing a photodetector and post-processing read-out electronics on a chip in accordance with the so-called “camera-on-a-chip” approach is one big advantage of CMOS image sensorics compared to CCD technology. 
One special field of CMOS image sensorics relates to contactless distance measurement based on the so-called time-of-flight (ToF) principle.  Here, active illumination in the form of pulsed or continuously modulated optical radiation is used to determine a distance of the sensor to an object of measurement directly or indirectly using the runtime of the optical radiation reflected.  Depending on the field of application, high performance requirements may result for such a ToF sensor.  Intelligent distance-measuring systems in automobiles, for example, have to operate at very high speeds so as to be able to provide real-time measurement data.  At the same time, the sensor must be able to work perfectly under greatly varying environmental influences, such as, for example, fog, darkness, bad weather conditions or extreme backlight situations.  These requirements can be fulfilled using highly dynamic CMOS image sensors and efficient read-out methods for ambient light suppression. 
Apart from the usage as a distance sensor in automobiles, there are other fields of application for ToF sensors, such as, for example, three-dimensional inspection/positioning systems or automotive systems.  Monitoring an automobile's interior, airbag control, theft protection systems, lane recognition systems, pre-crash sensorics, pedestrian protection and parking aids are potential fields of application for this.  Additionally, ToF sensors may be used for topography measurements, for monitoring systems in the security field, for imaging systems for medical technology, in consumer electronics (such as, for example, gaming consoles) or for functional machine security. 
Various methods of ToF-based distance or depth measurement have evolved over the last few years.  FIG.  1 shows a setup of a measuring arrangement 100 which, however, is identical for all ToF systems, in a schematic illustration. 
A modulated radiation or light source 102 here illuminates an object of measurement 104 in a space with an illumination intensity Elight,source.  After being reflected at the object of measurement 104, a light beam or light pulse hits a CMOS image sensor 106 after passing a distance 2d.  d represents a distance between the measurement system and the object of measurement 104.  The light impinging on the image sensor 106 consists of, on the one hand, a reflected light portion Elight,source,r and, on the other hand, ambient light Eamb.  A runtime delay Td of the light emitted can be determined by a synchronization of the light source 102 and the image sensor 106, which, according to:
                    d        =                              c            2                    ⁢                      T            d                                              (        1        )            is directly proportional to the distance d between the sensor 106 and the object of measurement 104.  In equation (1), represents the speed of light. 
Typically, the image sensor 106 is implemented as a semiconductor photo sensor and image converter, which is also referred to as a so-called active pixel sensor (APS) and comprises active circuit elements associated to each pixel, in particular semiconductor image converters employing photodiodes and/or photogates including so-called Correlated Double Sampling (CDS).  Active pixel sensors are semiconductor image converters in which each pixel contains typical semiconductor pixel elements, among other things photo-sensitive regions, resetting means, means for converting charge to voltage and, additionally, all parts of an amplifier.  The photo charge generated in a pixel by illumination is converted to a corresponding voltage or a corresponding current. 
Known ToF pixel structures which may be employed and/or modified in connection with the present invention will be described below using FIGS.  2 and 3. 
FIG.  2 shows a pixel structure 200 based on a pinned photodiode in a schematic cross-section. 
The ToF pixel structure 200 is formed in a p−-epi layer or lightly doped epitaxial p-type layer 202 which is arranged on a p+-type or heavily doped p-type substrate 204.  As can be seen, the pinned photodiode 206 is formed of an n-doped well 208 in the layer 202 at the exposed side of which in turn a p+-doped region 210 is arranged.  The p+-doped region 210 has the effect that a potential well of the photodiode 206 is removed from the surface and that dark currents, for example, can be minimized.  The p+-doped region 210 is connected to ground for this.  A space charge zone forms at the edges between the n-doped well 208, the p+-doped region 210 and the p−-epi layer 202, the n-well being depleted of free charge carriers.  The reference numeral 218 characterizes a parasitic capacity the spectral sensitivity of which is negligible.  Two more n+-doped regions 220 and 222 are formed at the exposed surface of the layer 202, the first one of these forming a transistor or MOS (metal-oxide semiconductor) transistor in connection with the n-well 210, which will subsequently also be referred to as transfer switch or transfer gate TG, and comprising a gate electrode 224 extending above the n-well 210 and the n+-type region 220 and therebetween and being separated from the n-well 210 and the n+-type region 220 by a silicon dioxide layer 226.  Similarly, a layer arrangement of a silicon dioxide layer 228 and a gate electrode 230 extends between the n+-type region 220 and the n+-type region 222 so as to form a transistor or MOS transistor functioning as a reset switch.  As is shown in FIG.  2, the pn junction between the n+-type region 220 and the p−-type layer 202 forms a read-out or evaluating capacity CFD which is also referred to as a floating-diffusion capacity.  The transfer gate TG and the reset transistor can be driven by a control circuit (not shown) such that, caused by an illumination of a photoactive region formed by the pinned photodiode, charge carriers flow from the photoactive region via the transfer gate to the evaluating capacity CFD and can, from there be processed in order to perform distance measurement based thereon.  The transfer gate consequently is a switch which connects the photoactive region formed by the photodiode 214 to the evaluating capacity CFD formed by the junction between the n+-type region 220 and the p−-type layer 202, when the switch is closed, i.e. when the gate electrode 224 is driven correspondingly. 
