Abstract:
The present invention discloses an optoelectronic detector for light sensing. The optoelectronic detector has a photosensitive element that converts light into electrons. Efficient collection of these electrons at readout nodes, embedded in the photosensitive element, is required to make correct measurements of light characteristics such as, phase shift and intensity. This collection of electrons is achieved by applying a voltage gradient across an electrode within the optoelectronic detector. The optoelectronic detector can have multiple readout nodes. Further, the present invention discloses methods for detecting intensity and phase shift of impinging light and for suppression of background illumination while detecting the characteristics of light.

Description:
BACKGROUND  
       [0001]     The present invention broadly relates to electronic light sensing devices. More specifically, the present invention relates to an optoelectronic detector for detecting radiation synchronized with pulsed illumination and its use thereof for suppressing background illumination and phase shift calculation.  
         [0002]     Electronic light sensing involves conversion of light into an electronically readable form. Optoelectronic detector has a light sensitive component. Light sensing component creates electron-hole pairs when light impinges on its surface. These electrons are subsequently integrated and readout to understand the characteristics of the light.  
         [0003]     The light sensing devices can detect spatial distribution of impinging radiation from an object as well as gather information related to the distance of an object. Parameters such as intensity and phase of the modulated light reflected from an object measure the distance of a light-reflecting object. Light sensing devices that can measure such parameters are known as Photo mixing devices or gated views.  
         [0004]     Light sensitive component of these Photo mixing devices are integrated in Charge Coupled Devices (CCD) technologies or in standard Complementary Metal Oxide Semiconductor (CMOS) processes. Both these technologies use the concept of photoelectric effect wherein light or photons interact with photosensitive materials, such as crystallized silicon, to create electron-hole pairs in the material.  
         [0005]     CCD is a light-sensitive integrated circuit comprising a single detector cell or a one-dimensional or two-dimensional array of photocells each of which is a capacitor. It stores and displays data relating to an image by converting each picture element (also referred to as pixel) in the image into an electrical charge. Further, CCD based devices can add charges collected over several integration cycles. This leads to a better signal over noise ratio.  
         [0006]     High fabrication cost and limited system integration on chip limits the use of CCD based devices in favor of CMOS based light sensing devices.  
         [0007]     An optoelectronic detector based on CMOS technology comprises a single detector or one-dimensional or two-dimensional arrays of CMOS-based photodiodes/photogates and readout amplifiers.  
         [0008]     Most known standard CMOS process based optoelectronic detectors, convert collected charges to voltage and buffer the voltage for readout. These optoelectronic detectors can implement a synchronous electronic shutter pixel that allows integration of charges corresponding to impinging light during very short integration times. This electronic shutter typically is a sample and hold switch. This switch requires sampling of free charges in the form of voltage, after every integration time. It is possible to add these voltages over multiple integration times to measure the free charges collected over multiple integration times. However, charge to voltage conversion introduces noise during each integration time. This noise gets added to the measured voltage in each integration time. Consequently, large amount of noise gets added leading to low precision in measuring the properties of impinged light.  
         [0009]     Photogate based detector architecture overcomes this drawback of CMOS technology based optoelectronic detectors. In Photogate based optoelectronic detectors, photo generated charges are collected in a depletion region underneath the photogate. This depletion region is built by means of applying a suitable potential gradient to the photogate. This method of charge collection is similar to that in a CCD type detector.  
         [0010]     After integration over a pre-defined time, using photogate architecture, collected charges are transferred to a readout node by means of changing the potential of the photogate. Photogate type devices feature the disadvantage of low charge transport efficiency compared to CCD type detectors.  
         [0011]     Charge transport efficiency is the ratio of the number of electrons read by a sensing arrangement, coupled to the optoelectronic detector, to the number of electrons created in the photosensitive area of the optoelectronic detector. The number of electrons sensed by the sensing arrangement is lower than the number actually produced in the detector because a significant number of electrons are lost in their transport from the detector to the sensing instrument. This leads to poor measurements of light features such as phase separation and intensity. Therefore, quality of images produced using the optoelectronic detectors is not very good and often has low contrast and poor brightness.  
