Patent Publication Number: US-7217982-B2

Title: Photodiode having voltage tunable spectral response

Description:
CROSS-REFERENCE TO A RELATED PATENT APPLICATION 
   This patent application is a divisional of and allowed U.S. patent application Ser. No. 10/256,835, filed on Sep. 27, 2002 and now U.S. Pat. No. 6, 803,557. 

   TECHNICAL FIELD: 
   These teachings relate generally to detectors of electromagnetic radiation and, more specifically, relate to photodiode detectors that are responsive to electromagnetic radiation in more than one spectral band. Even more specifically, these teachings relate to detectors of electromagnetic radiation that have an electrically tunable response to light of different wavelengths. 
   BACKGROUND: 
   Electromagnetic radiation detectors that are responsive to light in more than one wavelength band, also referred to as multi-spectral or multi-color detectors, provide a number of advantages in modern imaging systems. In general, the light that is detected may be visible light or light that is not visible to the human eye (e.g., infrared (IR) radiation). 
   Early efforts to detect IR radiation within more than one spectral band have relied on the use of multiple detector arrays, each having a different spectral filter. Multiple detector arrays with different spectral responses have also been used. The use of a continuously variable wedge filter in conjunction with a detector array is also known in the art, as is the use of a mechanical spectral filter selector. For reasons related at least to increased cost, complexity and weight, these conventional approaches to multi-spectral imaging are disadvantageous for many applications. 
   It was thus realized that the detection of IR radiation in two or more spectral bands with a single integrated detector device was a very desirable alternative to the conventional approaches. Representative examples of such detectors can be found in the following commonly assigned U.S. patents. 
   In U.S. Pat. No.: 5,113,076 by Eric Schulte, “Two terminal multi-band infrared radiation detector”, there is described a radiation detector that includes a first heterojunction and a second heterojunction that are electrically coupled together in series between a first electrical contact and a second electrical contact. The detector contains at least a three regions or layers, including a first layer having a first type of electrical conductivity, a second layer having a second type of electrical conductivity and a third layer having the first type of electrical conductivity. The first and second heterojunctions are coupled in series and function electrically as two back-to-back diodes. During use the detector is coupled to a switchable bias source that includes a source of positive bias (+Vb) and a source of negative bias (−Vb). With +Vb applied across the detector the first heterojunction is in far forward bias and functions as a low resistance conductor, thereby contributing no significant amount of photocurrent to the circuit. The second heterojunction is in a reverse bias condition and modulates the circuit current in proportion to the photon flux of an associated spectral region or color. Conversely, with −Vb applied across the detector the second heterojunction is in forward bias and contributes no significant photocurrent to the circuit while the first heterojunction is reversed biased and produces a current modulation proportional to the incident flux, where the flux is associated with a different spectral region. 
   In U.S. Pat. No.: 5,731,621 to Kenneth Kosai, “Three band and four band multispectral structures having two simultaneous signal outputs”, there is described a solid state array that has a plurality of radiation detector unit cells, wherein each unit cell includes a bias-selectable two color photodetector in combination with either a second bias-selectable two color detector or a single photodetector. Each unit cell is thus capable of simultaneously outputting charge carriers resulting from the absorption of electromagnetic radiation within two spectral bands that are selected from one of four spectral bands or three spectral bands. 
   In U.S. Pat. No.: 5,751,005 by Richard Wyles et al., “Low-crosstalk column differencing circuit architecture for integrated two-color focal plane arrays”, there is described an integrated two-color staring focal plane array having rows and columns of photodetector unit cells, each of which is capable of simultaneously integrating photocurrents resulting from the detection of two spectral bands. A readout circuit performs a subtraction function, and includes a differential charge-sensing amplifier in a one-per-column arrangement. The amplifier works in cooperation with circuitry located in each unit cell. The subtraction function is employed to create a separate Band 1  signal from a Band 2  and (Band 1 +Band 2 ) signals generated by each simultaneous two-color detector. The circuit offers low spectral crosstalk between the two spectral bands. 
