Patent Publication Number: US-10770504-B2

Title: Wide spectrum optical sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of and claims the benefit to U.S. patent application Ser. No. 15/803,591, filed Nov. 3, 2017, which is a continuation of U.S. patent application Ser. No. 15/248,618, filed Aug. 26, 2016, which claims the benefit of U.S. Provisional Patent Application No. 62/210,946, filed Aug. 27, 2015, U.S. Provisional Patent Application No. 62/210,991, filed Aug. 28, 2015, U.S. Provisional Patent Application No. 62/211,004, filed Aug. 28, 2015, U.S. Provisional Patent Application No. 62/216,344, filed Sep. 9, 2015, U.S. Provisional Patent Application No. 62/217,031, filed Sep. 11, 2015, U.S. Provisional Patent Application No. 62/251,691, filed Nov. 6, 2015, and U.S. Provisional Patent Application No. 62/271,386, filed Dec. 28, 2015, which are incorporated by reference herein. 
    
    
     BACKGROUND 
     This specification relates to detecting light using a semiconductor based light absorption apparatus. 
     Light propagates in free space or an optical medium is coupled to a semiconductor based light absorption apparatus that converts an optical signal to an electrical signal for processing. 
     SUMMARY 
     A semiconductor based light absorption apparatus, such as a photodiode, may be used to detect optical signals and convert the optical signals to electrical signals that may be further processed by another circuitry. Such light absorption optical sensor may be used in consumer electronics products, image sensors, data communications, time-of-flight (TOF) applications, medical devices, security/surveillance, and many other suitable applications. Conventionally, silicon is used as an image sensor material, but silicon has a low optical absorption efficiency for wavelengths in the near-infrared (NIR) spectrum or longer. Other materials and/or material alloys such as germanium, germanium-silicon, or germanium-tin may be used as image sensor materials with innovative optical device structure design described in this specification. According to one innovative aspect of the subject matter described in this specification, a photodiode is formed using materials such as germanium or germanium-silicon to increase the speed and/or the sensitivity and/or the dynamic range and/or the operating wavelength range of the device. In one embodiment, photodiodes formed using germanium or germanium-silicon and photodiodes formed using silicon may be integrated on a common substrate to yield a photodiode array having a greater operating wavelength range. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in an optical that includes a semiconductor substrate; a first light absorption region formed in the semiconductor substrate, the first light absorption region configured to absorb photons at a first wavelength range and to generate photo-carriers from the absorbed photons; a second light absorption region on the first light absorption region, the second light absorption region configured to absorb photons at a second wavelength range and to generate photo-carriers from the absorbed photons; and a sensor control signal coupled to the second light absorption region, the sensor control signal configured to provide at least a first control level and a second control level. At the first control level, an energy band difference at an interface of the first light absorption region and the second light absorption region is above a threshold for blocking one specific polarity of the photo-carriers generated by the second light absorption region from entering the first light absorption region. At the second control level, the energy band difference at the interface of the first light absorption region and the second light absorption region is below the threshold for blocking the one specific polarity of the photo-carriers generated by the second light absorption region from entering the first light absorption region. 
     This and other implementations can each optionally include one or more of the following features. The first light absorption region may include an n-doped silicon region; and a p-doped silicon region on the n-doped silicon region. The second light absorption region may include an intrinsic region including germanium on the p-doped silicon region of the first light absorption region; and a p-doped region including germanium on the intrinsic region. The optical sensor may include an n-doped readout region coupled to a readout circuit; and a gate coupled to a gate control signal, the gate configured to control a carrier transit between the first light absorption region and the n-doped readout region. The second light absorption region may include a mesa including germanium. The second light absorption region may include a film including germanium. 
     Another innovative aspect of the subject matter described in this specification can be embodied in an optical sensor that includes a first diode formed using a first material, the first diode comprising an n-doped region and a p-doped region; a NMOS transistor that includes a source region coupled to the n-doped region of the first diode; a gate region coupled to a NMOS gate control signal; and a drain region; a second diode formed using a second material, the second diode including an n-doped region coupled to a first bias signal; and a p-doped region; and a PMOS transistor including a source region coupled to the p-doped region of the first diode and the p-doped region of the second diode; a gate region coupled to a PMOS gate control signal; and a drain region. 
     This and other implementations can each optionally include one or more of the following features. The drain region of the NMOS transistor may be coupled to a first readout circuit. The drain region of the PMOS transistor may be coupled to a second bias source, such that (i) the first readout circuit collects, stores, and processes electrons generated by the first diode, (ii) the drain region of the PMOS transistor transfers holes generated by the first diode to the second bias source, and (iii) the drain region of the PMOS transistor transfers holes generated by the second diode to the second bias source. The first diode may be configured to absorb light at visible wavelengths to generate electrons and holes. 
     The drain region of the PMOS transistor may be coupled to a second readout circuit. The drain region of the NMOS transistor may be coupled to a third bias source, such that (i) the drain region of the NMOS transistor transfers electrons generated by the first diode to the third bias source, (ii) the second readout circuit collects, stores, and processes holes generated by the first diode, and (iii) the second readout circuit collects, stores, and processes holes generated by the second diode. The second diode may be configured to absorb light at near-infrared or infrared wavelengths to generate electrons and holes. 
     The optical sensor may further include a substrate, where the first diode, the NMOS transistor, and the PMOS transistor are formed in the substrate. The second diode may further include an intrinsic region, where the p-doped region of the second diode is on the p-doped region of the first diode, where the intrinsic region of the second diode is on the p-doped region of the second diode, and where the n-doped region of the second diode is on the intrinsic region of the second diode. The first diode may be a diode including silicon and the second diode may be a diode including germanium. 
     Another innovative aspect of the subject matter described in this specification can be embodied in an optical sensor that includes a semiconductor substrate; a first light absorption region formed in the semiconductor substrate, the first light absorption region configured to absorb photons at a first wavelength range and to generate photo-carriers from the absorbed photons, the first light absorption region including: a first carrier-collection region configured to collect electrons; and a second carrier-collection region configured to collect holes; a second light absorption region on a portion of the first light absorption region, the second light absorption region configured to absorb photons at a second wavelength range and to generate photo-carriers from the absorbed photons; a first readout region coupled to a first readout circuitry, the first readout region configured to provide the electrons collected by the first carrier-collection region to the first readout circuitry, where the electrons collected by the first carrier-collection region are provided by the first light absorption region; a first gate coupled to a first control signal that controls a carrier transport between the first carrier-collection region and the first readout region; a second readout region coupled to a second readout circuitry, the second readout region configured to provide the holes collected by the second carrier-collection region to the second readout circuitry, where the holes collected by the second carrier-collection region are provided by the second light absorption region; and a second gate coupled to a second control signal that controls a carrier transport between the second carrier-collection region and the second readout region. 
     This and other implementations can each optionally include one or more of the following features. The second light absorption region may include a p-doped region including germanium on the first light absorption region; an intrinsic region including germanium on the p-doped region; and an n-doped region including germanium on the intrinsic region. The p-doped region may have a first strain and a first area, and the intrinsic region may have a second strain that is lower than the first strain, and the intrinsic region may have a second area that is larger than the first area. 
     The second light absorption region may be on a portion of the second carrier-collection region but not on the first carrier-collection region. The first light absorption region and the second light absorption region may be configured to receive light at different locations. The first light absorption region and the second light absorption region may be coupled by one or more interconnects formed by bonding two donor wafers. 
     The optical sensor may include a third readout region coupled to a third readout circuitry, the third readout region configured to provide the holes collected by the second carrier-collection region to the third readout circuitry; and a third gate coupled to a third control signal that controls a carrier transport between the second carrier-collection region and the third readout region. The optical sensor may include a fourth readout region coupled to a fourth readout circuitry, the fourth readout region configured to provide the holes collected by the second carrier-collection region to the fourth readout circuitry; and a fourth gate coupled to a fourth control signal that controls a carrier transport between the second carrier-collection region and the fourth readout region. 
     Advantageous implementations may include one or more of the following features. Germanium is an efficient absorption material for near-infrared wavelengths, which reduces the problem of slow photo-carriers generated at a greater substrate depth when an inefficient absorption material, e.g., silicon, is used. An increased device bandwidth allows the use of a higher modulation frequency in an optical sensing system, giving advantages such as a greater depth resolution and a higher frame rate with innovative device design. An alloy germanium-silicon material as the optical absorption layer with innovative device design provides higher optical absorption efficiency over conventional Si material, which may provide a more sensitive sensor in the visible and near-infrared spectrums, may reduce crosstalk between neighboring pixels, and may allow for a reduction of pixel sizes. A punch-through (or reach-through) sensor design may enable a single-pixel photodiode or an array-pixel photodiodes to detect light in both the visible and near-infrared spectrums. A hybrid sensor design may support time-of-flight (TOF), near-infrared, and visible image sensing within the same sensing array. An increased device bandwidth allows the use of a higher modulation frequency in a time-of-flight system, giving a greater depth resolution. The punch-through sensor design and the hybrid sensor design may be controlled to prevent dark currents from the germanium-silicon region to leak into the silicon region and so avoids performance degradation. Having one sensor with two modes (e.g., VIS and NIR modes) instead of needing two separate sensors may reduce manufacturing cost, may increase the number of usable pixels in a given area, and may reduce packaging complexity. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other potential features and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an example of a photodiode array. 