         [0012]     The overall performance of both CCD and CMOS based optoelectronic detectors suffer from background illumination measured along with the actual signal, at the charge sensing devices. The background illumination can saturate the readout channel if its intensity is very high, or can deteriorate the contrast between the charge sensing devices. As a result, the precision for the detection of the phase and the spatial distribution of the impinging radiation is low.  
         [0013]     In the existing CCD and CMOS based optoelectronic detectors, background illumination is suppressed by compressing the energy emitted by a light source in a short pulse and integrating the light only for short pulse&#39;s duration. Compression of energy involves reducing the on time of the light source as compared to the off time of the light source. This is effective for light sources with a low mean power relative to the background illumination but which can deliver pulse intensities above background illumination for short pulse times such as LED or LASER light sources.  
         [0014]     The above method of background suppression requires modification (in this case compression) of the emitted light before the light impinges on the optoelectronic detector. There is no component in the optoelectronic detector that can contribute to reduction of background illumination. Thus, the amount of background illumination that is suppressed is limited, leading to imprecision in the detection of the properties of impinged light. Imprecision occurs as the effect of background illumination is also recorded with the properties of light.  
         [0015]     Several patents disclose the use of abovementioned technologies for light sensing, some of which are explained hereinafter.  
         [0016]     U.S. Pat. No. 5,856,667 titled “Apparatus and method for detection and demodulation of an intensity modulated radiation field”, assigned to Leica A G, Heerbrugg, Switzerland discloses a CCD based image sensor with multiple image sensing elements. For sensing the light coming from an image, the system switches between the various image-sensing elements. Switching enables recording different parameters of the light at the different image sensing elements.  
         [0017]     PCT patent application no. 98/10255 titled “Method and device for determining the phase and/or amplitude data of an electromagnetic wave”, discloses a CCD based device for the calculation of phase and amplitude of an electromagnetic wave. The device collects electrons with the help of staircase shaped depletion region.  
         [0018]     Although the abovementioned patents disclose light sensing devices, they suffer from one or more of the disadvantages cited earlier, i.e., low transport efficiency, background illumination and inaccurate measurement of light parameters.  
         [0019]     Keeping the above discussion in consideration, there is a need for an invention that features the possibility of adding charges created during several integration times. Further, there is a need for an invention that can correctly detect the phase of impinging modulated radiation with respect to a reference phase by reducing background illumination of the impinging light. Also, there is a need for an invention that has a high transport efficiency and therefore, provides an increase in precision to detect the phase shift of the pulsed impinging radiation over other devices produced using standard CMOS technology.  
       SUMMARY  
       [0020]     An objective of the present invention is to provide an optoelectronic detector with multiple readout nodes that can efficiently readout electrons generated by light in a photosensitive material.  
         [0021]     Another objective of the present invention is to accurately measure characteristics of light such as phase and intensity.  
         [0022]     Yet another objective of the present invention is to suppress the background illumination associated with modulated light.  
         [0023]     The invention discloses architecture of an optoelectronic device comprising a photosensitive substrate with several readout nodes embedded in it. The photosensitive substrate and the readout nodes are oppositely doped, i.e., if the photosensitive substrate is n-type then the readout nodes are p-type and vice-versa. The substrate is covered with a dielectric layer, which in turn is covered with a layer of electrode. Several electrical contact sites are metallized to the electrode. Voltage is applied across the electrode through the electrical contact sites. Each readout node is connected to a charge-sensing device to read the charges integrated at the readout nodes.  