   Also by example, in U.S. Pat. No.: 5,959,339 by Chapman et al., “Simultaneous two-wavelength p-n-p-n infrared detector” there is disclosed an array that contains a plurality of radiation detectors. Each radiation detector includes a first photoresponsive diode (D 1 ) having an anode and a cathode that is coupled to an anode of a second photoresponsive diode (D 2 ). The first photoresponsive diode responds to electromagnetic radiation within a first band of wavelengths and the second photoresponsive diode responds to electromagnetic radiation within a second band of wavelengths. Each radiation detector further includes a first electrical contact that is conductively coupled to the anode of the first photoresponsive diode; a second electrical contact that is conductively coupled to the cathode of the first photoresponsive diode and to the anode of the second photoresponsive diode; and a third electrical contact that is conductively coupled to a cathode of each second photoresponsive diode of the array. The electrical contacts are coupled during operation to respective bias potentials. The first electrical contact conducts a first electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths, and the second electrical contact conducts a second electrical current induced by electromagnetic radiation within the second predetermined band of wavelengths, less an electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths. 
   The disclosures of these various commonly assigned U.S. Patents are incorporated by reference herein in so far as there is no conflict with the teachings of this invention. 
   Also of interest to the teachings of this invention is a p-i-i-n (p-type, intrinsic, intrinsic, n-type) detector that is described by Brüggermann et al., “The operational principle of a new amorphous silicon based p-i-i-n color detector”, J. Appl. Phys. 81(11), 1 Jun. 1997, 7666–7672. The device is constructed using two large band gap front layers of doped and intrinsic hydrogenated amorphous silicon carbide (a-SiC:H), followed by an intrinsic and a doped a-Si:H layer. These authors report that by band gap engineering an experimental red response is maximized at a large reverse bias voltage, whereas the green response has its maximum at low reverse bias voltage. The potential profile of the p-i-i-n structure is said to be of crucial importance to the color detection mechanism. At larger wavelengths the large potential drop across the two highly defective front layers assists recombination in the back part of the device, which leads to the drop in the red response at low reverse voltage. For the voltage-dependent shift in spectral sensitivity it is said to be important that photogenerated carriers, under green bias illumination, are lost by recombination in the front part of the device. 
   Also of interest is an n-i-p-i-i-n detector of a type described by H. Stiebig et al., “Transient Behavior of Optimized nipiin Three-Color Detectors”, IEEE Transactions on Electron Devices, Vol. 45, No. 7, Jul. 1998, 1438–1444. These authors report the detection of the fundamental components of visible light (blue, green, red) with a multi-spectral two-terminal photodiode that is based on amorphous silicon. The preferential carrier collection region of the two-terminal device shifts upon a change of the applied bias voltage, which leads to a color sensitivity. Structures with controlled bandgap and mobility-lifetime product exhibit a dynamic behavior above 96 dB. Three linearly independent spectral response curves can be extracted to generate a RGB (red-green-blue)-signal. Bias voltage switching experiments under different monochromatic illumination conditions were carried out to investigate the time-dependent behavior. 
   SUMMARY OF THE PREFERRED EMBODIMENTS 
   The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings. 
   A photodetector in accordance with the teachings of this invention includes a substrate having a surface; a first layer of semiconductor material that is disposed above the surface, the first layer containing a first dopant at a first concentration for having a first type of electrical conductivity; and a second layer of semiconductor material overlying the first layer. The second layer contains a second dopant at a second concentration for having a second type of electrical conductivity and forms a first p-n junction with the first layer. The second layer is compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from the p-n junction. The compositional grading can be done in a substantially linear fashion, or in a substantially non-linear fashion, e.g., in a stepped manner. Preferably the first dopant concentration is at least an order of magnitude greater than the second concentration, and more preferably is at least two orders of magnitude greater. When the first p-n junction is reverse biased, a depletion region exists substantially only within the second layer, and varying the magnitude of the bias shifts the wavelength at which a maximum spectral sensitivity or responsiveness is obtained. At least one electrical contact is provided for coupling the second layer to a source of variable bias voltage for reverse biasing the p-n junction. As the magnitude of the bias voltage is changed a wavelength of electromagnetic radiation to which the photodetector is responsive is changed. 
   As examples, the semiconductor material can be selected from a Group II-VI material or from a Group III-V material. The first type of electrical conductivity can be p-type, and the second type of electrical conductivity can be n-type, or the first type of electrical conductivity can be n-type, and the second type of electrical conductivity can be p-type. 
   The photodetector can further include a third layer of semiconductor material that is disposed above the second layer, the third layer containing a third dopant at a third concentration for having the first type of electrical conductivity and a fourth layer of semiconductor material overlying the third layer. The fourth layer contains a fourth dopant at a fourth concentration for having the second type of electrical conductivity and forming a second p-n junction with the third layer, the fourth layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap semiconductor material in a direction away from the second p-n junction. The third concentration is at least an order of magnitude greater than the fourth concentration, and when the second p-n junction is reverse biased a depletion region exists substantially only within the fourth layer. 