         FIGS. 2-4  are examples of a photodiode having a punch-through structure for detecting visible and/or NIR light. 
         FIG. 5  is an example of a photodiode for detecting visible and/or NIR light. 
         FIGS. 6A-6C  are examples of a circuit schematic that represents the photodiode described in reference to  FIG. 5 . 
         FIGS. 7-10  are examples of a multi-gate photodiode for detecting visible and/or NIR light. 
         FIG. 11  is an example of an integrated photodiode for detecting visible and/or NIR light. 
         FIG. 12  is an example of an integrated photodiode for detecting visible and/or NIR light. 
         FIGS. 13-14  are examples of a multi-gate photodiode for detecting visible and/or NIR light. 
         FIG. 15  is an example of a multi-gate photodiode. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. It is also to be understood that the various exemplary embodiments shown in the figures are merely illustrative representations and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     Semiconductor based optical sensors, such as photodiodes may be used to detect optical signals and convert the optical signals to electrical signals that may be further processed by another circuitry. In general, a material absorbs light at various wavelengths to generate free carriers depending on an energy bandgap associated with the material. For example, at room temperature, silicon (Si) may have an energy bandgap of 1.12 eV. As another example, at room temperature, germanium (Ge) may have an energy bandgap of 0.66 eV, and a germanium-silicon alloy (Ge x Si 1-x ) may have an energy bandgap between 0.66 eV and 1.12 eV depending on the composition. As another example, at room temperature, tin (Sn) may have an energy bandgap of about 0 eV, and a germanium-tin alloy (Ge x Sn 1-x ) may have an energy bandgap between 0 eV and 0.66 eV depending on the composition. In general, a material having a lower energy bandgap has a higher absorption coefficient at wider wavelength ranges. If the absorption coefficient of a material is too low, the optical signal cannot be converted to an electrical signal efficiently. However, if the absorption coefficient of a material is too high, free carriers will be generated near the surface of the material with reduced efficiency. Silicon is not an efficient sensor material for NIR wavelengths due to its large bandgap. On the other hand, germanium or tin has an absorption coefficient that may be too high for shorter wavelengths (e.g., blue), where free carriers may recombine at the surface. An optical sensor that integrates silicon and a different material, such as germanium or germanium-silicon, germanium-tin, on a common substrate, where the optical sensor uses silicon to detect visible light and uses the different material to detect NIR light, would enable the optical sensor to have a wide detection spectrum. In this application, the term “photodiode” may be used interchangeably as the term “optical sensor”. In this application, the term “germanium-silicon (GeSi)”, “silicon-germanium (SiGe)” may be used interchangeably, and both include all suitable SiGe composition combinations from 100% germanium (Ge) to more than 90% silicon (Si). In this application, the term “germanium-tin (GeSn)” includes all suitable GeSn composition combinations from 100% germanium (Ge) to more than 90% tin (Sn). In this application, the GeSi layer may be formed using blanket epitaxy, selective epitaxy, or other applicable techniques. Furthermore, the GeSi layer may be formed on a planar surface, a mesa surface, or a trench/hole region at least partially surround be insulator (ex: oxide, nitrite), or semiconductor (ex: Si, Ge), or their combinations. Also note that in this application, lightly doped region may have doping level from 10 15  to 10 19  cm −3 . Furthermore, a strained super lattice structure including multiple layers such as alternating GeSi layer with different compositions may be used for the absorption or forming a quantum well structure. 
       FIG. 1  is an example a photodiode array  100  where photodiodes are formed by integrating silicon with germanium-silicon. An optical image sensor array is an example of a photodiode array. The photodiode array  100  includes a substrate  102 , an integrated circuit layer  104 , an interconnect layer  106 , a sensor layer  108 , a filter layer  110 , and a lens layer  112 . In general, light of a single wavelength or multiple wavelengths enters the lens layer  112 , where the light may be focused, collimated, expanded, or processed according to the lens design. The shape of the lens could be concave, convex, or planar with surface structure such as Fresnel lens, or other shapes, and its shape should not be limited by the exemplary drawings here. 
     The light then enters the filter layer  110 , where the filter layer  110  may be configured to pass light having one or more specific wavelength ranges. A filter may be formed by depositing layers of dielectric materials, such that light having a wavelength within a specific wavelength range would pass through the filter and light having a wavelength outside the specific wavelength range would be reflected by the filter. For example, a filter of the filter layer  110  may be configured to pass light at a blue wavelength range (e.g., 460 nm±40 nm) and a NIR wavelength range (e.g., 850 nm±40 nm, 940 nm±40 nm, or &gt;1 μm), while another filter of the filter layer  110  may be configured to pass light at a green wavelength range (e.g., 540 nm±40 nm) and the NIR wavelength range. In some implementation, one or more filters of the filter layer  110  may pass wavelengths in a tunable fashion by including a capacitive microelectromechanical systems (MEMS) structure. The photodiodes in the sensor layer  108  converts the incident light into free carriers. The integrated circuit layer  104  collects the free carriers through the interconnect layer  106  and processes the free carriers according to the specific application. 
     In general, the substrate  102  may be a silicon substrate, a silicon-on-insulator (SOI) substrate, or any other suitable carrier substrate materials. The integrated circuits of the integrated circuit layer  104  and the interconnects of the interconnect layer  106  may be fabricated using CMOS processing techniques. For example, the interconnects may be formed by dry-etching a contact region through a dielectric layer in the form of holes or trenches and filling the contact region by copper using electroplating process. 
     The sensor layer  108  includes multiple photodiodes for detecting visible and/or NIR light. Each photodiode may be isolated by insulating sidewall spacers, trenches, or other suitable isolation structures. In some implementations, the wavelength range that a photodiode is configured to detect may be controlled by an optical filter in the filter layer  110 . For example, a photodiode may be configured to receive a red wavelength range (e.g., 620 nm±40 nm) and a NIR wavelength range, where the center wavelengths and the limits of the wavelength ranges are controlled by the characteristics of the filter above the photodiode. In some implementations, the wavelength range that a photodiode is configured to detect may be controlled by a material composition of the photodiode. For example, an increase in germanium composition in a germanium-silicon alloy may increase the sensitivity of the photodiode at longer wavelengths. In some implementations, the wavelength range that a photodiode is configured to detect may be controlled by a combination of the optical filter and the material composition of the photodiode. 
     As described in more details in reference to  FIGS. 2-4 , in some implementations, the photodiodes in the photodiode array  100  may be controlled to transfer photo-carriers generated by either the visible or the NIR light to a readout circuit for further processing. For example, the photodiodes in the photodiode array  100  may be controlled to transfer photo-carriers generated by the visible light to the readout circuit for further processing during daytime, and the photodiodes may be controlled to transfer photo-carriers generated by the NIR light to the readout circuit for further processing during night time for night-vision applications. As another example, the optical sensor can be controlled to collect both the visible light and the NIR light simultaneously or in an alternating sequence. 
     As described in more details in reference to  FIGS. 5-10 , in some implementations, the photodiodes in the photodiode array  100  may be controlled to transfer photo-carriers generated by either the visible or the NIR light to different readout circuits for further processing. For example, the photodiodes in the photodiode array  100  may be controlled to transfer free-electrons generated by the visible light to a first readout circuit and to transfer free-holes generated by the NIR light to a second readout circuit for further processing. As another example, the optical sensor can be controlled to collect both the visible light and the NIR light simultaneously or in an alternating sequence. 
     As described in more details in reference to  FIGS. 13-14 , in some implementations, different photodiodes in the photodiode array  100  may be controlled to detect the visible and/or the NIR light. For example, one pixel of the photodiodes in the photodiode array  100  may be controlled to transfer free-electrons generated by the visible light to a first readout circuit, while another pixel of the photodiode in the photodiode array  100  may be controlled to transfer free-holes generated by the NIR light to a second readout circuit for further processing. In some implementations, both pixels may share part of the structures such as the doping region. 
       FIG. 2  illustrates an example photodiode  200  having a punch-through structure for detecting visible and near-infrared optical signals. In general, the example photodiodes  200  includes a first absorption region for converting a visible optical signal to photo-carriers (i.e., electron-hole pairs) and a second absorption region for converting a NIR optical signal to photo-carriers. A sensor control signal  226  may control the transfer of either photo-carriers generated by the visible optical signal or photo-carriers generated by the NIR optical signal to the readout circuit  224 . The photodiode  200  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     The photodiode  200  includes a p-Si substrate  202 , a p+ Si region  204 , an n-Si region  206 , a p-Si region  208 , an intrinsic GeSi region  210 , a p+ GeSi region  212 , a gate  214 , an n+ Si region  216 , a gate control signal  222  coupled to the gate  214 , a sensor control signal  226  coupled to the p+ GeSi region  212 , and a readout circuit  224  coupled to the n+ Si region  216 . In some implementations, the first absorption region for converting a visible optical signal to photo-carriers may include the p+ Si region  204 , the n-Si region  206 , and the p-Si region  208 . The p+ Si region  204  may have a p+ doping, where the activated dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  with boron. The n-Si region  206  may be lightly doped with an n-dopant, e.g., phosphorus. The p-Si region  208  may be lightly doped with a p-dopant, e.g., boron. In some implementations, the second absorption region for converting a NIR optical signal to photo-carriers may include the intrinsic GeSi region  210  and the p+ GeSi region  212 . The p+ GeSi region  212  may have a p+ doping, where the dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  when the intrinsic GeSi region  210  is germanium and doped with boron. Here, the intrinsic GeSi region  210  is a germanium-silicon film. The p-Si substrate  202  may be lightly doped with a p-dopant, e.g., boron. 