         [0024]     The voltage gradient across the two electrical contact sites creates a potential gradient between suitably doped readout nodes. The voltage gradient applied is continuous and therefore, the potential gradient created by it, in the photosensitive substrate, is continuous. Light is impinged on the photosensitive substrate and consequently, free electrons are generated in the region between the readout nodes. The potential gradient forces these free electrons to collect at the readout node with higher potential. Continuity in the potential gradient between the readout nodes improves the efficiency in collection of electrons or the charge transport efficiency. The collected electrons are sensed using voltage or current measuring devices. For suppression of background illumination one of the readout nodes in the architecture is continuously drained out. The measured voltage and current values are used to calculate parameters such as intensity and phase shift. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote like elements, and in which:  
         [0026]      FIG. 1  illustrates a block diagram of the environment in which the present invention operates;  
         [0027]      FIG. 2  illustrates a cross-sectional view of the architecture of an optoelectronic detector in accordance with preferred embodiment of the invention;  
         [0028]      FIG. 3  illustrates an example circuit for readout circuits used in conjunction with the optoelectronic detector;  
         [0029]      FIG. 4  illustrates an alternative example circuit for readout circuits used in conjunction with the optoelectronic detector;  
         [0030]      FIG. 5  is a flowchart illustrating the application of optoelectronic detector as a photomixing device;  
         [0031]      FIG. 6  illustrates a top-view of an optoelectronic detector with three readout nodes that suppresses background illumination; and  
         [0032]      FIG. 7  is a flowchart illustrating the method for suppression of background illumination. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]     The present invention illustrates an optoelectronic detector, with high charge transport efficiency, for sensing light. These optoelectronic detectors are used as single detectors or as an individual unit of an array of such detectors for sensing light. Further, the present invention presents an optoelectronic detector to detect phase shift of modulated light and to suppress background illumination from the emitted light. The detector has a photosensitive material in which electron and hole pairs are created when light impinges on the substrate. The number of electrons generated depends on the characteristics of light. These characteristics can be amplitude, frequency and phase shift of the light. To read the information conveyed by these electrons, they are collected at specific locations in the detector. Electrons are collected at these specific locations by applying a voltage gradient across an electrode. The collected electrons can be read through voltage or current sensing detectors.  
         [0034]      FIG. 1  illustrates a block diagram of the environment in which the present invention operates. Light source  101  is a source of light such as lasers, LED and the like. Optoelectronic detector  103 , in accordance with the present invention, is exposed to light source  101 . These light rays that fall on optoelectronic detector  103  convert light into free electrons. These free electrons are sensed by charge sensing devices  105  that are linked to optoelectronic detector  103 .  
         [0035]      FIG. 2  illustrates a cross-sectional view of the architecture of an optoelectronic detector in accordance with a preferred embodiment of the invention. Optoelectronic detector  103  has a doped photosensitive substrate  201 . Photosensitive substrate  201  is a doped semiconductor material such as Si, αSi, SiGe, GaAs and InPh. Photosensitive substrate  201  produces electron-hole pairs when light falls on it. Photosensitive substrate  201  is doped to become either n-type or p-type material. If substrate  201  is n-type, the major charge carriers are electrons. If substrate  201  is p-type, the major charge carriers are holes. Photosensitive substrate  201  has two oppositely doped regions, referred to as readout nodes  203  and  205 , embedded on its surface. For example, in case photosensitive substrate  201  is p-type, readout nodes  203  and  205  are n-type and vice-versa. Although only two readout nodes have been shown in  FIG. 2 , it must be apparent to one skilled in the art that optoelectronic detector  103  can have any number of readout nodes. Photosensitive substrate  201  also has an electrical contact site  207  metallized on its surface to maintain photosensitive substrate  201  at a constant potential.  
         [0036]     Dielectric layer  209  on top of photosensitive substrate  201  is a material with negligible electrical and thermal conductivity such as SiO 2 , SiN x , glass or porcelain. Dielectric  209  is transparent to the impinging light (e.g. IR, X-Ray, UV, Visible) from light source  101 . Dielectric  209  layer prevents any electrical contact between photosensitive substrate  201  below it and electrode  211  layer above it.  