   Also disclosed is an array of IR radiation responsive photodetectors wherein each photodetector includes a photodiode having a p-n junction. A wavelength at which a maximum spectral response of the photodiode occurs is determined at least in part by a magnitude of a reverse bias voltage applied to the p-n junction. Each of the photodiodes includes a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from the p-n junction. The compositionally graded layer confines substantially all of a depletion region of the photodiode. 
   A method is also disclosed for operating an array of electromagnetic radiation responsive photodetectors. The method includes providing the array such that each photodetector includes a photodiode having a p-n junction, where a wavelength at which a maximum spectral response of the photodiode occurs is determined at least in part by a magnitude of a reverse bias voltage applied across the p-njunction. Each of the photodiodes includes a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from the p-n junction, where the layer confines substantially all (e.g., preferably more than about 95%, and more preferably more than about 99%) of a depletion region of the photodiode. During operation of the array the method establishes for each photodetector a predetermined magnitude of reverse bias voltage; and detects a signal generated from each photodetector that results from incident electromagnetic radiation having wavelengths that correspond to the maximum spectral response that is determined at least in part by the magnitude of the reverse bias voltage. The step of establishing may establish approximately the same magnitude of reverse bias voltage for each photodetector of the array, or it may establish approximately the same magnitude of reverse bias voltage for some of the photodetectors of the array while establishing at least one different magnitude of reverse bias voltage for other photodetectors of the array, or the step of establishing may establish a different magnitude of reverse bias voltage for each photodetector of the array. For a case where the array contains rows and columns of photodetectors, the step of establishing may establish a different magnitude of reverse bias voltage for individual ones of rows or columns of the array. The step of establishing can include varying the magnitude of the reverse bias potential during operation of the array. For a case where the layer of semiconductor material is compositionally graded in a stepped fashion, increments of reverse bias voltage can have a magnitude that is related to the steps. 
   An alternating current signal can be superimposed on the reverse DC bias voltage and a synchronous detection technique used to detect photons corresponding to a certain bandgap energy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein: 
       FIG. 1  is a simplified cross-sectional view of a photodiode that is constructed in accordance with the teachings of this invention; 
       FIG. 2  is an energy band diagram that corresponds to the photodiode shown in  FIG. 1 ; 
       FIGS. 3A ,  3 B and  3 C are graphs depicting the composition profile, depletion depth vs. bias, and spectral cutoff vs. bias for the photodiode shown in  FIG. 1 ; 
       FIG. 4  is an energy band diagram of a multi-layer device with two independently tunable two color spectral responses; 
       FIG. 5  is a simplified cross-sectional view of a tunable detector in accordance with the energy band diagram of  FIG. 4 ; 
       FIG. 6  is an energy band diagram of a detector having a fixed cut-off LWIR region and a variable cut-on LWIR region that is a function of a variable cut-off of an MWIR responsive region; 
       FIG. 7  is an energy band diagram of a detector having a fixed LWIR bandpass and a variable cut-off MWIR bandpass; 
       FIGS. 8A ,  8 B and  8 C are graphs depicting the band diagram, depth composition profile and depth doping profile for a non-linear or stepped profile photodiode detector; 
       FIG. 9  illustrates a graph that depicts the spectral response vs. reverse bias (normalized to unity) for the stepped profile photodiode detector; 
       FIGS. 10A ,  10 B and  10 C depict an energy band diagram, depth composition profile and depth doping profile, respectively, for a device fabricated using the opposite polarity (p on n) of semiconductor material to the embodiments described by  FIGS. 1–9 ; 
       FIG. 11A  is a simplified top view of a staring type photodetector array that is biased so as to provide a uniform spectral response indicated by wavelength λ 1 ; 
       FIG. 11B  is a simplified top view of a staring type photodetector array that is biased so as to provide a bandpass response indicated by wavelengths λ 1 , λ 2  and λ 3 ; 
       FIG. 11C  is a simplified top view of a multi-column scanning array where each column is biased differently to provide a graded spectra response indicated by wavelengths λ 1 , λ 2 , λ 3 , λ 4  and λ 5 ; 