     In general, the first absorption region of the photodiode  200  receives the optical signal  220 . The optical signal  220  may be a visible optical signal or an NIR optical signal. In some implementations, the optical signal  220  may be filtered by a wavelength filter not shown in this figure, such as a filter in the filter layer  110  as described in reference to  FIG. 1 . In some implementations, a beam profile of the optical signal  220  may be shaped by a lens not shown in this figure, such as a lens in the lens layer  112  as described in reference to  FIG. 1 . In the case where the optical signal  220  is a visible optical signal, the first absorption region may absorb the visible optical signal and generate photo-carriers. In the case where the optical signal  220  is a NIR optical signal, the first absorption region may be transparent or nearly transparent to the NIR optical signal, and the second absorption region may absorb the NIR optical signal and generate photo-carriers. The sensor control signal  226  may control whether the photodiode  200  operates in a visible light mode or in a NIR light mode when a tunable wavelength filter is applied. 
     In the visible light mode, the optical signal  220  is a visible optical signal, where the first absorption region absorbs the visible optical signal and generates photo-carriers. A built-in potential between the p+ Si region  204  and the n-Si region  206  creates an electric field between the two regions, where free electrons generated from the n-Si region  206  are drifted/diffused to a region below the p+ Si region  204  by the electric field. The p-Si region  208  is configured to repel the free-electrons in the intrinsic GeSi region  210  from entering the first absorption region, such that the dark current of the photodiode  200  may be reduced. The sensor control signal  226  is configured to be able to apply a first sensor control signal level to further prevent free-electrons in the intrinsic GeSi region  210  from entering the first absorption region, such that the dark current of the photodiode  200  may be further reduced. For example, the p+ GeSi region  212  may be coupled to a voltage source, where the sensor control signal  226  may be a time-varying voltage signal from the voltage source. The first sensor control signal level may be 0V, such that an energy band difference between the p-Si region  208  and the intrinsic GeSi region  210  further prevents free-electrons in the intrinsic GeSi region  210  from entering the p-Si region  208 . In some implementations, an intrinsic Si layer or unintentionally doped Si layer may be inserted in between the n-Si layer  206  and p-Si layer  208  to increase the optical absorption of the first absorption region. 
     The gate  214  may be coupled to the gate control signal  222 . For example, the gate  214  may be coupled to a voltage source, where the gate control signal  222  may be a time-varying voltage signal from the voltage source. The gate control signal  222  controls a flow of free electrons from the region below the p+ Si region  204  to the n+ Si region  216 . For example, if a voltage of the gate control signal  222  exceeds a threshold voltage, free electrons accumulated in the region below the p+ Si region  204  will drift or diffuse to the n+ Si region  216 . 
     The n+ Si region  216  may be coupled to the readout circuit  224 . The readout circuit  224  may be in a three-transistor configuration consisting of a reset gate, a source-follower, and a selection gate, or any suitable circuitry for processing free carriers. In some implementations, the readout circuit  224  may be fabricated on a substrate that is common to the photodiode  200 . In some other implementations, the readout circuit  224  may be fabricated on another substrate and co-packaged with the photodiode  200  via die/wafer bonding or stacking. For example, the readout circuit  224  may be fabricated on the integrated circuit layer  104  as described in reference to  FIG. 1 . 
     In the NIR light mode, the optical signal  220  is a NIR optical signal, where the optical signal  220  passes through the first absorption region because of the low NIR absorption coefficient of silicon, and the second absorption region then absorbs the NIR optical signal to generate photo-carriers. In general, the intrinsic GeSi region  210  receives an optical signal  220  and converts the optical signal  220  into electrical signals. In some implementations, a thickness of the intrinsic GeSi region  210  may be between 0.05 μm to 2 μm. In some implementations, the intrinsic GeSi region  210  may include a p+ GeSi region  212 . The p+ GeSi region  212  may repel the photo-electrons generated from the intrinsic GeSi region  210  to avoid surface recombination and thereby may increase the carrier collection efficiency. 
     In the NIR light mode, the sensor control signal  226  is configured to apply a second sensor control signal level to allow free-electrons in the intrinsic GeSi region  210  to enter the first absorption region. For example, the second sensor control signal level may be −1.2V, such that the p-Si region  208  is depleted, and the free-electrons in the intrinsic GeSi region  210  may enter the n-Si region  206  and drift or diffuse to below the p+ Si region  204 . The free-electrons accumulated in the region below the p+ Si region  204  may be transferred to the readout circuit  224  by controlling the gate control signal  222  in similar manners as described earlier in reference to the visible mode. 
     Although not shown in  FIG. 2 , in some other implementations, the first absorption region and the second absorption region of the photodiode  200  may alternatively be designed into opposite polarity to collect holes. In this case, the p-Si substrate  202  would be replaced by an n-Si substrate, the p+ Si region  204  would be replaced by an n+ Si region, the n-Si region  206  would be replaced by a p-Si region, the p-Si region  208  would be replaced by an n-Si region, the p-Si substrate  202  would be replaced by an n-Si substrate, the n+ Si region  216  would be replaced by a p+ Si region, and the p+ GeSi region  212  would be replaced by an n+ GeSi region. 
       FIG. 3  illustrates an example photodiode  300  having a punch-through structure for detecting visible and near-infrared optical signals. In general, the example photodiodes  300  includes a first absorption region for converting a visible optical signal to photo-carriers and a second absorption region for converting a NIR optical signal to photo-carriers. A sensor control signal  326  may control the transfer of either photo-carriers generated by the visible optical signal or photo-carriers generated by the NIR optical signal to the readout circuit  324 . The photodiode  300  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     The photodiode  300  includes a p-Si substrate  302 , a p-Si region  304 , an n-Si region  306 , an intrinsic GeSi region  310 , a p+ GeSi region  312 , a gate  314 , an n+ Si region  316 , a gate control signal  322  coupled to the gate  314 , a sensor control signal  326  coupled to the p+ GeSi region  312 , and a readout circuit  324  coupled to the n+ Si region  316 . In some implementations, the first absorption region for converting a visible optical signal to photo-carriers may include the p-Si region  304  and the n-Si region  306 . The n-Si region  306  may be lightly doped with an n-dopant, e.g., phosphorus. The p-Si region  304  may be lightly doped with a p-dopant, e.g., boron. In some implementations, the second absorption region for converting a NIR optical signal to photo-carriers may include the intrinsic GeSi region  310  and the p+ GeSi region  312 . The p+ GeSi region  312  may have a p+ doping, where the dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  when the intrinsic GeSi region  310  is germanium and doped with boron. Here, the intrinsic GeSi region  310  is a germanium-silicon mesa. The p-Si substrate  302  may be lightly doped with a p-dopant, e.g., boron. 
     In general, the first absorption region of the photodiode  300  receives the optical signal  320 . The optical signal  320  may be a visible optical signal or an NIR optical signal similar to the optical signal  220  as described in reference to  FIG. 2 . In the case where the optical signal  320  is a visible optical signal, the first absorption region may absorb the visible optical signal and generate photo-carriers. In the case where the optical signal  320  is a NIR optical signal, the first absorption region may be transparent or nearly transparent to the NIR optical signal, and the second absorption region may absorb the NIR optical signal and generate photo-carriers. The sensor control signal  326  may control whether the photodiode  300  operates in a visible light mode or in a NIR light mode when a tunable wavelength filter is applied. 
     In the visible light mode, the optical signal  320  is a visible optical signal, where the first absorption region absorbs the visible optical signal and generates photo-carriers. A built-in potential between the p-Si region  304  and the n-Si region  306  creates an electric field between the two regions, where free electrons generated from the n-Si region  306  are drifted/diffused to a region below the p-Si region  304  by the electric field. The p-Si region  304  is configured to repel the free-electrons in the intrinsic GeSi region  310  from entering the first absorption region, such that the dark current of the photodiode  300  may be reduced. The sensor control signal  326  is configured to be able to apply a first sensor control signal level to further prevent free-electrons in the intrinsic GeSi region  310  from entering the first absorption region, such that the dark current of the photodiode  300  may be further reduced. For example, the first sensor control signal level may be 0V, such that an energy band difference between the p-Si region  304  and the intrinsic GeSi region  310  further prevents free-electrons in the intrinsic GeSi region  310  from entering the p-Si region  304 . In some implementations, an i-Si or unintentionally doped Si layer may be inserted between the n-Si layer  304  and p-Si layer  306  to increase the optical absorption of the first absorption region. 
     The gate  314  may be coupled to the gate control signal  322 . The gate control signal  322  controls a flow of free electrons from the region below the p-Si region  304  to the n+ Si region  316 . For example, if a voltage of the gate control signal  322  exceeds a threshold voltage, free electrons accumulated in the region below the p+ Si region  304  will drift or diffuse to the n+ Si region  316 . The n+ Si region  316  may be coupled to the readout circuit  324 . The readout circuit  324  may be similar to the readout circuit  224  as described in reference to  FIG. 2 . 
