Abstract:
A photodiode structure is based on the use of a double junction sensitive to different wavelength bands based on a magnitude of a reverse bias applied to the photodiode. The monolithic integration of a sensor with double functionality in a single chip allows realization of a low cost ultra-compact sensing element in a single packaging useful in many applications which require simultaneous or spatially synchronized detection of optical photons in different spectral regions.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to photodiodes and particularly to monolithic photodetectors sensitive to radiation in at least two different wavelength bands ranging from ultraviolet to infrared. 
     Description of the Related Art 
     Photodiodes are semiconductor devices that contain a P-N junction, and often an intrinsic (undoped) layer between n and p layers. Devices with an intrinsic layer are called P-i-N photodiodes. Alternatively, a Schottky photodiode uses a metal-semiconductor junction as a Schottky junction rather than a semiconductor-semiconductor junction as in conventional photodiodes. Light absorbed in depletion regions or the intrinsic region in P-i-N photodiodes generates electron-hole pairs, most of which contribute to a photocurrent. Conventional photodiodes operate in different regions of the electromagnetic radiation spectrum. The particular semiconductor materials that make up the photodiode determine the particular wavelength or wavelength range of the radiation to which the photodiode responds. Photodiodes can be fabricated from elemental semiconductors, such as silicon, as well as compound semiconductors, such as gallium-arsenide, gallium nitride, or silicon carbide. 
     Broadband photodetectors having detection capabilities ranging from ultraviolet to near infrared can be useful in many applications such as data transport and storage in optical communications. Moreover there are several applications, mainly in military and industrial domains, which aspire to simultaneous or spatially synchronized detection of optical photons in different spectral regions. Photons emitted by fires, jet or rocket nozzles or stellar luminaries for example have typical wavelengths ranging from ultraviolet to infrared. All these emissions are detected over the ambient light background by two or more detectors having different sensitivity ranges, or better by a single fast multi-range photodetector giving the possibility to perform time resolved measurements in different optical bands. These known devices can have false-alarm warnings. To avoid the false-alarm warnings it is important to know an appropriate photodetector spectral range and appropriate device speed and spatial resolution. For all these applications, photomultiplier tubes (PMT) are typically used as photodetectors. For instance Hamamatsu PMT UV Tron R2868 is commonly used for flame and fire alarm detection. However, although PMTs have high sensitivity and good timing response, these devices are bulky, expensive, sensitive to magnetic fields, require high operating bias, and have low mechanical and temperature strength. 
     Another possibility could be the use of two or more discrete solid state detectors having different sensitivity ranges in a single housing. However, this solution does not allow for detection of multiband optical signals with high spatial resolution and, furthermore, metallic or ceramic packages used to house these devices cannot withstand temperatures as high as those achieved in flame monitoring systems. An additional possibility could be the use of different external (and expensive) interferential filters to make a single broadband photodiode sensitive only in selected portions of the electromagnetic spectrum. However, this solution typically implies the use of a broadband photodetector and, what&#39;s more, does not allow a fast detection of photons with different wavelengths due to the use of the external filters usually housed in appropriate filter wheels controlled, for instance, by remote. 
     Moreover the capability of using a single detector as a receiver makes the detection system less expensive and more compact, and, not least, could make it easier for the system vendors to reduce the inventory. 
     A low operating bias is particularly relevant in low power consumption applications. Electro-tunable multiband photodiodes have known a growing success in last years. The optical absorption of these devices realized by stacking multilayers of different materials with different optical properties and energy gaps (i.e., high cut off wavelength) are modulated by varying the applied reverse bias. 
     The possibility to modulate the absorption and eventually increase the photodetector sensitivity wavelength range by changing the applied reverse bias has been found to be a powerful tool to reduce the overall number of photodetectors used for the detection of photons in a wide wavelength range. 
     A multiband spectral infrared photodetector and imager are disclosed by Mitra in US Pat. App. No. 2004/0108461, in which two or more different bands in the infrared wavelength range are detected by a diffractive resonant optical cavity. Furthermore, a device and the relative fabrication method for a two-color infrared detector are disclosed by Park et al. in U.S. Pat. No. 6,049,116, and a multilayer junction photodiode for multiple infrared wavelengths detection is presented by Dutta in US Pat. App. No. 2009/0189207. 
     On the other hand, Tsang, in US Pat. App. No. 2009/0159785 presents an optical sensing device with multiple photodiode elements and multi-cavity Fabry-Perot ambient light structure in order to detect light signals with different wavelength spectrums. 
     Korona et al., in the paper “Multiband GaN/AlGaN UV Photodetector, Acta Physica Polonica A, Vol. 110, No 2, pp. 211-217, 2006,” present a multiband GaN/AlGaN photodetector structure capable of detecting three UV ranges, tuned by external voltage. The multilayer structures were grown by a Metal Organic Chemical Vapor Deposition (MOCVD) technique, while the device was designed as a Schottky photodiode with a semitransparent continuous gold Schottky contact evaporated on its surface. 
     However, at the moment these devices experience a poor technological maturity, as is highlighted by their high leakage (dark) current and often by a not optimized control of the optical absorption (i.e., the photoresponse) in different wavelength bands with the bias. 
     BRIEF SUMMARY 
     One embodiment is a multiband photodiode capable of detecting light in distinct wavelength bands. By applying a first voltage between an anode and a cathode of the photodiode, the photodiode is sensitive to light in a first wavelength band and insensitive to light in a second wavelength band. By applying a second voltage between the anode and cathode, the photodiode becomes sensitive to light in the second wavelength band. 
     In one embodiment, the photodiode includes a first layer of semiconductor material, a second layer of semiconductor material positioned on top of the first layer of semiconductor material, and a contact material positioned on the first and second layers of semiconductor material. The photodiode includes a first photodiode junction at an interface between the first layer of semiconductor material and the contact material. The first photodiode junction is positioned below a portion of a first electrode that is opaque to light. The photodiode includes a second photodiode junction at an interface of the second layer of semiconductor material and the contact material. An aperture in the first electrode allows light to pass to respective light receiving areas of the first and second layers of semiconductor material. 