         [0037]     Electrode  211  is transparent to impinging radiation (e.g. IR, X-Ray, UV, Visible) and is a conductive material. Voltage gradient between two points on the surface of electrode  211  causes flow of current across it. To apply the voltage gradient, two electrical contact sites  213  and  215  are metallized on the surface of electrode  211 . Current generated by the applied voltage gradient must be low to reduce power consumption of the detector. Using a high resistivity material such as polysilicon, indium titane oxide and the like for electrode  211  ensures generation of a small current. A continuous voltage gradient is maintained across contact sites  213  and  215  by applying different voltages on contact sites  213  and  215 . This voltage gradient creates a potential gradient between readout nodes  203  and  205 . The details regarding the creation of potential gradient is provided later in the description. A switching unit  217  controls voltage gradient across contact sites  213  and  215 .  
         [0038]     Switching unit  217  varies the voltage gradient according to a switching sequence. Switching sequence defines variation in voltages at the electrical contact sites in a switching cycle and depends on the characteristics (e.g. modulation frequency) of incoming light. This switching sequence, implemented by switching unit  217 , is synchronized to the modulation frequency of light emitted by light source  101 . For example, if the light has a frequency of 10 Mhz (time period ˜100 ns), in the first 50 ns cycle a positive voltage gradient is applied between electrical contact sites  213  and  215  and during the next 50 ns cycle a negative voltage gradient is applied between them.  
         [0039]     When the light impinges on photosensitive substrate  201 , after passing through transparent dielectric  209  and electrode  211  layers, it generates free electrons in the region between readout nodes  203  and  205 . Light can also impinge directly on photosensitive substrate  201 . This process of impingement directly on the surface of photosensitive substrate  201  is called back-illumination. Back illumination is useful in cases where electrode  209  and dielectric layer  211  are not transparent to impinging light.  
         [0040]     Voltage gradient across contact sites  213  and  215  generates a potential gradient between readout nodes  203  and  205  forcing the free electrons towards one of the readout nodes. These free electrons contain information about the light such as amplitude, intensity and phase shift. Free electrons collected at readout nodes  203  and  205  are sensed using charge-sensing devices  219  and  221 , respectively. Charge sensing device  219  corresponds to readout node  203  and charge-sensing device  221  corresponds to readout node  205 . Charge sensing devices  219  and  221  can be voltage sensing or current sensing devices, i.e., the free electrons can be converted into an equivalent voltage or an equivalent current by the charge sensing devices. An example of a charge-sensing device is described later in conjunction with  FIG. 3 .  
         [0041]     The collection of free electrons at a readout node requires generation of a depletion region between the readout nodes. Suppose, for p-type substrate  201 , a continuous voltage gradient is applied across two contact sites  213  and  215  such that contact site  215  is at a higher voltage than contact site  213 . This voltage gradient generates a region deficient in p-type carriers (holes) between readout nodes  203  and  205 . This p-type deficient region is called the depletion region. Applied voltage is so selected that the depletion region extends over as large a surface as possible between readout nodes  203  and  205 . However, the voltages are chosen such as to prevent a direct connection (by means of building a continuous channel) between the readout nodes  205  and  203 . The applied voltage gradient also generates a continuous potential across the depletion region such that the higher potential is towards readout node  205 .  
         [0042]     For p-type substrate  201 , the free electrons generated upon impingement of light collect at readout node  205  since readout node  205  is at a higher potential. Similarly, if the voltage gradient is reversed i.e. electrical contact site  213  has a higher voltage than contact site  215 , the free electrons collect at readout node  203 .  
         [0043]     Time for which the free electrons are collected at a readout node is called integration time. For the example switching sequence, described already, integration time for both readout nodes  203  and  205  would be 50 ns. For optoelectronic detectors  103  with very short integration times, one of readout nodes  203  and  205  is drained constantly to prevent electron accumulation in optoelectronic detector  103 . Connecting it to a reset potential, described later in description, drains the readout node.  
         [0044]     In the present invention, the steering of free electrons to one of readout nodes  203  and  205  can be repeated over several cycles wherein each cycle consists of a switching sequence. In particular, for detectors with very small integration times, charge sensing devices  219  and  221  sense electrons collected at readout nodes  203  and  205  accumulated over multiple switching cycles. Several switching cycles are used to collect sufficient electrons, which are more than the free electrons created by background illumination, at the readout nodes. The number of cycles needed to collect sufficient electrons reduces with increase in transport efficiency of optoelectronic detector  103 . Charge sensing devices  219  and  221  sense the charges collected on readout nodes  203  and  205  in the form of current or voltage.  