     In the NIR light mode, the optical signal  320  is a NIR optical signal, where the optical signal  320  passes through the first absorption region because of the low NIR absorption coefficient of silicon, and the second absorption region then absorbs the NIR optical signal to generate photo-carriers. In general, the intrinsic GeSi region  310  receives an optical signal  320  and converts the optical signal  320  into electrical signals. The p+ GeSi region  312  may repel the photo-electrons generated from the intrinsic GeSi region  310  to avoid surface recombination and thereby may increase the carrier collection efficiency. 
     In the NIR light mode, the sensor control signal  326  is configured to apply a second sensor control signal level to allow free-electrons in the intrinsic GeSi region  310  to enter the first absorption region. For example, the second sensor control signal level may be −1.2, such that the p-Si region  304  is depleted, and the free-electrons in the intrinsic GeSi region  310  may enter the n-Si region  306 . The free-electrons collected in the n-Si region  306  may be transferred to the readout circuit  324  by controlling the gate control signal  322  in similar manners as described earlier in reference to the visible mode. 
     Although not shown in  FIG. 3 , in some other implementations, the first absorption region and the second absorption region of the photodiode  300  may alternatively be designed into opposite polarity to collect holes. In this case, the p-Si substrate  302  would be replaced by an n-Si substrate, the p-Si region  304  would be replaced by an n-Si region, the n-Si region  306  would be replaced by a p-Si region, the p-Si substrate  302  would be replaced by an n-Si substrate, the n+ Si region  316  would be replaced by a p+ Si region, and the p+ GeSi region  312  would be replaced by an n+ GeSi region. 
       FIG. 4  illustrates an example photodiode  400  having a punch-through structure for detecting visible and near-infrared optical signals. In general, the example photodiodes  400  includes a first absorption region for converting a visible optical signal to photo-carriers and a second absorption region for converting a NIR optical signal to photo-carriers. A sensor control signal  426  may control the transfer of either photo-carriers generated by the visible optical signal or photo-carriers generated by the NIR optical signal to the readout circuit  424 . The photodiode  400  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     The photodiode  400  includes a p-Si substrate  402 , a p+ Si region  404 , an n-Si region  406 , a p-Si region  408 , an intrinsic GeSi region  410 , a p+ GeSi region  412 , a gate  414 , an n+ Si region  416 , a gate control signal  422  coupled to the gate  414 , a sensor control signal  426  coupled to the p+ GeSi region  412 , and a readout circuit  424  coupled to the n+ Si region  416 . In some implementations, the first absorption region for converting a visible optical signal to photo-carriers may include the p+ Si region  404 , the n-Si region  406 , and the p-Si region  408 . The p+ Si region  404 , the n-Si region  406 , and the p-Si region  408  are similar to the p+ Si region  204 , the n-Si region  206 , and the p-Si region  208  as described in reference to  FIG. 2 . In some implementations, the second absorption region for converting a NIR optical signal to photo-carriers may include the intrinsic GeSi region  410  and the p+ GeSi region  412 . The p+ GeSi region  412  may have a p+ doping, where the dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  when the intrinsic GeSi region  410  is germanium and doped with boron. Here, the intrinsic GeSi region  410  is a mesa structure that is surrounded by an insulator layer  430 , and may be fabricated by a selective Ge epitaxial growth. 
     In general, the first absorption region of the photodiode  400  receives the optical signal  420 . The optical signal  420  may be a visible optical signal or an NIR optical signal similar to the optical signal  220  as described in reference to  FIG. 2 . In the case where the optical signal  420  is a visible optical signal, the first absorption region may absorb the visible optical signal and generate photo-carriers. In the case where the optical signal  420  is a NIR optical signal, the first absorption region may be transparent or nearly transparent to the NIR optical signal, and the second absorption region may absorb the NIR optical signal and generate photo-carriers. The sensor control signal  426  may control whether the photodiode  400  operates in a visible light mode or in a NIR light mode similar to the visible light mode and the NIR light mode operations described in reference to  FIG. 2 . 
     Although not shown in  FIG. 4 , in some other implementations, the first absorption region and the second absorption region of the photodiode  400  may alternatively be designed into opposite polarity to collect holes. In this case, the p-Si substrate  402  would be replaced by an n-Si substrate, the p+ Si region  404  would be replaced by an n+ Si region, the n-Si region  406  would be replaced by a p-Si region, the p-Si region  408  would be replaced by an n-Si region, the p-Si substrate  402  would be replaced by an n-Si substrate, the n+ Si region  416  would be replaced by a p+ Si region, and the p+ GeSi region  412  would be replaced by an n+ GeSi region. 
       FIG. 5  illustrates an example photodiode  500  having a hybrid structure for detecting visible and NIR optical signals. The example photodiodes  500  includes a first diode for converting a visible optical signal to photo-carriers, a second diode for converting a NIR optical signal to photo-carriers, an NMOS transistor for transporting the photo-carriers, primarily electrons, generated by the first diode to a first readout circuit, and a PMOS transistor for transporting the photo-carriers, primarily holes, generated by the second diode to a second readout circuit. The photodiode  500  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     The first diode may include an n-Si region  504  and a p-Si region  506  fabricated in a p-Si substrate  502  that is lightly doped with a p-dopant, e.g., boron. The n-Si region  504  may be lightly doped with an n-dopant, e.g., phosphorus. The p-Si region  506  may be lightly doped with a p-dopant, e.g., boron. The NMOS transistor may include a first n+ Si region  514 , a p-Si region  518 , a second n+ Si region  516 , and an NMOS gate  520 . An NMOS gate control signal  522  may be coupled to the NMOS gate  520 , and a first readout circuit  524  may be coupled to the second n+ Si region  516 . 
     The second diode may include a p-GeSi region  508 , an intrinsic GeSi region  510 , and an n+ GeSi region  512 . In some implementations, a thickness of the intrinsic GeSi region  510  may be between 0.05 μm to 2 μm. The n+ GeSi region  512  may have a n+ doping, where the dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  when the intrinsic GeSi region  510  is germanium and doped with phosphorus. The p-GeSi region  508  may be lightly doped with a p-dopant, e.g., boron, when the intrinsic GeSi region  510  is germanium. The PMOS transistor may include a first p+ Si region  534 , an n-Si region  528 , a second p+ Si region  536 , and a PMOS gate  530 . A PMOS gate control signal  538  may be coupled to the PMOS gate  530 , and a second readout circuit  532  may be coupled to the second p+ Si region  536 . Although not shown in  FIG. 5 , the n+ GeSi region  512  may have opposite polarity, namely become a p+ GeSi region to form a p-i-p vertical doping profile in the GeSi region. 
       FIGS. 6A-6C  illustrate an example circuitry  600  for illustrating operations of the photodiode  500  or other structures with similar device design concepts. Referring to  FIG. 6A , the circuitry  600  includes a silicon diode  606 , a germanium-silicon diode  604 , a PMOS transistor  602 , and a NMOS transistor  608 , which may correspond to the first diode, the second diode, the PMOS transistor, and the NMOS transistor as described in reference to  FIG. 5 , respectively. The source of the NMOS transistor  608  is coupled to the n-end of the silicon diode  606 , the drain of the NMOS transistor  608  is coupled to a first readout circuit  624 , and the gate of the NMOS transistor  608  is coupled to an NMOS gate control signal  622 . The source of the PMOS transistor  602  is coupled to the p-end of the silicon diode  606  and the p-end of the germanium-silicon diode  604 , the drain of the PMOS transistor  602  is coupled to a second readout circuit  632 , and the gate of the PMOS transistor  602  is coupled to a PMOS gate control signal  638 . The n-end of the germanium silicon diode  604  is coupled to a voltage source V DD . Although not shown in  FIG. 6A , the first readout circuit  624  and the second readout circuit  632  may each be in a three-transistor configuration consisting of a reset gate, a source-follower, and a selection gate, or any suitable circuitry for processing free carriers. 
     Referring to  FIG. 6B , in the visible light mode, the drain of the PMOS transistor  602  is coupled to a voltage source V SS . This may be achieved by activating the reset gate of the second readout circuit  632  to couple the drain of the PMOS transistor  602  to the voltage source V SS  of the second readout circuit  632 . In the visible light mode, the silicon diode  606  absorbs the incoming optical signal and generates electron-hole pairs. For example, referring to  FIG. 5 , if the optical signal  540  is in the visible wavelength spectrum, the n-Si region  504  would absorb the optical signal  540  to generate electron-hole pairs. Example values of V DD  and V SS  may be 1.2V and −1.2V. 
     Referring back to  FIG. 6B , the free electrons generated by the silicon diode  606  may be transferred to the first readout circuit  624  by turning on the NMOS transistor  608  using the NMOS gate control signal  622 . For example, referring to  FIG. 5 , by turning on the NMOS gate  520  using the NMOS gate control signal  522 , the free electrons accumulated in the n-Si region  504  may be transferred from the first n+ Si region  514  to the second n+ Si region  516 , where the first readout circuit  524  may collect, store, and process the electrons. 