     In one embodiment, when a first voltage is applied between the first and second electrodes, a depletion area in the second layer of semiconductor material is positioned in the light receiving area of the second layer of semiconductor material, thereby rendering the photodiode sensitive to light in a first wavelength band. The first electrode may be an anode and the second electrode may be a cathode, as discussed in more detail below. When the first voltage is applied between the first electrode and the second electrode, a depletion region in the first layer of semiconductor material does not extend to the light receiving area of the first layer of semiconductor material, thereby rendering the photodiode insensitive to a second wavelength band. When a second voltage is applied between the first and second electrodes, the depletion region in the first layer of semiconductor material extends to the light receiving area of the first layer of semiconductor material, thereby rendering the photodiode sensitive to the second wavelength band. When the first voltage is applied between the first and second electrodes, a depletion region in the second layer of semiconductor material is positioned in the light receiving area of the first layer of semiconductor material, thereby rendering the photodiode sensitive to light in a first wavelength band. 
     In one embodiment, the contact material is a material that forms a photo diode junction at an interface of the contact material with the second or third layer of semiconductor material. Accordingly, in one embodiment, the contact material is a Schottky contact that forms a Schottky barrier at an interface with the second or third layer of semiconductor material. Alternatively, the contact material can be a semiconductor material of opposite conductivity type (P or N) with the second or third layer. For example, if the second and third layers of semiconductor material are doped with N-type conductivity, then the contact material can be semiconductor material that is doped with P-type conductivity. The contact material can be monocrystalline with the second or third layer of semiconductor material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  is cross section of an integrated circuit die including a multiband Schottky photodiode, according to one embodiment. 
         FIG. 1B  is a cross section of the integrated circuit die of  FIG. 1A  when a first voltage is applied to electrodes of the multiband Schottky photodiode, according to one embodiment. 
         FIG. 1C  is a graph illustrating responsivity of the photodiode of  FIG. 1A  when the first voltage is applied, according to one embodiment. 
         FIG. 1D  is a cross section of the integrated circuit die of  FIG. 1A  when a second voltage is applied to electrodes of the multiband Schottky photodiode, according to one embodiment. 
         FIG. 1E  is a graph illustrating responsivity of the photodiode of  FIG. 1A  when the second voltage is applied, according to one embodiment. 
         FIGS. 1F-1K  are cross sections of the integrated circuit die of  FIG. 1A  at intermediate stages of processing, according to one embodiment. 
         FIG. 2A  is a cross section of an integrated circuit die including a multiband P-N photodiode when a first voltage is applied to electrodes of the multiband P-N photodiode, according to one embodiment. 
         FIG. 2B  is a cross section of the integrated circuit die of  FIG. 2A  when a second voltage is applied to electrodes of the multiband P-N photodiode, according to one embodiment. 
         FIG. 2C  is a cross section of an integrated circuit die including a multiband P-N photodiode with a buffer layer positioned between a first and a second layer of semiconductor material, according to one embodiment. 
         FIG. 3  is a cross section of an integrated circuit die including a multiband Schottky photodiode having a layer of dielectric material positioned between a first layer of semiconductor material and a second layer of semiconductor material, according to one embodiment. 
         FIG. 4  is a block diagram of a system including a multiband photodiode, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a cross section of an integrated circuit die  20  including a multiband photodiode  22 , according to one embodiment. The integrated circuit die  20  includes a first layer of semiconductor material  24 , a second layer of semiconductor material  26  positioned on the first layer of semiconductor material  24 , and a third layer of semiconductor material  28  positioned on the second layer of semiconductor material  26 . The integrated circuit die  20  further includes a first Schottky contact  30   a  positioned on the second layer of semiconductor material  26 , a second Schottky contact  30   b  positioned on the second layer of semiconductor material  26 , and a third Schottky contact  30   c  positioned on the third layer of semiconductor material  28 . A first anode  32   a  is positioned on the first Schottky contact  30   a . A second anode  32   b  is positioned on the second Schottky contact  30   b . A third anode  32   c  is positioned on the third Schottky contact  30   c . Respective dielectric barriers  34   a ,  34   b  are positioned between the third layer of semiconductor material  28  and the first and second anodes  32   a ,  32   b . An ohmic contact  36  is positioned below the first layer of semiconductor material  24 . A cathode  38  is positioned below the ohmic contact  36 . A junction between the second layer of semiconductor material  26  and the first contact  30   a  is a first Schottky photodiode junction  40   a . A junction between the second layer of semiconductor material  26  and the second contact  30   b  is a second Schottky photodiode junction  40   b . A junction between the third contact  30   c  and the third layer of semiconductor material  28  is a third Schottky photodiode junction  40   c.    
     In one embodiment, the first layer of semiconductor material  24  is monocrystalline 4H—SiC about 300 μm thick. The first layer of semiconductor material  24  is highly doped with N-type dopant atoms at a concentration of about 1e19/cm 3 . The second layer of semiconductor material  26  is monocrystalline 4H—SiC about 5 μm thick. The second layer of semiconductor material  26  is moderately doped with N-type dopant atoms at a concentration between about 5e15/cm 3  and 5e16/cm 3 . The third layer of semiconductor material  28  is monocrystalline 4H—SiC about 5 μm thick. The third layer of semiconductor material  28  is lightly doped with N-type dopant atoms at a concentration between about 8e13/cm 3  and 2e14/cm 3 . The 4H—SiC material is sensitive in the UV region. Other materials, such as silicon are used in other embodiments to detect other wavelengths, such as infrared. The sensitivity changes based on the selected material. 
     In one embodiment, the Schottky contacts  30   a - 30   c  are formed of a very thin layer of metal selected to form a Schottky barrier with the second layer of semiconductor material  26  and the third layer of semiconductor material  28 . In one example, the Schottky contacts  30   a - 30   c  are a thin layer of Ni about 10-30 nm thick formed on top of respective portions of the second layer of semiconductor material  26  and the third layer of semiconductor material  28 . This nickel layer may also be a nickel silicide layer or other appropriate material. The nickel silicide layer may be formed by a thin nickel layer being deposited, followed by a low temperature rapid thermal annealing process at 700 C, for 20 seconds in a nitrogen environment. In an alternative embodiment, the ohmic contact may be a titanium layer that becomes a titanium silicide after a rapid thermal anneal, such as at 1000 degrees Celsius. The titanium silicide layer would be different from the cathode  38 . 