       FIG. 12A  is a graph that plots composition and doping profiles for an exemplary detector device;  FIG. 12B  illustrates the resulting energy band diagram and  FIG. 12C  shows the spectra response for two bias voltages; 
       FIGS. 13A ,  13 B and  13 C depict an energy band diagram, depth composition profile and depth doping profile for a device fabricated using a Group III-V material, specifically the material Al x Ga (1-x) As over the composition range for which it is a direct bandgap semiconductor; and 
       FIG. 14A  is a simplified schematic diagram of a single photodetector unit cell coupled to a readout and variable bias integrated circuit; 
       FIG. 14B  is a timing diagram for the unit cell of  FIG. 14A ; 
       FIG. 15A  is an enlarged top view of an exemplary 4×4 unit cell detector array; and  FIG. 15B  is an enlarged cross-sectional view of a portion of the array of  FIG. 15A ; 
       FIG. 16  is a block diagram, partly in schematic diagram form, of the 4×4 array of  FIGS. 15A and 15B ; and 
       FIG. 17  is an exemplary ROIC unit cell timing diagram. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As employed herein Short Wavelength Infrared (SWIR) radiation is considered to include a spectral region extending from approximately 1000 nanometers (nm) to approximately 3000 nm. Medium Wavelength Infrared (MWIR) radiation is considered to include a spectral region extending from approximately 3000 nm to approximately 8000 nm. Long Wavelength Infrared (LWIR) radiation is considered to include a spectral region extending from approximately 7000 nm to approximately 14000 nm. Very Long Wavelength Infrared (VLWIR) radiation is considered to include a spectral region extending from approximately 12000 nm to approximately 30000 nm. Although the bands overlap to some extent, for the purposes disclosed herein the overlap is not considered to be significant. Also, as employed herein a semiconductor material is considered to exhibit significant responsivity to a given spectral band if the semiconductor material exhibits a maximum or substantially maximum photosensitivity to wavelengths within the given spectral band. 
   Referring to the photodetector  10  shown in cross-section in  FIG. 1 , and to the bandgap diagram of  FIG. 2 , in accordance with the teachings of this invention a highly doped wide bandgap p-type Hg (1-x) Cd x Te layer  14  is grown on a substantially transparent (at the wavelengths of interest) substrate  12 . The layer  14  may be doped p-type using, for example, Arsenic (As) having a concentration of about  10   18  cm 3 . The substrate  12  could be a suitable type of Group II-VI substrate, such as a CdTe substrate, or it could be a Silicon or other type of substrate having appropriate accommodation and/or lattice matching layers grown thereon, if required. A lightly doped n-type Hg (1-x) Cd x Te layer  16  is grown over the p-type layer  14 , wherein the value of x is varied from a higher value (wider bandgap) to a lower value (narrower bandgap) in a direction away from the p-type layer  14 , as shown in  FIG. 3A . By so varying the value of x during layer growth the n-type layer  16  is compositionally graded through at least a portion of its thickness. The layer  16  may be doped n-type using, as an example, Indium (In) having a concentration in a range of about 1×10 14  cm 3  to about 3×10 14  cm 3 . Molecular Beam Epitaxy (MBE) is a presently preferred layer growth technique, although other layer growth techniques, such as Liquid Phase Epitaxy (LPE) or Vapor Phase Epitaxy (VPE), could be used as well. The resulting layers  14  and  16  form a p-n junction  15  where, because of the difference in the doping concentrations of layers  14  and  16  and the compositional grading profile of the n-type layer  16 , the depletion region  17  is predominantly and substantially exclusively located within the n-type layer  16 . When the p-n junction is reverse biased it is then observed that the depletion region  17  is driven progressively deeper into the n-type layer  16  as the magnitude of the reverse bias is increased, as shown in  FIG. 3B . This tends to overcome the electric field caused by the composition gradient of the Hg (1-x) Cd x Te layer  16 , and photo-generated holes can be captured by the electric field in the depletion region  17 , instead of recombining outside of the depletion region  17 . Thus, increasing the magnitude of the reverse bias increases the spectral response cutoff wavelength of the p-n junction  15  of the photodetector  10 , when illuminated through the substrate  12 , as shown in  FIG. 3C . 
   Representative, but not limiting, layer thicknesses are about 2–3 microns for p-type layer  14  and about 10–15 microns for the n-type layer  16 . 