     Referring back to  FIG. 6B , the free holes generated by the silicon diode  606  may be transferred to the voltage source V SS  by turning on the PMOS transistor  602  using the PMOS gate control signal  638 . In general, it is desirable to minimize or eliminate the noise-current, such as the dark-current, from the germanium-silicon diode  604  in the visible light mode because the dark-current of a germanium-silicon photodiode is generally much greater than the dark-current of a silicon photodiode. Since the n-end of the germanium-silicon diode  604  is coupled to a voltage source V DD , the free electrons generated by the germanium-silicon diode  604  would be transferred to the voltage source V DD . Similarly, since the p-end of the germanium-silicon diode  604  is coupled to a voltage source V SS  by letting the PMOS transistor on, the free holes generated by the germanium-silicon diode  604  would be transferred to the voltage source V SS . Consequently, the photo-carriers generated by the germanium-silicon diode  604  would not be transferred to the first readout circuit  624 , thereby improving the overall performance of photodiode  600  in the visible light mode. 
     Referring to  FIG. 6C , in the NIR light mode, the drain of the NMOS transistor  608  is coupled to a voltage source V DD . This may be achieved by activating the reset gate of the first readout circuit  624  to couple the drain of the NMOS transistor  608  to the voltage source V DD  of the first readout circuit  624 . In the NIR light mode, the germanium-silicon diode  604  absorbs the incoming optical signal and generates electron-hole pairs. For example, referring to  FIG. 5 , if the optical signal  540  is in the NIR wavelength spectrum, the n-Si region  504  and the p-Si region  506  would be transparent to the optical signal  540 , and the intrinsic GeSi region  510  would absorb the optical signal  540  to generate electron-hole pairs. Example values of V DD  may be 1.2V. 
     Referring back to  FIG. 6C , the free holes generated by the germanium-silicon diode  604  may be transferred to the second readout circuit  632  by turning on the PMOS transistor  602  using the PMOS gate control signal  638 . For example, referring to  FIG. 5 , by turning on the PMOS gate  530  using the PMOS gate control signal  538 , the free holes accumulated in the p-Si region  506  may be transferred from the first p+ Si region  534  to the second p+ Si region  536 , where the second readout circuit  532  may collect, store, and process the holes. 
     Referring back to  FIG. 6C , the free electrons generated by the germanium-silicon diode  604  may be transferred to the voltage source V DD  that is coupled to the n-end of the germanium-silicon diode  604 . Since the p-end of the silicon diode  606  is coupled to the PMOS transistor  602 , the free holes generated by the silicon diode  606  would also be transferred to the second readout circuit  632 . This is acceptable because the dark current of a silicon diode is generally much smaller than the dark current of a germanium-silicon diode. Since the n-end of the silicon diode  606  is coupled to a voltage source V DD  by letting the NMOS transistor on, the free electrons generated by the silicon diode  608  would be transferred to the voltage source V DD . 
     Although not shown in  FIGS. 5 and 6A , in some other implementations, the first diode (e.g., the silicon diode  606 ) and the second diode (e.g., the germanium-silicon diode  604 ) may alternatively be designed into opposite polarity to collect holes and electrons, respectively. In this case, the p-Si substrate  502  would be replaced by an n-Si substrate, the p-Si region  506  would be replaced by an n-Si region, the n-Si region  504  would be replaced by a p-Si region, the p+ GeSi region  512  would be replaced by an n+ GeSi region, the p-GeSi region  508  would be replaced by an n-GeSi region, the NMOS transistor would be replaced by a PMOS transistor, and the PMOS transistor would be replaced by an NMOS transistor. In some other implementations, the first diode and the second diode may be fabricated using other types of materials. For example, the second diode may be fabricated by a germanium-tin alloy to detect IR wavelength spectrum. 
       FIG. 7  illustrates an example photodiode  700  having a hybrid structure for detecting visible and NIR optical signals. The example photodiodes  700  includes a first absorption region for converting a visible optical signal to photo-carriers, and a second absorption region for converting a NIR optical signal to photo-carriers. A first gate control signal  722  controls the transfer of free-electrons generated by the first absorption region to a first readout circuit  724 , and a second gate control signal  738  controls the transfer of free-holes generated by the second absorption region to a second readout circuit  732 . The photodiode  700  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     In some implementations, the first absorption region may include an n-Si region  704  and a p-Si region  706  fabricated in a p-Si substrate  702  that may be lightly doped with a p-dopant, e.g., boron. The n-Si region  704  may be lightly doped with an n-dopant, e.g., phosphorus. The p-Si region  706  may be lightly doped with a p-dopant, e.g., boron. The second absorption region may include a p-GeSi region  708 , an intrinsic GeSi region  710 , and an n+ GeSi region  712 . In some implementations, a thickness of the intrinsic GeSi region  710  may be between 0.05 μm to 2 μm. The n+ GeSi region  712  may have a n+ doping, where the dopant concentration may be as high as a fabrication process may achieve, e.g., about 5×10 20  cm −3  when the intrinsic GeSi region  710  is germanium and doped with phosphorus. The p-GeSi region  708  may be lightly doped with a p-dopant, e.g., boron, when the intrinsic GeSi region  710  is germanium. 
     In general, the n-Si layer  704  receives an optical signal  740 . If the optical signal  740  is a visible optical signal, the n-Si region  704  absorbs the optical signal  740  and converts the optical signal  740  into free carriers. A built-in potential between the p-Si region  706  and the n-Si region  704  creates an electric field between the two regions, where free electrons generated from the n-Si region  704  are drifted/diffused towards the region below the p-Si region  706  by the electric field. 
     A first gate  720  may be coupled to the first gate control signal  722 . For example, the first gate  720  may be coupled to a voltage source, where the first gate control signal  722  may be a time-varying voltage signal from the voltage source. The first gate control signal  722  controls a flow of free electrons from the region below the p-Si region  706  to the n+ Si region  716 . For example, if a voltage of the control signal  722  exceeds a threshold voltage, free electrons accumulated in the region below the p-Si region  706  will drift or diffuse to the n+ Si region  716  for collection. The n+ Si region  716  may be coupled to the first readout circuit  724  that processes the collected electrical signal. The first readout circuit  724  may be similar to the readout circuit  224  as described in reference to  FIG. 2 . 
     If the optical signal  740  is a NIR optical signal, the NIR optical signal propagates through the first absorption region and is received by the second absorption region. The second absorption region receives the NIR optical signal and converts the NIR optical signal into electrical signals. Since the thickness of the p-GeSi region  708  is generally thin (e.g., 50˜150 nm), the optical signal  740  propagates into the intrinsic GeSi region  710 , where the intrinsic GeSi region  710  absorbs the optical signal  740  and converts the optical signal  740  into free carriers. The n+ GeSi region  712  may repel the holes generated from the intrinsic GeSi region  710  to avoid surface recombination and thereby may increase the carrier collection efficiency. 
     The photo-generated free holes in the intrinsic GeSi region  710  may drift or diffuse into the p-Si region  706 . The photo-generated free electrons in the intrinsic GeSi region  710  may be repelled by the p-GeSi region  708 , which prevents the free electrons from entering the p-Si region  706 . In some implementations, a drain supply voltage V DD  may be coupled to the n+ GeSi region  712  to create an electric field within the second absorption region, such that the free holes may drift or diffuse towards the p-Si region  706  while the free electrons may transport to the V DD  voltage source. 
     The second gate  730  may be coupled to the second gate control signal  738 . For example, the second gate  730  may be coupled to a voltage source, where the second gate control signal  738  may be a time-varying voltage signal from the voltage source. The second gate control signal  738  controls a flow of free holes from the p-Si region  706  to the p+ Si region  736 . For example, if a voltage of the second gate control signal  738  exceeds a threshold voltage, free holes accumulated in the p-Si region  706  will drift or diffuse towards the p+ Si region  736 . The p+ Si region  736  may be coupled to the second readout circuit  732  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 7 , in some other implementations, the first absorption region and the second absorption region may alternatively be designed into opposite polarity to collect holes and electrons, respectively. In this case, the p-Si substrate  702  would be replaced by an n-Si substrate, the p-Si region  706  would be replaced by an n-Si region, the n-Si region  704  would be replaced by a p-Si region, the n+ GeSi region  712  would be replaced by an p+ GeSi region, the p+ GeSi region  708  would be replaced by a n+ GeSi region, the n+ Si region  716  would be replaced by a p+ region, the n-Si region  728  would be replaced by a p-Si region, and the p+ Si region  736  would be replaced by an n+ region. Although not shown in  FIG. 7 , in some implementations, the n+ GeSi region  712  may have different polarity, namely become a p+ GeSi region to form a p-i-p vertical doping profile in the GeSi regions. 
       FIG. 8  illustrates an example photodiode  800  having a hybrid structure for detecting visible and NIR optical signals. Similar to the example photodiodes  700  as described in reference to  FIG. 7 , the photodiode  800  includes a first absorption region for converting a visible optical signal to photo-carriers, and a second absorption region for converting a NIR optical signal to photo-carriers. A first gate control signal  822  controls the transfer of free-electrons generated by the first absorption region to a first readout circuit  824 , and a second gate control signal  838  controls the transfer of free-holes generated by the second absorption region to a second readout circuit  832 . The photodiode  800  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     In some implementations, the first absorption region may include an n-Si region  804  and a p+ Si region  806  fabricated in a p-Si substrate  802 . The second absorption region may include a p+ GeSi region  808 , an intrinsic GeSi region  810 , and an n+ GeSi region  812 . The first absorption region and the second absorption region are bonded using a first donor wafer  850  and a second donor wafer  852 , and the first absorption region and the second absorption region are electrically coupled by one or more interconnects  842 . 