     In one embodiment, the first, second, and third anodes  32   a - 32   c  are formed of a layer of conductive material, such as AlSiCu, about 3 μm thick. The first, second, and third anodes  32   a - 32   c  are electrically connected to the respective Schottky contacts  30   a - 30   c  on which they are positioned. Though not shown, the first, second, and third anodes  32   a - 32   c  re electrically shorted together, thereby forming a single collective anode. Though features  32   a - 32   c  may be recited in this description as each being a separate anode, in one embodiment, the anodes  32   a - 32   c  are collectively a single anode. 
     In one embodiment, the dielectric barriers  34   a ,  34   b  are SiO 2  about 3-6 μm thick. The dielectric barriers  34   a ,  34   b  separate the third layer of semiconductor material  28  from the Schottky contacts  30   a ,  30   b  and the anodes  32   a ,  32   b.    
     In one embodiment, the ohmic contact  36  is a thin layer of conductive material selected to form an ohmic contact with the highly doped first layer of semiconductor material  24 . The ohmic contact  36  may be a nickel silicide layer (Ni2Si), which may be formed by sputtering a nickel layer to a thickness of 200 nm, followed by a rapid thermal annealing process (RTA) at 1000 C, 60 seconds in a nitrogen environment. 
     In one embodiment, the cathode  38  is a highly conductive material, such as a titanium, nickel, gold layer, positioned on the bottom of the ohmic contact  36 . The cathode  38  is about 550 nm thick. For example, the titanium layer may be 100 nm thick, the nickel layer may be 400 nm thick, and the gold layer may be 50 nm thick. Though the cathode  38  and the ohmic contact  36  are referred to separately, the ohmic contact and the cathode  38  can be considered collectively to be a cathode in ohmic contact with the first layer of semiconductor material  24 . 
     The junctions  40   a - 40   c  are Schottky barriers that have a rectifying function, i.e., current only flows in one direction across the junction. Thus, the junctions  40   a - 40   c  act as individual diodes that allow current to flow in one direction but not the other. 
     Under forward bias conditions, i.e., when the anodes  32   a - 32   c  have a higher voltage than the cathode  38 , a continuous current flows across the junctions  40   a - 40   c  from the Schottky contacts  30   a - 30   c  to the second and third semiconductor layers  26 ,  28 , and on to the cathode  38 . In other words, under forward bias conditions, current flows from the anodes  32   a - 32   c  to the cathode  38 . 
     Under reverse bias conditions, i.e., the anodes  32   a - 32   c  have a lower voltage than the cathode  38 , the junctions  40   a - 40   c  act as photodiode junctions. Under reverse bias conditions, a depleted region forms in the second and third layers of semiconductor material  26 ,  28  near the junctions  40   a - 40   c . The depleted region is characterized by a lack of free charges. Previously free electrons have combined with holes, leaving a region depleted of free charges. An electric field is present in the depleted region. The direction of the electric field is such that any free electrons are driven away from the junction toward the cathode  38 . Photo-generated holes can contribute to the measured signal. They are swept away towards the anode (top contact) while the electrons are moved towards the cathode (bottom contact). The Schottky diode is normally a unipolar device in normal operation conditions. However, during photo-generation holes can give a contribution to the photocurrent. 
     An electron hole pair can be generated if an electron in the valence band absorbs a photon in the depleted region. In particular, if a photon having an energy greater than or equal to the bandgap between the valence band and the conduction band of the second or third layer of semiconductor material  26 ,  28  is absorbed by an electron in the second or third layer of semiconductor material  26 ,  28 , then the electron gains enough energy to transition from the valence band to the conduction band. This generates a free electron and a hole. Because the electron is now in the conduction band, the electric field in the depletion region drives the electron to the cathode  38 . As more electrons absorb light and transition to the conduction band, a current is generated in the photodiode  22 . The current is proportional to the intensity of light having a particular energy incident on the photodiode  22 . 
     The energy E of a photon is given by the following equation:
 
 E=h*f,  
 
where f is the frequency of the light and h is Planck&#39;s constant. The frequency f of light is related to the wavelength λ of light by the following equation:
 
 f=c/λ,  
 
where c is the speed of light. Thus, the wavelength λ of the photon is related to the energy E of the photon by the following equation:
 
λ= h*c/E.  
 
     The bandgap is a property particular to a given semiconductor material. The bandgap is the energy difference between the valence band, in which an electron is not free to move through a crystal lattice, and the conduction band, in which the electron is free to move throughout the crystal lattice. If an electron in the valence band, i.e., an electron currently paired with a hole, absorbs a photon having an energy equal to or slightly greater than the bandgap of the semiconductor material, then the electron transitions across the bandgap from the valence band to the conduction band. Thus, photons in a particular range of wavelengths can cause electrons to transfer from the valence band to the conduction band, thereby generating current in the photodiode  22 . 
       FIG. 1B  is a cross section of the integrated circuit die  20  of  FIG. 1A  with a first voltage V 0  applied between the anodes  32   a - 32   c  and the cathode  38  by a power supply  46 . In one example, the first voltage V 0  is 0 V. Alternatively, the first voltage V 0  can be a small reverse bias voltage less than 2 V in absolute magnitude, but greater than 0 V in absolute magnitude. Under reverse bias conditions, the anodes  32   a - 32   c  have a lower voltage than the cathode  38 . 
     Dashed lines indicate depletion regions  42   a - 42   c  in the second and third layers of semiconductor material  26 ,  28 , i.e., regions where there are no free holes or electrons. The depletion region  42   a  corresponds to the depletion region in the second layer of semiconductor material  26  adjacent to the junction  40   a  with the Schottky contact  30   a . The depletion region  42   b  corresponds to the depletion region in the second layer of semiconductor material  26  adjacent to the junction  40   b  with the Schottky contact  30   b . The depletion region  42   c  corresponds to the depletion region in the third layer of semiconductor material  28  adjacent to the Schottky contact  30   c.    