         [0045]      FIG. 3  illustrates an example circuit for sensing the charges stored at readout nodes used in conjunction with the optoelectronic detector. This is a standard circuit for reading free electrons in the form of voltage. The example circuit for charge sensing device  219  comprises an amplifier  301  with a positive input node  303 , negative input node  305  and an output node  311 . Negative input node  305  is connected to one of readout node  203  while positive input node  303  is set at a voltage, V reset , the reset voltage that acts as a reference voltage. Negative input node  305  is also connected to output node  311  through a switch  307  and a feedback capacitor  309  with capacitance C f . Switch  307  and capacitor  309  are connected in parallel between negative input point  305  and output point  311 . To sense the charges from a readout node, capacitor  309  is initially discharged by closing switch  307 . The charges stored on the connected readout node Q read  are then stored on capacitor  309  by opening switch  307 . Stored charges develop a potential difference across capacitor  309  given by the following equation: 
 Voltage drop= Q   read   /C   f .  
         [0046]     The voltage at output node  311 , V out  gives the voltage corresponding to the free electrons created by the light, wherein V out  is defined as V out =V reset +Q read /C f .  
         [0047]      FIG. 4  is an alternative example circuit for readout circuits used in conjunction with the optoelectronic detector used for sensing the collected free charges at readout nodes  203  and  205 . The example circuit for charge sensing device  219  comprises a switch  401 , a parasitic capacitor  403  with capacitance C par , and a junction diode  405 . A node  407  and a node  409  are set at reset voltage V reset , Initially, the circuit is reset to set the readout node to voltage V reset  by closing switch  401 . After the reset, switch  401  is opened and the charges collected on the readout node are discharged into parasitic capacitor  403 . This charge is then sensed in the form of a voltage difference on readout node  203  after a certain number of switching cycles.  
         [0048]     The charge sensing circuits described using  FIG. 3  and  FIG. 4  can also drain a readout node of all the free charges collected at it without sensing it in the form of voltage. In case the circuit of  FIG. 3  is used, closing reset switch  307  drains the readout node. However, if the circuit of  FIG. 4  is used, then closing reset switch  401  and thereby connecting to the reset potential, V reset , achieves draining of the readout node.  
         [0049]     Optoelectronic detector  103  has various optoelectronic applications. In particular, optoelectronic detector  103  can act as a photomixing device to measure properties of the light such as intensity, phase shift and amplitude of the light.  
         [0050]      FIG. 5  is a flowchart illustrating the application of optoelectronic detector as a photomixing device. In particular, optoelectronic detector  103  can measure the phase shift of the light. For example, consider optoelectronic detector  103  in  FIG. 1 , with two readout nodes wherein charge-sensing devices  219  and  221 , respectively measure voltages V 1  and V 2  at the end of one switching sequence cycle. At step  501 , a two step switching sequence is determined. Let the switching sequence be the same as illustrated already for a light with frequency 10 MHz earlier in the description. At step  503 , the readout node at which the potential is higher in the first step of switching cycle, is determined. Suppose, a high voltage is applied to electrical contact site  215  and lower voltage is applied to electrical contact site  213  for the first 50 ns, i.e., the first step of switching cycle. In the next 50 ns, electrical contact site  213  is kept at a high voltage and electrical contact site  215  is kept at a lower voltage. This implies that at the first step of switching cycle readout node  205  will have a higher potential.  