     If the optical signal  840  is a visible optical signal, the operations of the photodiode  800  is similar to the operations of the photodiode  700  as described in reference to  FIG. 7 . If the optical signal  840  is a NIR optical signal, the NIR optical signal propagates through the first absorption region, the first donor wafer  850 , and the second donor wafer  852 , and is received by the second absorption region. The second absorption region receives the NIR optical signal and converts the NIR optical signal into electrical signals. Since the thickness of the p+ GeSi region  808  is generally thin (e.g., 50˜150 nm), the optical signal  840  propagates into the intrinsic GeSi region  810 , where the intrinsic GeSi region  810  absorbs the optical signal  840  and converts the optical signal  840  into free carriers. The n+ GeSi region  812  may repel the holes generated from the intrinsic GeSi region  810  to avoid surface recombination and thereby may increase the carrier collection efficiency. The photo-generated free holes in the intrinsic GeSi region  810  may transport to the p-Si region  806  via the one or more interconnects  842 . The photo-generated free electrons in the intrinsic GeSi region  810  may be repelled by the p+ GeSi region  808 , which prevents the free electrons from entering the p+ Si region  806 . 
     In some implementations, a drain supply voltage V DD  may be coupled to the n+ GeSi region  812  to create an electric field within the second absorption region, such that the free holes may drift or diffuse towards the p+ Si region  806  via the interconnects  842  while the free electrons may transport to the V DD  voltage source. The second gate  830  may be coupled to the second gate control signal  838 . The second gate control signal  838  controls a flow of free holes from the p+ Si region  806  to the p+ Si region  836 . The p+ Si region  836  may be coupled to the second readout circuit  832  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 8 , in some other implementations, the first absorption region and the second absorption region may alternatively be designed into opposite polarity to collect holes and electrons, respectively. In this case, the p-Si substrate  802  would be replaced by an n-Si substrate, the p+ Si region  806  would be replaced by an n+ Si region, the n-Si region  804  would be replaced by a p-Si region, the n+ GeSi region  812  would be replaced by a p+ GeSi region, the p+ GeSi region  808  would be replaced by an n+ GeSi region, the n+ Si region  816  would be replaced by a p+ region, the n-Si region  828  would be replaced by a p-Si region, and the p+ Si region  836  would be replaced by an n+ region. 
       FIG. 9  illustrates an example photodiode  900  having a hybrid structure for detecting visible and NIR optical signals. Similar to the example photodiodes  700  as described in reference to  FIG. 7 , the photodiode  900  includes a first absorption region for converting a visible optical signal to photo-carriers, and a second absorption region for converting a NIR optical signal to photo-carriers. A first gate control signal  922  controls the transfer of free-electrons generated by the first absorption region to a first readout circuit  924 , and a second gate control signal  938  controls the transfer of free-holes generated by the second absorption region to a second readout circuit  932 . The photodiode  900  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     In some implementations, the first absorption region may include an n-Si region  904  and a p-Si region  906  fabricated in a p-Si substrate  902 . The second absorption region may include a p-GeSi region  908 , an intrinsic GeSi region  910 , and an n+ GeSi region  912 . The p-GeSi region  908  may be formed in an etched region of an insulator layer  942  (e.g., oxide) using a lateral strain dilution technique or an aspect ratio trapping technique for forming a germanium or germanium-silicon mesa having reduced defects or being defect-free, which results into a lower dark current and a better sensitivity/dynamic range. The lateral strain dilution technique is described in U.S. patent application Ser. No. 15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fully incorporated by reference herein. 
     If the optical signal  940  is a visible optical signal, the operations of the photodiode  900  is similar to the operations of the photodiode  700  as described in reference to  FIG. 7 . If the optical signal  940  is a NIR optical signal, the NIR optical signal propagates through the first absorption region, and is received by the second absorption region. The second absorption region receives the NIR optical signal and converts the NIR optical signal into electrical signals. Since the thickness of the p-GeSi region  908  is generally thin (e.g., 50˜150 nm), the optical signal  940  propagates into the intrinsic GeSi region  910 , where the intrinsic GeSi region  910  absorbs the optical signal  940  and converts the optical signal  940  into free carriers. The n+ GeSi region  912  may repel the holes generated from the intrinsic GeSi region  910  to avoid surface recombination and thereby may increase the carrier collection efficiency. The photo-generated free holes in the intrinsic GeSi region  910  may transport to the p-Si region  906  via the p-GeSi region  908 . The photo-generated free electrons in the intrinsic GeSi region  910  may be repelled by the p-GeSi region  908 , which prevents the free electrons from entering the p-Si region  906 . 
     In some implementations, a drain supply voltage V DD  may be coupled to the n+ GeSi region  912  to create an electric field within the second absorption region, such that the free holes may drift or diffuse towards the p-Si region  906  via the p-GeSi region  908  while the free electrons may transport to the V DD  voltage source. The second gate  930  may be coupled to the second gate control signal  938 . The second gate control signal  938  controls a flow of free holes from the p-Si region  906  to the p+ Si region  936 . The p+ Si region  936  may be coupled to the second readout circuit  932  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 9 , in some other implementations, the first absorption region and the second absorption region may alternatively be designed into opposite polarity to collect holes and electrons, respectively. In this case, the p-Si substrate  902  would be replaced by an n-Si substrate, the p-Si region  906  would be replaced by an n-Si region, the n-Si region  904  would be replaced by a p-Si region, the n+ GeSi region  912  would be replaced by a p+ GeSi region, the p-GeSi region  908  would be replaced by an n-GeSi region, the n+ Si region  916  would be replaced by a p+ region, the n-Si region  928  would be replaced by a p-Si region, and the p+ Si region  936  would be replaced by an n+ region. 
       FIG. 10  illustrates an example photodiode  1000  having a hybrid structure for detecting visible and NIR optical signals. Similar to the example photodiodes  700  as described in reference to  FIG. 7 , the photodiode  1000  includes a first absorption region for converting a visible optical signal to photo-carriers, and a second absorption region for converting a NIR optical signal to photo-carriers. A first gate control signal  1022  controls the transfer of free-electrons generated by the first absorption region to a first readout circuit  1024 , and a second gate control signal  1038  controls the transfer of free-holes generated by the second absorption region to a second readout circuit  1032 . The photodiode  1000  may be one of the photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     In some implementations, the first absorption region may include an n-Si region  1004  and a p+ Si region  1006  fabricated in a p-Si substrate  1002 . The second absorption region may include a p+ GeSi region  1008 , an intrinsic GeSi region  1010 , and an n+ GeSi region  1012 . The first absorption region and the second absorption region are bonded using a first donor wafer  1050  and a second donor wafer  1052 , and the first absorption region and the second absorption region are electrically coupled by one or more interconnects  1044 . The n+ GeSi region  1012  may be formed in an etched region of an insulator layer (e.g., oxide)  1042  using a lateral strain dilution technique or an aspect ratio trapping technique for forming a germanium or germanium-silicon mesa having reduced defects or being defect-free, which results into a lower dark current and a better sensitivity/dynamic range. The lateral strain dilution technique is described in U.S. patent application Ser. No. 15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fully incorporated by reference herein. 
     If the optical signal  1040  is a visible optical signal, the operations of the photodiode  1000  is similar to the operations of the photodiode  700  as described in reference to  FIG. 7 . If the optical signal  1040  is a NIR optical signal, the NIR optical signal propagates through the first absorption region, and is received by the second absorption region. The second absorption region receives the NIR optical signal and converts the NIR optical signal into electrical signals. Since the thickness of the p+ GeSi region  1008  is generally thin (e.g., 50˜150 nm), the optical signal  1040  propagates into the intrinsic GeSi region  1010 , where the intrinsic GeSi region  1010  absorbs the optical signal  1040  and converts the optical signal  1040  into free carriers. The n+ GeSi region  1012  and the insulator layer  1042  may repel the holes generated from the intrinsic GeSi region  1010  to avoid surface recombination and thereby may increase the carrier collection efficiency. The photo-generated free holes in the intrinsic GeSi region  1010  may transport to the p+ Si region  1006  via the one or more interconnects  1044 . The photo-generated free electrons in the intrinsic GeSi region  1010  may be repelled by the p+ GeSi region  1008 , which prevents the free electrons from entering the p+ Si region  1006 . 
     In some implementations, a drain supply voltage V DD  may be coupled to the n+ GeSi region  1012  to create an electric field within the second absorption region, such that the free holes may drift or diffuse towards the p+ Si region  1006  via the one or more interconnects  1044  while the free electrons may transport to the V DD  voltage source. The second gate  1030  may be coupled to the second gate control signal  1038 . The second gate control signal  1038  controls a flow of free holes from the p+ Si region  1006  to the p+ Si region  1036 . The p+ Si region  1036  may be coupled to the second readout circuit  1032  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 10 , in some other implementations, the first absorption region and the second absorption region may alternatively be designed into opposite polarity to collect holes and electrons, respectively. In this case, the p-Si substrate  1002  would be replaced by an n-Si substrate, the p+ Si region  1006  would be replaced by an n+ Si region, the n-Si region  1004  would be replaced by a p-Si region, the n+ GeSi region  1012  would be replaced by a p+ GeSi region, the p+ GeSi region  1008  would be replaced by an n+ GeSi region, the n+ Si region  1016  would be replaced by a p+ region, the n-Si region  1028  would be replaced by a p-Si region, and the p+ Si region  1036  would be replaced by an n+ region. 