     The sizes of the depletion regions  42   a - 42   c  depend on the magnitude of the voltage V 0  and the concentration of dopant atoms in the second and third layers of semiconductor material  26 ,  28 . The higher the concentration of dopant atoms in a layer of semiconductor material, the smaller the depletion region will be at low-voltage. Because there is a much higher concentration of dopant atoms in the second layer of semiconductor material  26  (˜1e16/cm 3 ) than in the third layer of semiconductor material  28  (˜1e14/cm 3 ), the depletion regions  42   a ,  42   b  are much smaller than the depletion region  42   c  in the third layer of semiconductor material  28  at V 0 =0 V. 
     Light (photons)  44 , shown as straight arrows in  FIG. 1B , is incident on the photodiode  22 . The relatively thick conductive material of the anodes  32   a - 32   c  is opaque to the light  44 . Thus, the light  44  does not pass to those regions of the second and third layers of semiconductor material  26 ,  28  that are directly below the anodes  32   a - 32   c . However, the thin Schottky contact  30   c  is partially transparent to light, thereby allowing the light  44  to pass through the Schottky contact  30   c  to the second and third layers of semiconductor material  26 ,  28  directly below the portion of the Schottky contact  30   c  that is not covered by the anode  32   c . Thus, the gap between the anodes  32   a ,  32   b  is an aperture through which the light  44  can pass, except through the anode  32   c . Those areas of the second and third layers of semiconductor material  26 ,  28  directly below the aperture in the anodes  32   a - 32   c  are light receiving areas of the second and third layers of semiconductor material  26 ,  28 . 
     Some of the light  44  passing through the Schottky contact  30   c  will be absorbed by the third layer of semiconductor material  28 . Additionally, some of the light  44  will pass through the third layer of semiconductor material  28  into the second layer of semiconductor material  26  and will be absorbed by the second layer of semiconductor material  26 . When the light  44  is absorbed by an electron in the depletion region  42   c  of the third layer of semiconductor material  28 , the electron enters the conduction band and is driven to the cathode  38 , thereby generating current. However, because the depletion regions  42   a ,  42   b  do not extend directly below the Schottky contact  30   c , the photodiode  22  is not sensitive to longer wavelengths of light when the voltage V 0  is applied between the anodes  32   a - 32   c  and the cathode  38 . 
       FIG. 1C  is a graph of the responsivity of the photodiode  22  versus wavelength of light λ when the voltage V 0  is applied between the anodes  32   a - 32   c  and the cathode  38 . There is a peak responsivity at a wavelength of λ=250 nm. In other words, the photodiode  22  generates a photo current that is proportional to the intensity of light incident on the photodiode  22  with a wavelength λ of 250 nm. 
       FIG. 1D  is a cross section of the integrated circuit die  20  of  FIG. 1A  under conditions in which a higher reverse bias voltage V 1  is applied between the anodes  32   a - 32   c  and the cathode  38 . The photodiode  22  in  FIG. 1D  is substantially similar to the photodiode  22  of  FIG. 1B  except that the voltage V 1  is applied between the anodes  32   a - 32   c  and the cathode  38 . The application of the higher reverse bias voltage V 1  has caused the depletion regions  42   a ,  42   b  in the second layer of semiconductor material  26  to expand and merge together. Most notably, the depletion regions  42   a ,  42   b  now extend to the light receiving area below the Schottky contact  30   c.    
     Because the depletion regions  42   a ,  42   b  in the second layer of semiconductor material  26  now extend to the light receiving areas below the Schottky contact  30   c , the effective depth of the depletion region  42   c  has increased to include the depth of the second layer of semiconductor material  26 , thereby rendering the photodiode  22  responsive to light with a longer wavelength λ. In particular, the photodiode  22  now has two peak sensitivity wavelengths λ: 250 nm and 330 nm. This is most clearly illustrated by the graph of  FIG. 1E . 
       FIG. 1E  is a graph of the responsivity of the photodiode  22  versus wavelength of light λ when the voltage V 1  is applied between the anodes  32   a - 32   c  in the cathode  38 . There are now two peaks in the responsivity: one at λ=250 nm and one and λ=330 nm. Wavelengths between the two peaks are only weakly absorbed. The sensitivity of the photodiode  22  does not change for λ=250 nm when the voltage applied between the anodes  32   a - 32   c  in the cathode  38  increases from V 0  to V 1 . Thus, the current in the photodiode  22  at V 0  gives an indication of the intensity of light having a wavelength of λ=250 nm with little or no contribution from light having a wavelength of λ=330 nm. The current in the photodiode  22  at V 1  includes a contribution from light at both λ=250 nm and λ=330 nm. The difference in the currents in the photodiode at V 1  and V 0  gives an indication of the intensity of light having a wavelength of 330 nm. Thus, a single photodiode  22  can effectively measure two distinct bands of wavelengths by merely switching between a first voltage V 0  and a second voltage V 1  applied between the anodes  32   a - 32   c  in the cathode  38  and measuring the current when V 0  is applied as well as the difference between the currents when V 0  and V 1  are applied. 
     The particular values of the wavelength bands can be chosen by carefully selecting the thicknesses and doping profiles of the second and third layers of semiconductor material  26 ,  28 . The thickness of the third epitaxial layer can be suitably chosen in order to select a cut-off high wavelength for the lower range wavelengths. Likewise, the thickness of the second epitaxial layer  26  can be chosen to select a higher cut off wavelength for the higher range wavelengths. The doping and thickness of the second layer of semiconductor material are chosen suitably high in order to avoid depletion at low reverse bias. 
     Moreover the thick metal anodes  32   a ,  32   b  layer above the lateral junctions  40   a ,  40   b , acts as a blinding layer and thus does not allow the absorption of low wavelength photons at these junctions. The application of a higher reverse bias (typically anode contact set at ground, cathode set at a positive bias) depletes the second layer of semiconductor material  26  below the central Schottky contact  30   c . The application of this higher reverse bias is not able to modify the thickness of the depletion region  42   c  in the third layer of semiconductor material  28  because it is fully depleted already at very low reverse bias. However application of the higher reverse bias voltage V 1  expands the depletion regions  42   a ,  42   b  of the second layer of semiconductor material  26  to the light receiving areas underlying the fully depleted third layer of semiconductor material  28 . This increases the thickness of the active area of the photodiode, thereby making it sensitive to longer wavelengths (i.e. the range of sensitivity of the first junction is enlarged towards higher wavelength values due to the increase of the thickness of the active layer). In this way the photodiode  22  can be sensitive in different wavelength ranges simply by switching the reverse bias from a low value V 0  to a high value V 1  and can again become sensitive to only the first limited range of wavelengths by switching the bias from the high voltage V 1  to the low voltage V 0  chosen for the operation of the photodiode  22 . 