         [0051]     At step  505 , light source  101  that impinges modulated light on photosensitive substrate  201 , is activated. Impingement of light on photosensitive substrate  201  generates free electron-hole pairs. At step  507 , potential gradient is generated between readout nodes  203  and  205  in accordance with the first step of the switching sequence. The free electrons generated during step  507  are steered to readout node  203 . At step  509 , potential gradient is generated between readout nodes  203  and  205 , according to the second step of switching sequence. The free electrons generated during step  509  are steered to readout node  205 . The switching sequence carried out by steps  507  and  509  can be repeated several times to collect sufficient amount of charges at the respective readout nodes  203  and  205 . The sufficiency of the amount of charges is determined by the desired signal to noise ratio for a particular application of optoelectronic detector.  
         [0052]     For example, suppose an application desires a signal to noise ratio value as 5:1. Let the pulse duration of one pulse be 50 ns. The switching sequence for the detector is set so that the integration time for read out node  1  is 25 ns and read out node  2  is 25 ns. Therefore, the period of the entire switching cycle is 50 ns. If the photon to electron conversion rate of an optoelectronic detector is 50%, then for two impinging photons one electron is detected. Thus, if one pulse of the illumination creates an average of two photons on the photosensitive substrate, then according to the conversion rate one electron is created in the photosensitive substrate in 50 ns. Due to the switching sequence, during the first 25 ns of a light pulse, 0.5 electron is created at readout node  1  and during the next 25 ns of the light pulse, 0.5 electron is created at readout node  2 . Let the background illumination during the 50 ns of light pulse create 0.1 electron. In light of the switching sequence, background illumination creates 0.05 electrons for readout node  1  in the first 25 ns and 0.05 electrons for readout node  2  in the next 25 ns.  
         [0053]     Suppose, the noise created due to conversion of electrons into voltage be 100 electrons. In light of the above given specifications, the number of times the switching sequence needs to be repeated for 2000 cycles to achieve a signal to noise ratio of 5:1.  
         [0054]     At step  511 , the sufficiency of the amount of charges or the number of cycles corresponding to the collection of sufficient amount of charges at the readout nodes is checked. If the number of cycles required is not complete, then step  507  and  509  are repeated, else at step  513 , charge-sensing devices  219  and  221  read the free electrons collected at readout nodes  203  and  205 . At step  515 , voltages measured by charge sensing devices  219  and  221  are utilized to compute phase shift of the light according to the following equations: 
 
Phase shift=( V 1 −V 2)/( V 1 +V 2)   Equation 1 
 
         [0055]     The measured voltages can also be used to compute the intensity of light as follows: 
 
Intensity= V 1 +V 2   Equation 2 
 
         [0056]     The optoelectronic detector as described in  FIG. 2  can also suppress the background illumination associated with modulated light if one of the readout nodes is constantly drained and the other readout node used to collect free charges.  
         [0057]      FIG. 6  illustrates a top-view of an optoelectronic detector with three readout nodes, one of the applications of which is to suppress background illumination. Doped photosensitive substrate  201  forms the substrate of optoelectronic detector  103 . Readout nodes  203 ,  205  and  601  are embedded in photosensitive substrate  201 . Electrode  211  forms the top most layer of detector  103 . Between electrode  211  and photosensitive substrate  201  lies a layer of dielectric material to prevent flow of electrons between the two layers. Charge sensing devices  603 ,  219  and  221  are connected to readout nodes  601 ,  203  and  205  respectively. Electrical contact sites  605 ,  213 ,  215  and  207  allow application of voltage to various regions within optoelectronic detector  103 . Each electrical contact site corresponds to only one readout node, the number of electrical contact sites and readout nodes being the same. Switching unit  217  connects to electrical contact sites  605 ,  213  and  215 . Switching unit  217  changes the voltages applied to electrical contact sites  605 ,  213  and  215  depending on a predefined switching sequence.  