       FIG. 11  shows a top view of an example integrated photodiode array  1100  for detecting visible and NIR light as well as for a TOF application. The photodiode array  1100  includes a NIR/TOF/VIS pixel  1102 . The NIR/TOF/VIS pixel  1102  includes an NIR gate  1106 , a first TOF gate  1112 , a second TOF gate  1114 , and a VIS gate  1108 . The controls of the charge readout using the NIR gate  1106  and the VIS gate  1108  are similar to the multi-gate photodiode  200 ,  300 ,  400 ,  500 ,  700 ,  800 ,  900 , or  1000  as described in reference to  FIG. 2 , FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 7 ,  FIG. 8 ,  FIG. 9 , or  FIG. 10 , respectively. The controls of the charge readout using the TOF gates  1112  and  1114  are similar to the multi-gate photodiode  1500  as described in reference to  FIG. 15  and also are described in U.S. patent application Ser. No. 15/228,282 titled “Germanium-Silicon Light Sensing Apparatus,” which is fully incorporated by reference herein. The readout circuits coupled to the NIR gate  1106  and the TOF gates  1112  and  1114  would collect the same type of carriers, and the readout circuit coupled to the VIS gate  1108  would collect the opposite type of carriers. For example, if the readout circuits of the NIR gate  1106  and the TOF gates  1112  and  1114  are configured to collect electrons, the readout circuit coupled to the VIS gate  1108  would be configured to collect holes. Conversely, if the readout circuits of the NIR gate  1106  and the TOF gates  1112  and  1114  are configured to collect holes, the readout circuit coupled to the VIS gate  1108  would be configured to collect electrons. 
       FIG. 12  shows a top view of an example integrated photodiode array  1200  for detecting visible light and for a TOF application. The photodiode array  1200  includes a TOF/VIS pixel  1202 . The TOF/VIS pixel  1202  includes a first TOF gate  1212 , a second TOF gate  1214 , and a VIS gate  1208 . The controls of the charge readout using the VIS gate  1208  are similar to the multi-gate photodiode  200 ,  300 ,  400 ,  500 ,  700 ,  800 ,  900 , or  1000  as described in reference to  FIG. 2 , FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 7 ,  FIG. 8 ,  FIG. 9 , or  FIG. 10 , respectively. The controls of the charge readout using the TOF gates  1212  and  1214  are similar to the multi-gate photodiode  1500  as described in reference to  FIG. 15  and also are described in U.S. patent application Ser. No. 15/228,282 titled “Germanium-Silicon Light Sensing Apparatus,” which is fully incorporated by reference herein. The readout circuits coupled to the TOF gates  1212  and  1214  would collect the same type of carriers, and the readout circuit coupled to the VIS gate  1208  would collect the opposite type of carriers. For example, if the readout circuits of the TOF gates  1212  and  1214  are configured to collect electrons, the readout circuit coupled to the VIS gate  1208  would be configured to collect holes. Conversely, if the readout circuits of the TOF gates  1212  and  1214  are configured to collect holes, the readout circuit coupled to the VIS gate  1208  would be configured to collect electrons. 
       FIG. 13  illustrates example photodiodes  1300  for detecting visible and near-infrared optical signals. The example photodiodes  1300  includes an NIR pixel  1350  for collecting holes and a visible pixel  1352  for collecting electrons, where the NIR pixel  1350  and the visible pixel  1352  are formed on a common substrate. The NIR pixel  1350  and the visible pixel  1352  may not be separated by an isolation structure. The NIR pixel  1350  is configured to detect an optical signal having a wavelength in the NIR range. The visible pixel  1352  is configured to detect an optical signal having a wavelength in the visible range (e.g., blue and/or green and/or red). The NIR pixel  1350  and the visible pixel  1352  may be photodiodes in the sensor layer  108  as described in reference to  FIG. 1 , for example. 
     The visible pixel  1350  is configured to collect free electrons generated from photo-generated carriers, and includes an n-Si region  1304 , an n+ Si region  1316 , an p-Si region  1306 , a first gate  1320 , a first gate control signal  1322  coupled to the first gate  1320 , and a first readout circuit  1324  coupled to the n+ Si region  1316 . In general, the p-Si layer  1306  receives a visible optical signal  1342 . Since the thickness of the p-Si layer  1306  is generally thin (e.g.,  50 - 150  nm), the optical signal  1342  propagates into the n-Si region  1304 , where the n-Si region  1304  absorbs the optical signal  1342  and converts the optical signal  1342  into free carriers. In some implementations, the optical signal  1342  may be filtered by a wavelength filter not shown in this figure, such as a filter in the filter layer  110  as described in reference to  FIG. 1 . In some implementations, a beam profile of the optical signal  1342  may be shaped by a lens not shown in this figure, such as a lens in the lens layer  112  as described in reference to  FIG. 1 . 
     In general, a built-in potential between the p-Si region  1306  and the n-Si region  1304  creates an electric field between the two regions, where free electrons generated from the n-Si region  1304  are drifted/diffused towards the region below the p-Si region  1306  by the electric field. The first gate  1320  may be coupled to the first gate control signal  1322 . For example, the first gate  1320  may be coupled to a voltage source, where the first gate control signal  1322  may be a time-varying voltage signal from the voltage source. The first gate control signal  1322  controls a flow of free electrons from the region below the p-Si region  1306  to the n+ Si region  1316 . For example, if a voltage of the first gate control signal  1322  exceeds a threshold voltage, free electrons accumulated in the region below the p-Si region  1306  will drift or diffuse to the n+ Si region  1316  for collection. The n+ Si region  1316  may be coupled to the first readout circuit  1324  that processes the collected electrical signal. The first readout circuit  1324  may be similar to the readout circuit  224  as described in reference to  FIG. 2 . 
     The NIR pixel  1350  is configured to collect free holes generated from photo-generated carriers, and includes an n-Si region  1328 , a p+ Si region  1336 , a second gate  1330 , a second gate control signal  1338  coupled to the second gate  1330 , a second readout circuit  1332  coupled to the p+ Si region  1336 , a n+ GeSi region  1312 , an intrinsic GeSi region  1310 , and a p-Ge region  1308 . In addition, the NIR pixel  1350  shares the p-Si region  1306  with the VIS pixel  1352 , but the germanium-silicon mesa may not be formed on the n-Si region  1304 . 
     The n+ GeSi region  1312  receives a NIR optical signal  1340  and converts the NIR optical signal  1340  into electrical signals. Since the thickness of the n+ GeSi layer  1312  is generally thin (e.g., 50˜150 nm), the optical signal  1340  propagates into the intrinsic GeSi region  1310 , where the intrinsic GeSi region  1310  absorbs the optical signal  1340  and converts the optical signal  1340  into free carriers. In some implementations, a thickness of the intrinsic GeSi region  1310  may be between 0.05 μm to 2 μm. In some implementations, the n+ GeSi region  1312  may repel the holes generated from the intrinsic GeSi region  1310  to avoid surface recombination and thereby may increase the carrier collection efficiency. 
     The photo-generated free holes in the intrinsic GeSi region  1310  may drift or diffuse into the p-Si region  1306 . The photo-generated free electrons in the intrinsic GeSi region  1310  may be repelled by the p-GeSi region  1308 , which prevents the free electrons from entering the p-Si region  1306 . In some implementations, a drain supply voltage V DD  may be applied to the NIR pixel  1350  to create an electric field between the n+ GeSi region  1312  and the p-Si region  1308 , such that the free holes may drift or diffuse towards the p-Si region  1306  while the free electrons may drift or diffuse towards the n+ GeSi region  1312 . 
     The second gate  1330  may be coupled to the second gate control signal  1338 . The second control signal  1338  controls a flow of free holes from the p-Si region  1306  to the p+ Si region  1336 . For example, if a voltage of the second gate control signal  1338  exceeds a threshold voltage, free holes accumulated in the p-Si region  1306  will drift or diffuse towards the p+ Si region  1336 . The p+ Si region  1336  may be coupled to the second readout circuit  1332  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 13 , in some other implementations, the visible pixel  1352  may alternatively be designed into opposite polarity to collect holes instead of electrons and the NIR pixel  1350  may alternatively be designed into opposite polarity to collect electrons instead of holes. In this case, the p-Si substrate  1302  would be replaced by an n-Si substrate, the p-Si region  1306  would be replaced by an n-Si region, the n-Si regions  1304  and  1328  would be replaced by p-Si regions, the p+ Si region  1336  would be replaced by an n+ Si region, the n+ Si region  1316  would be replaced by a p+ Si region, the n+ GeSi region  1312  would be replaced by a p+ GeSi region, and the p-GeSi region  1308  would be replaced by an n-GeSi region. In some implementations, the direction of light signal shown in  FIG. 13  may be reversed depending on designs, packaging, and applications. For example, the NIR optical signal  1340  may enter the NIR pixel  1350  through the p-Si substrate  1302 , and the visible optical signal  1342  may enter the visible pixel  1352  through the p-Si substrate  1302  and the n-Si region  1304 . 