     The photodiode structure of  FIG. 1A  is extremely useful in all the applications where two or more wavelength photon fluxes in different wavelength ranges have to be detected. 
     Though particular layer thicknesses, layer arrangements, doping schemes, and materials have been described in relation to  FIGS. 1A-1E , those of skill in the art will recognize that many alternate arrangements are possible. For example, the layers of semiconductor material can be doped P-type and the material for the Schottky contacts  30   a - 30   c  can be selected to form a Schottky barrier with the P-type semiconductor materials. 
     While  FIGS. 1A, 1B, 1D  have shown the Schottky contacts  30   a ,  30   b  as being distinct, in practice the Schottky contacts  30   a ,  30   b  can be a single lateral Schottky contact surrounding the center Schottky contact  30   c . The single lateral Schottky contact can be annular in shape, rectangular in shape, or any other suitable shape. Likewise, the anodes  32   a ,  32   b  can be a single integral anode having an annular or rectangular aperture therein in which the central Schottky contact  30   c  can be positioned. 
       FIGS. 1F-1K  illustrate sequential process steps for manufacturing the photodiode  22  of  FIG. 1A . In  FIG. 1F , the first layer of semiconductor material  24 , which could be monocrystalline 4H—SiC about 300 nm thick, is doped with N-type donor atoms with a concentration of about 1e19/cm 3 . The second layer of semiconductor material  26  is epitaxially grown from the first layer of semiconductor material  24 . The second layer of semiconductor material  26  may be doped with N-type donor atoms with a concentration between about 5e15/cm 3  and 5e16/cm 3 . The second layer of semiconductor material  26  can be doped in situ during the epitaxial growth. The second layer of semiconductor material  26  may be about 5 μm thick. 
     In  FIG. 1G  the top surface of the integrated circuit die  20  is cleaned to remove any impurities or any dielectric growth on the surface of the second layer of semiconductor material  26 . A sacrificial layer, such as tantalum carbide  48  is formed on the second layer of semiconductor material  26 . The layer of tantalum carbide  48  is about 5 μm thick. The layer of tantalum carbide  48  can be formed by chemical vapor deposition (CVD) or by any other suitable method. 
     In  FIG. 1H  the layer of tantalum carbide  48  is patterned and etched via common photolithography techniques to remove a central portion of the layer of tantalum carbide  48 . A selective epitaxial growth is then performed whereby the third layer of semiconductor material  28  is grown from the second layer of semiconductor material  26  in a gap opened in the layer of tantalum carbide  48 . The third layer of semiconductor material  28  may be about 5 μm thick. The third layer of semiconductor material  28  may be doped with N-type donor atoms with a concentration of about 1e14/cm 3 . The third layer of semiconductor material  28  can be doped in situ during the epitaxial growth or by other implantation techniques. 
     The ohmic contact  36  is formed on the backside of the first layer of semiconductor material  24  after the tantalum carbide layer is etched and after the third layer of semiconductor material is formed. A protective silicon nitride layer may be formed over the tantalum carbide layer and the third layer of semiconductor material to protect these layers as the ohmic contact is formed. As noted above, the ohmic contact may be a nickel silicide or other appropriate material. 
     In  FIG. 1I , dielectric barriers  34   a ,  34   b  are formed adjacent to the sides of the third layer of semiconductor material  28 . The dielectric barriers  34   a ,  34   b  are for example silicon dioxide or TEOS. The silicon dioxide can be formed by CVD. The dielectric barriers  34   a ,  34   b  are patterned to exposed portions of the second layer of semiconductor material  26 . While the dielectric barriers  34   a ,  34   b  are shown as distinct dielectric barriers, in practice the dielectric barriers  34   a ,  34   b  can be a single continuous annular or rectangular dielectric barrier surrounding the third layer of semiconductor material  28 . 
     In  FIG. 1J , Schottky contacts  30   a ,  30   b ,  30   c  are formed by depositing a thin layer of conductive material, such as nickel by physical vapor deposition on the exposed surfaces of the second layer of semiconductor material  26 , the dielectric barriers  34   a ,  34   b , and the third layer of semiconductor material  28 . The thin layer of conductive material can be annealed to form a silicide and can be patterned and etched using common photolithography techniques to leave the Schottky contacts  30   a - 30   c . Alternatively, depending on the materials used, portions of the conductive material that have not reacted with the underling layer of semiconductor material can be removed with a wet etch. The Schottky contacts  30   a ,  30   b ,  30   c  are formed from semitransparent material, which nickel silicide can be. The nickel may be sputtered to be 10 nm thick and then be subjected to a rapid thermal anneal at 700 degrees Celsius for 20 seconds in an N2 environment. The non-annealed nickel layer can be removed by a wet etch. 
     In  FIG. 1K , anodes  32   a - 32   c  are formed by depositing another layer of conductive material, such as AlSiCu or a Ti/AlSiCu on the exposed surfaces of the Schottky contacts  30   a - 30   c  and on the dielectric barriers  34   a ,  34   b . The layer of conductive material is then patterned and etched using common photolithography techniques to leave anodes  32   a - 32   c . Patterning of the anodes  32   a - 32   c  leaves an aperture  50  in the anode through which light can pass to light receiving areas of the second and third layers of semiconductor material  26 ,  28 . The anodes  32   a - 32   c  are optically blinding such that they prevent photons from entering the semiconductor layer below the anode. The anode is thick to ensure the anode is “photon blind,” the junction below the anode is not optically active. 
     At this point, the cathode  38  is formed on the ohmic contact on the backside of the first layer of semiconductor material  24  in order to obtain the structure shown in  FIG. 1A . The cathode  38  is a conductive layer, which may include titanium, nickel, and gold, or other suitable materials. The cathode can be formed by depositing, via physical vapor deposition, a thin layer of titanium followed by a thin layer of nickel and another thin layer of gold. The titanium is for example 100 nm thick. The nickel is for example 400 nm thick. The gold is, for example, 50 nm thick. 