   It is noted that the compositional grading profile of the layer  16  is opposite to that most often encountered (i.e., where the composition x would be varied from a lower value (narrower bandgap) to a higher value (wider bandgap) in a direction away from the p-type layer  14  and towards a surface of the device.) 
   General reference in this regard can be made to U.S. Pat. No.: 5,466,953 by Rosbeck and Cockrum,  A Denuded zone field effect photoconductive detector@, where a compositionally graded HgCdTe radiation detector is constructed to have a high purity denuded zone that is formed adjacent to a radiation absorbing region. The compositional grading results in an internally generated electric field that is orthogonally disposed with respect to an externally generated electric field applied between contacts. The internally generated electric field has the effect of injecting photogenerated minority charge carriers into the denuded zone, thereby reducing recombination with photogenerated majority charge carriers and increasing carrier lifetime. The detector further includes a wider bandgap surface passivation region that functions to trap, or “getter”, impurities from the denuded zone and also to reduce surface recombination effects. 
   Reference can also be made to U.S. Pat. No.: 5,936,268 by Cockrum et al.,  A Epitaxial passivation of group II-VI infrared photodetectors@, where an array of photodiodes includes a radiation absorbing base layer of Hg 1-x Cd. x Te, where the value of x determines the responsivity of the array to either LWIR, MWIR or SWIR. The upper surface of the array is provided with a passivation layer comprised of an epitaxial layer of Group II-VI material which forms a heterostructure with the underlying Group II-VI material and which has a wider bandgap than the underlying Hg 1-x Cd. x Te, and which thereby repels both holes and electrons from the diode junctions. 
   Further reference can also be had to U.S. Pat. No.: 5,880,510 by Cockrum et al.,  A Graded layer passivation of group II-VI infrared photodetectors@, where a Group II-VI IR photodiode has a passivation layer overlying at least exposed surfaces of a p-n diode junction. The passivation layer is a compositionally graded layer comprised of Group II atoms diffused into a surface of the p-n diode junction. The passivation layer has a wider energy bandgap than the underlying diode material thereby repelling both holes and electrons away from the surface of the diode and resulting in improved diode operating characteristics. In the passivation layer the energy bandgap gradually decreases in value as a function of depth from the surface until the bandgap energy equals that of the underlying bulk material. 
   In the instant photodiode  10  the compositional grading, from wider bandgap to narrower bandgap in a direction away from the p-n junction  15 , in combination with the difference in doping concentrations between layers  14  and  16  (at least about one and preferably at least about two or more orders of magnitude), causes the depletion region to reside or be confined almost entirely within the n-type layer  16 , and to grow or extend further into the layer  16  upon an increase in reverse bias, as opposed to growing into both the p-type layer  14  and the n-type layer  16 . Suitable dopant concentrations for the p-type layer  14  can be from about 10 17  atoms/cm 3  to about 10 18  atoms/cm 3 , and suitable dopant concentrations for the n-type layer  16  can be from about 3×10 14  atoms/cm 3  to about 3×10 15  atoms/cm 3 . Generally speaking, and by example, the dopant concentration of the p-type layer  14  is greater by about two to three orders of magnitude than the dopant concentration of the n-type layer  16 . 
   It should be appreciated that this growth technique and the electrical operation of the resulting photodetector  10  may be used with other variable composition semiconductor materials to vary the photosensitivity wavelength. 
   Furthermore, these teachings are not limited to the n-on-p photodetector  10  embodiment shown in  FIG. 1 . For example,  FIGS. 10A ,  10 B and  10 C depict an energy band diagram, depth composition profile and depth doping profile, respectively, for a device fabricated using the opposite polarity (p-on-n) of Group II-VI semiconductor material to the embodiment described by  FIGS. 1 ,  2  and  3 A– 3 C, as well as those described below in reference to  FIGS. 4–9 . In this case the n-type material is more heavily doped than the p-type material, and the compositional profile of the p-type material is graded. 