       FIG. 14  illustrates example photodiodes  1400  for detecting visible and near-infrared optical signals. Similar to the photodiodes  1300  as described in reference to  FIG. 13 , the example photodiodes  1400  includes an NIR pixel  1450  for collecting holes and a visible pixel  1452  for collecting electrons, where the NIR pixel  1450  and the visible pixel  1452  are formed on a common substrate. The visible pixel  1450  includes an n-Si region  1404 , an n+ Si region  1416 , an p-Si region  1406 , a first gate  1420 , a first gate control signal  1422  coupled to the first gate  1420 , and a first readout circuit  1424  coupled to the n+ Si region  1416 . The operations of the visible pixel  1450  is similar to the operations of the visible pixel  1350  as described in reference to  FIG. 13 . 
     The NIR pixel  1450  is configured to collect free holes generated from photo-generated carriers, and includes an n-Si region  1428 , a p+ Si region  1436 , a second gate  1430 , a second gate control signal  1438  coupled to the second gate  1430 , a second readout circuit  1432  coupled to the p+ Si region  1436 , a n+ GeSi region  1412 , an intrinsic GeSi region  1410 , and a p-Ge region  1408 . The p-GeSi region  1408  may be formed in an etched region of an insulator layer (e.g., oxide)  1442  using a lateral strain dilution technique or an aspect ratio trapping technique for forming a germanium or germanium-silicon mesa having reduced defects or being defect-free, which results into a lower dark current and a better sensitivity/dynamic range. The lateral strain dilution technique is described in U.S. patent application Ser. No. 15/216,924 titled “High Efficiency Wide Spectrum Sensor,” which is fully incorporated by reference herein. 
     The n+ GeSi region  1412  receives a NIR optical signal  1440  and converts the NIR optical signal  1440  into electrical signals. Since the thickness of the n+ GeSi layer  1412  is generally thin (e.g., 50˜150 nm), the optical signal  1440  propagates into the intrinsic GeSi region  1410 , where the intrinsic GeSi region  1410  absorbs the optical signal  1440  and converts the optical signal  1440  into free carriers. In some implementations, a thickness of the intrinsic GeSi region  1410  may be between 0.05 μm to 2 μm. In some implementations, the n+ GeSi region  1412  may repel the holes generated from the intrinsic GeSi region  1410  to avoid surface recombination and thereby may increase the carrier collection efficiency. 
     The photo-generated free holes in the intrinsic GeSi region  1410  may drift or diffuse into the p-Si region  1406  via the p-GeSi region  1408 . The photo-generated free electrons in the intrinsic GeSi region  1410  may be repelled by the p-GeSi region  1408 , which prevents the free electrons from entering the p-Si region  1406 . In some implementations, a drain supply voltage V DD  may be applied to the NIR pixel  1450  to create an electric field between the n+ GeSi region  1412  and the p-Si region  1408 , such that the free holes may drift or diffuse towards the p-Si region  1406  while the free electrons may drift or diffuse towards the n+ GeSi region  1412 . 
     The second gate  1430  may be coupled to the second gate control signal  1438 . The second control signal  1438  controls a flow of free holes from the p-Si region  1406  to the p+ Si region  1436 . The p+ Si region  1436  may be coupled to the second readout circuit  1432  for further processing of the collected electrical signal. 
     Although not shown in  FIG. 14 , in some other implementations, the visible pixel  1452  may alternatively be designed into opposite polarity to collect holes instead of electrons and the NIR pixel  1450  may alternatively be designed into opposite polarity to collect electrons instead of holes. In this case, the p-Si substrate  1402  would be replaced by an n-Si substrate, the p-Si region  1406  would be replaced by an n-Si region, the n-Si regions  1404  and  1428  would be replaced by p-Si regions, the p+ Si region  1436  would be replaced by an n+ Si region, the n+ Si region  1416  would be replaced by a p+ Si region, the n+ GeSi region  1412  would be replaced by a p+ GeSi region, and the p-GeSi region  1408  would be replaced by an n-GeSi region. In some implementations, the direction of light signal shown in  FIG. 14  may be reversed depending on designs, packaging, and applications. For example, the NIR optical signal  1440  may enter the NIR pixel  1450  through the p-Si substrate  1402 , and the visible optical signal  1442  may enter the visible pixel  1452  through the p-Si substrate  1402  and the n-Si region  1404 . 
       FIG. 15  is an example multi-gate photodiode  1500  for converting an optical signal to an electrical signal. The multi-gate photodiode  1500  includes an absorption layer  1506  fabricated on a substrate  1502 . The substrate  1502  may be any suitable substrate where semiconductor devices can be fabricated on. For example, the substrate  1502  may be a silicon substrate. The coupling between the absorption layer  1506  and a first p+ Si region  1512  is controlled by a first gate  1508 . The coupling between the absorption layer  1506  and a second p+ Si region  1514  is controlled by a second gate  1510 . 
     In general, the absorption layer  1506  receives an optical signal  1512  and converts the optical signal  1512  into electrical signals. Although not shown in  FIG. 15 , in some implementations, the direction of the optical signal  1512  may be reversed depending on designs, packaging, and applications. For example, the optical signal  1512  may enter the multi-gate photodiode  1500  through the substrate  1502 . The absorption layer  1506  is selected to have a high absorption coefficient at the desired wavelength range. For NIR wavelengths, the absorption layer  1506  may be a GeSi mesa, where the GeSi absorbs photons in the optical signal  1512  and generates electron-hole pairs. The material composition of germanium and silicon in the GeSi mesa may be selected for specific processes or applications. In some implementations, the absorption layer  1506  is designed to have a thickness t. For example, for 850 nm wavelength, the thickness of the GeSi mesa may be approximately 1 μm to have a substantial quantum efficiency. In some implementations, the surface of the absorption layer  1506  is designed to have a specific shape. For example, the GeSi mesa may be circular, square, or rectangular depending on the spatial profile of the optical signal  1512  on the surface of the GeSi mesa. In some implementations, the absorption layer  1506  is designed to have a lateral dimension d for receiving the optical signal  1512 . For example, the GeSi mesa may have a circular shape, where d can range from 1 μm to 50 μm. 
     In some implementations, the absorption layer  1506  may include an n+ GeSi region  1531 . The n+ GeSi region  1531  may repel the holes from the surface of the absorption region  1506  and thereby may increase the device bandwidth. The multi-gate photodiode  1500  includes a p-well region  1504  implanted in the substrate  1502 . 
     The first gate  1508  is coupled to a first gate control signal  1522  and a first readout circuit  1524 . The second gate  1510  is coupled to a second control signal  1532  and a second readout circuit  1534 . The first gate  1508 , the first gate control signal  1522 , the first readout circuit  1524 , the second gate  1510 , the second gate control signal  1532 , and the second readout circuit  1534  are similar to the second gate  1428 , the second gate control signal  1438 , and the second readout circuit  1432  as described in reference to  FIG. 14 . 
     The first control signal  1522  and the second control signal  1532  are used to control the collection of holes generated by the absorbed photons. For example, when the first gate  1508  is turned “on” and the second gate  1510  is turned “off”, holes would drift from the p-well region  1504  to the p+ Si region  1512 . Conversely, when the first gate  1508  is turned “off” and the second gate  1510  is turned “on”, holes would drift from the p-well region  1504  to the p+ Si region  1514 . In some implementations, a voltage may be applied between the n+ GeSi region  1531  and the p-well  1504  to increase the electric field inside the absorption layer  1506  for drifting the holes towards the p-well region  1504 . 
     Although not shown in  FIG. 15 , in some other implementations, the photodiode  1500  may alternatively be designed into opposite polarity to collect electrons. In this case, the n-Si region  1502  would be replaced by an n-Si region, the p-well region  1504  would be replaced by an n-well region, the p+ Si regions  1512  and  1514  would be replaced by n+ Si regions, and the n+ GeSi region  1531  would be replaced by a p+ GeSi region. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, for the example photodiodes described in this application, a germanium-silicon alloy may be replaced by a germanium-tin alloy for applications where longer operating wavelengths at the infrared region is required. As another example, for the example photodiodes described in this application, the germanium concentration of the germanium-silicon alloy may be varied based on application or process constraints and/or requirements. The drawings shown in this application are for illustration and working principle explanation purpose. For example,  FIG. 2  does not limit that the orientation of the p+ GeSi region  212  to be at the bottom and the p+ Si region  204  to be at the top for packaging or operation purposes. Rather, the direction of the optical signal  220  would inform the orientation of the photodiode  200 , i.e., the first absorption region would receive the optical signal  220  before the second absorption region. 
     Various implementations may have been discussed using two-dimensional cross-sections for easy description and illustration purpose. Nevertheless, the three-dimensional variations and derivations should also be included within the scope of the disclosure as long as there are corresponding two-dimensional cross-sections in the three-dimensional structures. 
     While this specification contains many specifics, these should not be construed as limitations, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims may be performed in a different order and still achieve desirable results.