     While particular materials, layers, configurations, processes, and dimensions have been described in relation to forming the photodiode  22  of  FIG. 1A , those of skill in the art will recognize, in light of the present disclosure, that many other materials, arrangements, processes, and dimensions can be used to obtain the photodiode  22  in accordance with principles of the present disclosure. 
       FIG. 2A  is a cross section of an integrated circuit die  220  including a multiband photodiode  222 , according to one embodiment. The integrated circuit die  220  includes a first layer of semiconductor material  224 , a second layer of semiconductor material  226  positioned on the first layer of semiconductor material  224 , and a third layer of semiconductor material  228  positioned on the second layer of semiconductor material  226 . The first, second, and third layers of semiconductor material  224 ,  226 ,  228  are doped, for example, N. The integrated circuit die  220  further includes a first P-type contact  230   a  positioned on the second layer of semiconductor material  226 , a second P-type contact  230   b  positioned in the second layer of semiconductor material  226 , and a third p-type contact  230   c  positioned in the third layer of semiconductor material  228 . A first anode  232   a  is positioned on the first P-type contact  230   a . A second anode  232   b  is positioned on the second P-type contact  230   b . A third anode  232   c  is positioned on the third P-type contact  230   c . Respective dielectric barriers  234   a ,  234   b  are positioned between the third layer of semiconductor material  228  and the first and second anodes  232   a ,  232   b . An ohmic contact  236  is positioned below the first layer of semiconductor material  224 . A cathode  238  is positioned below the ohmic contact  236 . A junction between the N-type portion of the second layer of semiconductor material  226  and the first P-type contact  230   a  is a first P-N photodiode junction  240   a . A junction between the N-type portion of the second layer of semiconductor material  226  and the second P-type contact  230   b  is a second P-N photodiode junction  240   b . A junction between the third P-type contact  230   c  and the N-type third layer of semiconductor material  228  is a third P-N photodiode junction  240   c.    
     In one embodiment, the first layer of semiconductor material  224  is monocrystalline Si about 500 μm thick. The first layer of semiconductor material  224  is highly doped with N-type dopant atoms with a concentration of about 1e19/cm 3 . The second layer of semiconductor material  226  is monocrystalline Si about 5 μm thick. The second layer of semiconductor material  226  is moderately doped with N-type dopant atoms at a concentration of about 1e16/cm 3 . The third layer of semiconductor material  228  is monocrystalline Si about 5 μm thick. The third layer of semiconductor material  228  is lightly doped with N-type dopant atoms at a concentration of about 1e14/cm 3 . 
     In one embodiment, the P-type contacts  230   a - 230   c  are doped with P-type acceptor atoms with a concentration between about 1e18/cm 3  and 1e19/cm 3 . 
     In one embodiment, the first, second, and third anodes  232   a - 232   c  are formed of a layer of conductive material, such as AlSiCu, about 3 μm thick. The first, second, and third anodes  232   a - 232   c  are electrically connected to the respective P-type contacts  230   a - 230   c  on which they are positioned. The first, second, and third anodes  232   a - 232   c  are electrically shorted together, thereby forming a single collective anode. 
     In one embodiment, the dielectric barriers  234   a ,  234   b  are SiO 2  about 3-6 μm thick. The dielectric barriers  234   a ,  234   b  separate the third layer of semiconductor material  228  from the P-type regions  230   a ,  230   b  and the anodes  232   a ,  232   b.    
     In one embodiment, the ohmic contact  236  is a thin layer of conductive material selected to form an ohmic contact with the highly doped first layer of semiconductor material  224 . The ohmic contact  236  can include a metal, such nickel silicide in direct contact with the bottom side of the first layer of semiconductor material  224 . 
     In one embodiment, the cathode  238  is a highly conductive material, such as a combination layer of titanium, nickel, and gold, positioned on the bottom of the ohmic contact  236 . The cathode  238  is about 550 nm thick. 
     The photodiode  222  of  FIG. 2A  operates according to substantially similar principles of the photodiode  22  of  FIG. 1B . In particular, when a low-voltage V 0  (for example 0 V) is applied between the anodes  232   a - 232   c  and the cathode  238  by the power supply  246 , the lightly doped third layer of semiconductor material is nearly entirely depleted. In other words, the depletion region  242   c  of the third layer of semiconductor material  228  takes up nearly the entire third layer of semiconductor material  228 . Thus light passes through the top surface of the P-type region  230   c  or through the top of the third layer of semiconductor material  228  and can be absorbed in the third layer of semiconductor material  228  or in the second layer of semiconductor material  226 . If light is absorbed in the depletion region  242   c  of the third layer of semiconductor material  228 , then current passes between the anode  232   c  and the cathode  238 . Because the depletion regions  242   a ,  242   b  do not extend directly below an aperture  251  in the anodes  232   a - 232   c  when the low-voltage V 0  is applied between the anodes  232   a - 232   c  and the cathode  238 , light that passes to the second layer of semiconductor material  226  will not generate current in the photodiode  222 . The photodiode  222  absorbs light with a wavelength of about 300 nm when the voltage at V 0  is applied between the anodes  232   a - 232   c  and the cathode  238 . 
     In  FIG. 2B , a high reverse bias voltage V 1  (for example, −10 V) is applied between the anodes  232   a - 232   c  and the cathode  238 . As a result, the depletion regions  242   a ,  242   b  in the second layer of semiconductor material  226  expand until they join and take up nearly the entirety of the second layer of semiconductor material  226 . Thus, the depletion regions  242   a ,  242   b  extend directly below the third P-type contact  230   c  through which light can pass to the third layer of semiconductor material  228  and the second layer of semiconductor material  226 . Because depletion regions  242   a ,  242   b  have increased the effective depth of the depletion region  242   c , the photodetector  222  is able to detect light at two different peaks in two different wavelength bands. In particular, the photodiode  222  is sensitive to light at 300 nm and 800 nm in wavelength in a similar fashion as described previously with respect to  FIG. 1D . 
     The photodiode  222  of  FIGS. 2A, 2B  can be manufactured by growing the second and third layers of semiconductor material  226 ,  228  epitaxially. The second and third layers of semiconductor material  226 ,  228  can be doped in situ during the epitaxial growth. 