   In addition, these teachings may be applied to other than Group II-VI detectors of IR radiation. For example, these teachings may be applied as well to photodetectors constructed using Group III-V materials. Examples of III-V semiconductor materials that are suitable for supporting the bias tuneable bandgap in accordance with these teachings include, but need not be limited to, the following:
     Al x In (1-x) P   Ga x In (1-x) P   InP x As (1-x)      Al x Ga (1-x) As   Ga x In (1-x) As   InAs x Sb (1-x)      Al x In (1-x) As   Ga x In (1-x) Sb   InGaN   Al x Ga (1-x)      SbGaP x As (1-x)      Ga x In (1-x) As y P (1-y)      Al x In (1-x) Sb   GaAs x Sb (1-x)      

   As but one example,  FIGS. 13A ,  13 B and  13 C illustrate an energy band diagram, depth composition profile and depth doping profile for a photodetector device fabricated using the material Al x Ga (1-x) As over the composition range for which it is a direct bandgap semiconductor. This device as well features the significant doping concentration difference across the p-n junction and the compositionally graded layer that cooperate to support the confinement and growth of the depletion region  17  within the more lightly doped layer under increasing reverse bias conditions. 
   By superimposing a smaller AC voltage on the DC bias voltage, the resulting AC diode current can be treated as a spectral bandpass signal. The magnitude of the AC voltage determines the spectral width of the bandpass. By sweeping the DC bias voltage, the spectral bandpass is then swept across a range of the IR spectrum. By using phase sensitive AC detection, such as synchronous detection techniques that are phase locked to the phase of the AC bias voltage, the signal-to-noise ratio (SNR) of the detector  10  may be significantly improved. 
   In this case a p-n junction has a graded composition in a low doped layer in which the depletion region can be pushed into increasingly narrower bandgap material as the reverse bias is increased. Synchronous detection of the small AC current resulting from the small AC bias that is superimposed on the swept DC reverse bias is used to obtain a variable spectral response to infrared photon stimulation. 
   In one non-limiting example a −1 volt DC bias has a 20 mV RMS, 1 kHz AC signal superimposed thereon. A synchronous detector is used to detect only the AC component, where the synchronous detector is phase and frequency locked to the AC component of the bias signal. The DC bias voltage can be held at a constant value, or it can be swept over a range of voltage values. In this aspect of the invention one essentially measures only those IR photons having an energy that corresponds to a bandgap energy that is related to the AC bias component and the present value of the DC bias. 
   A multilayer device with two independently tunable two color spectral responses may also be fabricated. For example,  FIG. 4  is an energy band diagram of a multi-layer device with two independently tunable two color spectral responses (e.g., LWIR and MWIR), and  FIG. 5  is a simplified cross-sectional view of a tunable detector  30  in accordance with the energy band diagram of  FIG. 4 . In  FIG. 5  the substrate  12  and p+ layer  14  may be as described above for the photodetector  10  of  FIG. 1 , as may the n− layer  16 . Representative, but not limiting, layer thicknesses are about 2–3 microns for p+ layer  14  and about 10–15 microns for the n− layer  16 . 
   Over the n− layer  16  is then grown a p-type barrier layer  18 , and then another compositionally graded n-type layer  20 . Representative, but not limiting, layer thicknesses are about 2–3 microns for the p barrier layer  18  and about 10–15 microns for the n− layer  20 . The resulting layered structure is photolithographically processed into pixel or unit cell mesa structures each comprised of a first, primary mesa  30 A and a secondary mesa  30 B. Suitable electrical contact pads and interconnects, such as Indium bumps  22 A and  22 B are then added. Indium bump  22 A provides electrical contact for a readout and variable biasing integrated circuit (ROVBIC)  32 . Metallization  24  provides an electrically conductive path from Indium bump  22 B to the p barrier layer  18  contained with the primary mesa  30 A wherein incident MWIR and LWIR radiation is detected. In this embodiment, as shown in  FIG. 4 , the detection wavelength of both the MWIR and the LWIR radiation is bias tuneable. 
     FIG. 14A  is a simplified schematic diagram of a single photodetector unit cell  10 , as in  FIG. 1 , coupled to the ROVBIC  32 . A Detector Common Bias  32 C attaches to a ground ring of the detector array (shown more clearly in  FIG. 15A ), and sets the detector common voltage to a level that is optimum for a capacitance transimpedance amplifier (CTIA). A Detector Bias DAC (digital to analog converter)  32 A sets the bias voltage on the detector diode  10  through the CTIA  32 B, when an Integration Switch (S 1 ) is closed. This bias voltage sets the spectral response of the detector array. The above-mentioned AC component of the bias voltage can be added by the DAC  32 A by varying the input digital value at a rate and over a range of values to provide the desired AC modulation of the DC bias. The AC component would then also be provided to a synchronous detector enabling phase and frequency lock to the AC bias component. The DC bias voltage value can be varied or swept as well, as was also mentioned previously. 