Another exemplary ToF pixel structure 300 based on a photogate is shown in FIG.  3 in cross-section. 
Like the pixel structure 200 in accordance with FIG.  2, the pixel structure 300 in accordance with FIG.  3, too, is formed on a lightly-doped epitaxial p-type layer 302 which is arranged on a heavily doped p-type substrate 304.  Only exemplarily, a layer 306 referred to as a p-well, in which different components of the ToF pixel structure 300 are arranged, is arranged above the p−-type layer 302, as will be described below. 
Two so-called n+-type floating-diffusion regions 310 and 312 (diffusion regions) are formed between the p-well 306 and a silicon dioxide layer 308 arranged thereon.  These floating-diffusion regions are of a capacitive nature, i.e. are able to store charge carriers, which is indicated by CFD.  The pixel element 300 can be controlled using three electrodes or gates 314, 316 and 318 which are typically made of polycrystalline silicon (polysilicon) and arranged above the silicon dioxide layer 308.  Conventional STI (Shallow Trench Isolation) structures 320 are arranged laterally outside the n+-type floating-diffusion regions 310 and 312 so as to avoid leakage currents to a neighboring pixel element (not shown). 
The pixel structure 300 functions as a photodetector so as to generate electron-hole pairs, depending on the penetration depth of the incident photons, in the crystalline structure of the silicon semiconductor material by photons reflected by an object of measurement 104 and entering the pixel structure 300.  The structure 300 is generally referred to as a photogate photodetector since an electrical field starting from the photogate 314 results in a depletion zone 322 in the underlying p-well 306, so that charge carriers will accumulate here caused by the action of light.  The depletion zone 322 starts at the surface of the p-well 306 and extends in the direction of the underlying p+-type substrate 304.  The depth of the depletion zone 322 depends on a thickness of the silicon dioxide layer 308, a voltage applied to the photogate 314, the p-well doping and substrate doping.  When the transfer gates 316, 318 are driven, a flow of charge carriers is enabled between the depletion zone 322, i.e. the photoactive region, and the respective floating-diffusion region 310, 312.  The charge carriers transferred in turn cause, at the capacities CFD formed by the floating-diffusion regions, voltage drops which in turn are directly proportional to the charge transferred and can be measured.  When the transfer gates 316, 318 are driven at suitable points in time and the resulting charges or voltages are measured, the beginning and end of a radiation pulse reflected by the object of measurement 104 can be determined.  This allows drawing conclusions as to the distance d to the object of measurement 104. 
However, superpositioning useful signals Elight,source,r and ambient light Eamb makes exact determination of the runtime delay Td and thus the precision of the distance measurement extremely difficult.  Different methods have established themselves in order to efficiently suppress ambient light.  DE 198 33 207 and U.S. Pat.  No.  7,186,965 each describe a read-out method operating using pulsed laser sources.  By double exposure of a 3D scene (once with a laser and again without a laser), the ambient light is determined and eliminated by subsequent subtraction in this method.  Additionally, two electronic “shutter” time windows of different lengths are employed so as to take the reflectance of the object of measurement and the attenuation caused by the distance of the laser pulse reflected into consideration.  All in all, two corresponding laser pulse cycles are needed for determining a distance, which, on the one hand, limits the speed of the method and, on the other hand, increases the laser energy needed.  Additionally, the temporal offset between the two laser pulse cycles may result in errors in the distance measurement when the sensor or the object move. 
In accordance with the patent application PCT/EP2009/002570, an optimized method including an electronic double “shutter” consisting of two sample-and-hold stages is presented, which allows determining the measured values which depend on the distance and the reflectivity values using only one laser pulse.  Both methods employ a standard pn photodiode and at the same time use the barrier layer capacity of the pn junction as an integration and read-out node.  Other ToF methods, such as, for example, in US 2007/0158770 A1 and U.S. Pat.  No.  7,436,496 B2, however, use pixel structures based on photogates or pinned photodiodes including at least two transfer gates in which the photoactive region is separated from the read-out node, i.e. the floating diffusion.  By suitably synchronizing the laser pulse and driving the two transfer gates, the pixel structure is operated as a kind of “charge swing” using which the time delay Td, of the reflected laser pulse and, thus, the distance d can be determined. 
Efficient ambient light suppression, however, is not described in this method. 
Similar photogate structures are presented in U.S. Pat.  No.  6,825,455 or U.S. Pat.  No.  7,060,957.  In contrast to the pulsed ToF methods, however, a continuously modulated sinusoidal light source is used here.  The distance information here may be reconstructed using the phase shift of the light reflected back.  Due to the continuous illumination of the space, however, problems with keeping security aspects for humans may result when using this method.  Only decreasing the illumination intensity can be used as a remedy, which in turn results in problems for ambient light suppression.  Additionally, the scanning algorithm proves to be susceptible for ambiguities when determining the phase position.