     After the epitaxial growth of the two epitaxial layers, a sacrificial layer can be deposited, such as a TEOs layer that is 8 um thick. The sacrificial layer can be patterned to form a mask to define a shape of the third layer of semiconductor material. A photoresist layer may be used as the mask. The third layer of semiconductor material can be etched, such as a trench etch around the central junction to form the remaining third layer of semiconductor material  228 . The trench depth is at least higher than a thickness of the third semiconductor  228  and lower than a sum of the thicknesses of the second and third semiconductor layers  226  and  228 . Once the shape of the third layer of semiconductor is formed, the sacrificial layer is removed. 
     A layer of dielectric material, such as silicon dioxide about 1 μm thick, is then deposited by chemical vapor deposition on the second and third layers of semiconductor material  226 ,  228 . The layer of the dielectric material is patterned and etched to expose the portions of the second and third layers of semiconductor material  226 ,  228  that will become the P-type contacts  230   a - 230   c . A very thin oxide layer is then grown on the exposed portions of the second and third layers of semiconductor material  226 ,  228 . Acceptor atoms are then implanted through the thin oxide layer into the second and third layers of semiconductor material  226 ,  228 , thereby leaving P-type contacts  230   a - 230   c  on the second and third layers of semiconductor material  226 ,  228 . The thin layer of dielectric material is then removed and anodes  232   a - 232   c  are formed on the P-type contacts  230   a - 230   c  in substantially the same manner as the anodes  30   a - 30   c  described with respect to  FIG. 1A . The ohmic contact  236  and the cathode  238  are likewise formed in substantially the same manner as the ohmic contact  36  and the cathode  38  of  FIG. 1A . 
       FIG. 2C  includes a photodiode  222  according to an alternate embodiment. The photodiode  222  of  FIG. 2C  is substantially similar to the photodiode  222  of  FIGS. 2A, 2B . However, the photodiode  222  of  FIG. 2C  includes a fourth layer of semiconductor material  250  positioned between the second layer of semiconductor material  226  and the third layer of semiconductor material  228 . The fourth layer of semiconductor material  250  is a highly doped buffer layer between about 100 nm and 1 μm in thickness. The buffer layer  250  is moderately doped with N-type donor atoms at a concentration between about 5e15/cm 3  and 6e15/cm 3 . When the high reverse bias voltage V 1  (e.g. =10 V is applied between the anodes  232   a - 232   c  and the cathode  238 , the buffer layer  250  is not depleted and is therefore not optically active. This results in a further decrease in sensitivity to light in the range between the first and second peaks of 300 nm and 800 nm in wavelength, if made with silicon. This equates to an increase in the sensitivity of the photodiode  222 . To have the wavelengths of 250 nm and 330 nm for peaks, the devices described in  FIGS. 2A, 2B, and 2C  are formed using 4H—SiC. 
     The buffer layer  250  can also be implemented in the photodiode  222  of  FIG. 1A  with a similar result as that described above with respect to  FIG. 2C . 
     In an alternative embodiment, the dielectric barriers  234   a ,  234   b  may have extensions such that a portion of the dielectric barrier extends over a top surface of the third layer of semiconductor material  228  from both the left and the right side. This may make the region exposed for the contact  230   c  smaller. In addition, along a bottom surface of the dielectric barriers  234   a ,  234   b , a small extension, thinner than the dielectric barriers can extend to the left and right away from the a third layer of semiconductor material  228  to cover a larger portion of the top surface of the second layer of semiconductor material  226 . This would make a larger space between the contacts  230   a ,  230   b , and the region directly below the third layer of semiconductor material  228 . 
       FIG. 3  is a cross section of an integrated circuit die  320  including a photodiode  322  according to one embodiment. The integrated circuit die  320  includes a first layer of semiconductor material  324 , a second layer of semiconductor material  326  positioned on the first layer of semiconductor material  324 , a dielectric layer  352 , and a transparent conductive oxide  354  positioned on the dielectric layer  352 . A layer of polysilicon  356  is positioned on the transparent conductive oxide  354 . The integrated circuit die  320  further includes a first Schottky contact  330   a  positioned on the second layer of semiconductor material  326 , a second Schottky contact  330   b  positioned on the second layer of semiconductor material  326 , and a third Schottky contact  330   c  positioned on the layer of polysilicon  356 . A first anode  332   a  is positioned on the first Schottky contact  330   a . A second anode  332   b  is positioned on the second Schottky contact  330   b . A third anode  332   c  is positioned on the third Schottky contact  330   c . Respective dielectric barriers  334   a ,  334   b  are positioned between the layer of polysilicon  356  and the first and second anodes  332   a ,  332   b . An ohmic contact  336  is positioned below the first layer of semiconductor material  324 . A cathode  338  is positioned below the ohmic contact  336 . 
     A junction between the second layer of semiconductor material  326  and the first contact  330   a  is a first Schottky photodiode junction  340   a . A junction between the second layer of semiconductor material  326  and the second contact  330   b  is a second Schottky photodiode junction  340   b . A junction between the third contact  330   c  and the layer of polysilicon  356  is a third Schottky photodiode junction  340   c.    
     In one embodiment, the first layer of semiconductor material  324  is monocrystalline silicon about 300 μm thick. The first layer of semiconductor material  324  is highly doped with N-type dopant atoms, such as phosphorus or arsenic, at a concentration of about 1e19/cm 3 . The second layer of semiconductor material  326  is monocrystalline silicon between about 3 μm and 15 μm thick. Using silicon in this embodiment allows the device to detect radiation in the infrared. It is noted that 4H—SiC is sensitive to the UV range. The second layer of semiconductor material  326  is lightly doped with N-type dopant atoms at a concentration between about 5e13/cm 3  and 5e14/cm 3 . The dielectric layer  352  is, for example, silicon dioxide between about 100 nm and 300 nm thick. The transparent conductive dielectric  354  is, for example, indium tin oxide (ITO) a between about 50 and 200 nm thick. The layer of polysilicon material  356  is between about 50 nm and 200 nm thick. The polysilicon layer  356  is lightly doped with N-type dopant atoms at a concentration between about 1e13/cm 3  and 1e14/cm 3 . 