   In operation, and referring also to  FIG. 14B  (which shows two integration periods), the Integration Switch S 1  and a Reset Switch (S 2 ) are closed, and a Row Switch (S 3 ) is open. This puts the CTIA  32 B into a unity gain mode, and the voltage produced by the Detector Bias DAC  32 A appears across the Detector Diode  10 . Then the Reset Switch S 2  is released, and electrical charge from the Detector Diode  10  flows into an integration capacitor (Cint), since the charge cannot flow into the inverting input of the CTIA  32 B. As the charge flows into Cint the voltage across it increases. However, the CTIA  32 B operates to maintain its + and  B  inputs at the same voltage, which in this case is the voltage output by the Detector Bias DAC  32 A. The result is that as charge continues to flow into Cint, the voltage on the output node of the CTIA  32 B increases negatively. When the integration time is over, the Integration Switch S 1  is opened. At some time later the Row Switch S 3  is closed, and the output voltage of the CTIA  32 B is output from the ROVBIC  32 . The output waveform in  FIG. 14B  shows twice the amount of voltage change between 20 ms and 29 ms as between 4 ms and 13 ms. This would occur if the detector photo current were to double between the two integration periods. 
   A photodetector device with a fixed cutoff LWIR and a variable LWIR Acuton@, depending on the variable cutoff of the MWIR, is shown in  FIG. 6 . Only one indium bump  22  per pixel is required for this embodiment, as opposed to the use of two Indium bumps  22 A and  22 B in  FIG. 5 . In essence, in this embodiment the secondary mesa  30 B of  FIG. 5  is eliminated. 
     FIG. 7  is an energy band diagram of a detector having a fixed LWIR bandpass and a variable cut-off MWIR bandpass. In this embodiment the n-barrier prevents carriers generated to the right of it from reaching the LWIR junction when it is accessed. Only one Indium bump  22  per pixel is required. Applying a negative bias to the left terminal (the terminal opposite the entry of the light) with respect to the right terminal reverse biases the left p-n junction  15 A and activates the LWIR mode. Applying a positive bias activates the right p-n junction  15 B, with tuneable cutoff wavelength by varying the positive bias voltage. 
   Any of the preceding structures that implement the compositional gradient may also employ the non-linear or step profile shown in  FIGS. 8A–8C , where  FIGS. 8A ,  8 B and  8 C show the band diagram, the depth composition profile and the depth doping profile, respectively, for the stepped profile photodiode detector, as opposed to the smoothly graded or linear composition profile photodiode detector.  FIG. 9  shows a graph that depicts the spectral response vs. reverse bias (normalized to unity) for the stepped profile photodiode detector. Each step in the composition of the n-type layer  16  is bounded on the left edge by a minority carrier reflector, to improve quantum efficiency. The resulting photodiode exhibits a stepped spectral cutoff response, and small variations of bias voltage does not affect the response. By varying the bias voltage between two steps, a spectral bandpass response results from the difference signal detected at each of the two steps. This embodiment gives the same spectral response across an array of detectors, even if the bias voltage varies a small amount between detectors, and/or if the depletion depth varies because of doping variations. A detector array fabricated with this step approach may be more producible, and may require less scene correction. In addition, the steps may be adjusted to make the detector sensitive to specific spectral bands or spectral cutoffs. 
   While the stepped structure illustrated in  FIGS. 8A–8C  and  9  may appear at first glance to be superficially similar to that of Brüggermann et al. (J. Appl. Phys. 81, 7666 (1997)), Brüggermann et al. did not rely on the barrier height of the isotype heterojunction to provide separation of photocarriers according to the wavelength of the absorbed light. 
   A two dimensional focal plane array  40  may be fabricated from any of the previously described structures. The array  40  may be biased uniformly so it responds uniformly in spectral response, as in  FIG. 11A , or the spectral response may be electrically graded across the array  40 , as in  FIG. 11  B. The graded response may be useful for scanning applications, as in  FIG. 11C . 
   More specifically,  FIG. 11A  is a simplified top view of a staring type photodetector array  40  that is biased so as to provide a uniform spectral response indicated by wavelength λ 1 . 