         [0058]      FIG. 7  is a flowchart illustrating the method for suppression of background illumination. The set up in  FIG. 6  with three readout nodes  601 ,  203  and  205  can suppress background illumination in light. First step  701  is determination of switching sequence for the method. The switching sequence is decided keeping into consideration the periods in which light is present along with background illumination and another period in which only background illumination is present. At step  703 , the readout node at which electrons corresponding to background illumination is identified. Suppose, the duration of light pulse generated by a source is 50 ns and the delay between two pulses is 1 μs. Switching unit  217  is set so that electrical contact site  213  is at a high voltage for the first 50 ns and electrical contact site  215  is at a high voltage for the next 50 ns. Finally, electrical contact site  605  is at a high voltage until a new light pulse is emitted by light source  101 . Therefore, electrical contact sites  213  and  215  are at high voltages during the periods when light is present along with the background illumination. On the other hand, electrical contact site  605  is at a high voltage when only background illumination is present.  
         [0059]     At step  705 , light source  101  is activated to impinge light on photosensitive substrate  201 . At step  707 , the potential gradients at electrical contact sites  213 ,  215  and  605  are applied according to the switching sequence. Readout node  601 , corresponding to electrical contact site  605 , collects free electrons generated by background illumination when the potential at readout node  601  is highest according to the switching sequence. These free electrons collected at readout node  601  are constantly drained at step  709 .  
         [0060]     While one of the contact sites is at a high voltage, the other two contact sites are at ground voltage. The switching sequence as already described is repeated until sufficient electrons are collected at readout nodes  203  and  205 .  
         [0061]     After completion of sufficient number of switching cycles, charge sensing devices  219  and  221  sense charges collected at the respective readout nodes  203  and  205 . The free electrons collected at readout node  601  are drained out by connecting charge sensing device  603  to reset potential. The period during which electrons collect at draining readout node  601  is longer than the period during which electrons collect at nodes  203  and  205 . Longer period for draining achieves significant suppression of the background illumination. The drained out charges correspond to the portion of light that contribute to the background illumination. The electrons collected at readout nodes  203  and  205  are subsequently used to rebuild the light signal with significantly reduced background illumination.  
         [0062]     Although the method, in  FIG. 7 , for reducing background illumination is illustrated using optoelectronic detector  103 , the method can also be implemented using any optoelectronic device that has a photosensitive substrate, a plurality of readout nodes and a means for steering free electrons (e.g. an electrical contact site) to one of the readout nodes.  
         [0063]     Although optoelectronic detectors with two and three readout nodes have been described above, it should be clear to anyone ordinarily skilled in the art that the optoelectronic detectors can have any number of nodes. In particular, the optoelectronic detectors with at least two readout nodes (not necessarily three readout nodes) can suppress background illumination by draining the charges collected over several switching cycles at one of the readout nodes. Further, photosensitive substrate  201  can be either p-type or n-type and correspondingly the readout nodes can be n-type or p-type respectively. In addition, dielectric  209  and electrode  211  need not be transparent. Further, optoelectronic detector  103  can detect light of any frequency.  
         [0064]     The current invention has many advantages. First, the optoelectronic detector disclosed in the detailed description has high transport efficiency. Higher transport efficiency results from the continuous potential gradient across the depletion region. Therefore, the detector measures characteristics of light such as phase shift and intensity with greater accuracy.  
         [0065]     Another advantage of the disclosed optoelectronic detector is that it can extend to any number of readout nodes by making changes to geometry of the detector. This extension to greater number of readout nodes further improves the accuracy of the optoelectronic detector. For example, multiple readout nodes can suppress background illumination when measuring phase shift and intensity of modulated light. Further, multiple readout nodes can more accurately measure higher frequency components of a light with greater accuracy or to extend the range over which phase shifts is measured.  
         [0066]     Another advantage of the invention is that charges can accumulate over several cycles of switching sequence at the readout nodes. In particular, if the integration time is very low, charges are collected at readout nodes for several cycles before being measured by the charge sensing devices. This reduces the effect of charge to voltage conversion noise in the measurement of the properties of the light. Consequently, the optoelectronic detector measures the properties of light with very high frequency, i.e., very low integration times, with greater accurately.  
         [0067]     Yet another advantage of the invention is that switching between the different contact points for applying high voltage need not continue in either anti-clockwise or clockwise direction. The switching sequence can follow any direction between the electrical contact sites. Therefore, motion of the electrons can be steered in any direction, at any point in time.  
         [0068]     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.