     In one embodiment, the Schottky contacts  330   a - 330   c  are formed of a very thin layer of metal selected to form a Schottky barrier with the second layer of semiconductor material  326  and the layer of polysilicon  356 . In one example, the Schottky contacts  330   a - 330   c  are a thin layer of Ni about 10-30 nm thick formed on top of respective portions of the second layer of semiconductor material  326  and the layer of polysilicon  356 . 
     In one embodiment, the first, second, and third anodes  332   a - 332   c  are formed of a layer of conductive material, such as AlSiCu, about 3 μm thick. The first, second, and third anodes  332   a - 332   c  are electrically connected to the respective Schottky contacts  330   a - 330   c  on which they are positioned. The first, second, and third anodes  332   a - 332   c  are electrically shorted together, thereby forming a single collective anode. 
     In one embodiment, the dielectric barriers  334   a ,  334   b  are SiO 2  about 3-6 μm thick. The dielectric barriers  334   a ,  334   b  separate the layer of polysilicon  356  from the Schottky contacts  330   a ,  330   b  and the anodes  332   a ,  332   b.    
     In one embodiment, the ohmic contact  336  is a thin layer of conductive material selected to form an ohmic contact with the highly doped first layer of semiconductor material  324 . 
     In one embodiment, the cathode  338  is a highly conductive material, such as a combination layer of titanium, nickel, and gold, positioned on the bottom of the ohmic contact  336 . The combination layer may be formed such that all 3 layers are formed by the same equipment, where the titanium is in direct contact with the ohmic contact. The cathode  338  is about 550 nm thick. 
     The photodiode  322  functions in a similar manner to the photodiode  22  of  FIG. 1A . When a low-voltage V 0 , for example 0 V, is applied between the anodes  332   a - 332   c  and the cathode  338  by the power supply  346 , depletion regions  342   a ,  342   b  in the second layer of semiconductor material  326  do not extend below the Schottky contact  330   c . When a high voltage V 1 , for example −10 V, is applied between the anodes  332   a - 332   c  and the cathode  338 , the depletion regions  342   a ,  342   b  extend below the Schottky contact  330   c . At both V 0  and V 1 , the photodiode  322  is sensitive to a first peak wavelength of about 300 nm. However, at V 1  the photodiode  322  is further sensitive to a second peak wavelength of about 800 nm. The photodiode  322  is substantially blind (insensitive) to wavelengths between 300 and 800 nm due to the presence of the dielectric layer  352  and the conductive transparent oxide  354 . This represents a tremendous improvement over previous photodiodes because both infrared and ultraviolet light can be detected by the photodiode  322  by switching between V 0  and V 1 . 
     The photodiode  322  can be manufactured by first epitaxially growing the second layer of semiconductor material  326  from the first layer of semiconductor material  324 . Next the dielectric layer  352 , for example silicon dioxide about 200 nm thick, is deposited on the second layer of semiconductor material  326 . A via (not shown) is etched in the dielectric layer  352  exposing the second layer of semiconductor material  326 . The transparent conductive oxide  354 , about 100 nm thick, is deposited on the dielectric layer  352  and in the via contacting the second layer of semiconductor material  326 , which will allow contact of the second layer of semiconductor material to the conductive oxide. A polysilicon layer  356  is then deposited on the transparent conductive oxide. The polysilicon layer  356  is electrically connected to the cathode  338  through the transparent conductive oxide. Subsequent steps to deposit the Schottky contacts  330   a - 330   c , before the anodes  332   a - 332   c , the dielectric barriers  334   a ,  334   b , the ohmic contact  336 , and the anodes  338  can be performed in substantially the same manner as similar structures are formed with respect to  FIGS. 1F-1K . 
     Although Schottky contacts and the P-type contacts have been described as being shorted together in the various embodiments, those of skill in the art will recognize that the central Schottky contact, or the central P-type contacts, can be connected to separate voltages from the lateral Schottky contacts or the lateral P-type contacts. 
       FIG. 4  is a block diagram of a system  400 . The system  400  includes a power supply  421 , a photodiode  422 , an amplifier  423 , and a microcontroller  425 . This power supply  421  provides power to the photodiode  422 . According to one embodiment, the power supply first applies a first low-voltage V 0 , then a high reverse bias voltage V 1 , to the photodiode  422 . The photodiode  422  outputs a photo current to the amplifier  423 . When the power supply  421  supplies the low-voltage V 0 , then the photo current includes only current generated by a first wavelength of light incident on the photodiode  422 . When the power supply  421  supplies the higher reverse bias voltage, then the photo current includes current generated from the first peak wavelength of light incident on the photodiode  422  and from a second peak wavelength of light incident on the photodiode  422 . The amplifier  423  includes only a single trans-impedance amplifier in an embodiment in which all of the anode terminals are shorted together. The trans-impedance amplifier amplifies the photo current and passes it to the microcontroller  425 . The microcontroller  425  measures the photo current at both V 0  and V 1 . The photo current at V 0  indicates the intensity of light at the first peak wavelength. The difference between the photo currents at V 1  and V 0  indicates the intensity of light of the second peak wavelength. 
     The embodiments described here provide several advantages with respect to the current state of the art. According to principles of the present disclosure, it is possible to more precisely define the lateral extension of a central junction through the lateral confinement structure (deep trenches all around the central junction). In this way the condition of absorption of the photons at low wavelengths does not change at a higher reverse bias value since all the region below the central junction is fully depleted at 0V. Due to the multiplexed anode contact layout, the eventual increase of the current signal produced at higher reverse bias can be attributed exclusively to the absorption of high wavelength photons and not to the increase of the lateral depleted area at the first junction as can happen for example in devices without any lateral confinement of the depletion layer. Furthermore, it is possible to separate the signals arising from the central junction and portion of first epitaxial layer depleted at high reverse bias below the central region through independent anode metallization. In this way the signals coming from the two junctions can be extracted and separated at any time. This concept can be extended to any number of junctions, each one sensitive to a different range of wavelengths. It is also possible to electrically and optically insulate two superimposing junctions through an intermediate buffer layer (dielectric/combination of dielectric layers, or highly doped semiconductor layer) not optically and electrically active, whose nature and thickness can be suitably tailored to make the device sensitive to two not adjacent wavelengths bands (for example in the UV and NIR ranges). 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.