   An AC component may be applied to the detector substrate  12  to achieve a bandpass response. For example,  FIG. 11B  is a simplified top view of the staring type photodetector array  40  that is selectively biased so as to provide a bandpass response indicated by wavelengths λ 1 , λ 2  and λ 3 .  FIG. 11C  is a simplified top view of a multi-column scanning type of array  40 , where each column is biased differently, indicated by voltages V 1 , V 2 , V 3 , V 4  and V 5 , to provide a graded spectral response indicated by wavelengths λ 1, λ   2 , λ 3 , λ 4  and λ 5 , respectively. In this embodiment the scanning direction of the incident light is perpendicular to the columns, and each column is sensitive to a different range of wavelengths arriving from the scene being viewed. In other embodiments each row may be biased differently. 
   In general, in various embodiments in accordance with this invention each of n photodetectors of the array  40  could be reverse biased differently so as provide a maximum spectral response to individual ones of λ 1  through λ n  different wavelengths. In another embodiment some of the photodetectors of the array could have a fixed spectral response, i.e., one that is not bias voltage tuneable, while others of the photodetectors of the array could have the adjustable spectral response, i.e., one that is bias voltage tuneable. In any of these embodiments the reverse bias voltage could be varied and then set prior to operation, or the reverse bias voltage may be varied during operation (for example at the frame rate or at a multiple of the frame rate). 
   Reference is now made to  FIG. 12A  for showing a graph that plots composition and doping profiles for an exemplary photodetector constructed in accordance with these teachings, while  FIG. 12B  illustrates the resulting energy band diagram and  FIG. 12C  shows the spectral response for two different (and exemplary) bias voltages (i.e., V DET =0V then λ c =2.23 μm and V DET =5V then λ c =3.98 μm). 
   An exemplary 4×4 detector array  100  may be fabricated as shown in  FIGS. 15A and 15B . The 4×4 array of pixels are surrounded by a ring of ground contacts  102  to the detector common p+ layer  14 . The detector array  100  may be connected to the readout integrated circuit (ROIC), or ROVBIC  32  in this case, with the indium bumps  22 A that mate with corresponding bumps  104 A. The ground contacts  102  mate with corresponding bumps  104 B. 
   An electrical diagram for the ROVBIC  32  is shown in  FIG. 16 . Note that each of the unit cell schematic diagrams corresponds to the one discussed above with respect to  FIG. 14A . Each detector diode cathode  16  connects to a unit cell input circuit, which in this case is the CTIA  32 B. The detector anodes  14  all form the p+ common connection, which attaches to the ROVBIC  32  by the ground contact ring  102  shown in  FIGS. 15A and 15B . 
   Also shown in  FIG. 16  is a clock generator block  34  that generates the Integrate and Reset timing signals for switches S 1  and S 2 , respectively, and a row shift register block  36  that generates the Row switch S 3  timing signals sequentially in a row-by-row manner. The unit cell outputs are input to one of four column amplifiers  38  that in turn feed a chip output amplifier  40  via a column shift register  42 . A detector bias generator block  44  includes the Detector Bias DAC  32 A and also generates the Detector Bias Common  32 C, both shown in  FIG. 14A . 
   A Digital block  46  receives digital signals (clock and data) from an external controller or data processor (not shown), and converts the digital signals into column readout signals that are applied to the column shift register for controlling the outputting of unit cell signals. Another output of the Digital block  46  is applied to the Detector Bias Generator block  44  for setting at least the level of the output of the Detector Bias DAC  32 A. 
     FIG. 17  shows the timing waveforms for the 4×4 array  100  of  FIGS. 15A and 15B , and illustrates the operation of the circuitry shown in  FIG. 16 . The output of the array  100  (the bottom trace) results in this example from imaging the red and blue scenes shown below the timing diagram. The blue scene is assumed to cover every other pixel, while the red scene is assumed to cover the center two pixel columns of the array  100 . The time from the first high pulse on Data to the next high pulse is referred to as a frame; and the exemplary frame period in  FIG. 17  is thus 11 ms. The Data may be used to program the Detector Bias DAC  32 A, among other necessary functions, as described above. In this illustration the Detector Bias DAC  32 A is programmed to provide detector sensitivity to a relatively short wavelength scene in the first frame. In the second frame, the Detector Bias DAC  32 A is programmed to provide longer wavelength sensitivity (−0.5 volts versus −1.0 volts, respectively). The Reset and Integrate timing signals, and the Bias voltage, affect all of the detectors  10  on the array  100  simultaneously. After reset and integration, all sixteen unit cell voltages are multiplexed to the single output amplifier  40 , producing a video stream of analog voltages. This video stream may then be converted back into a viewable image, and/or it may be processed using any of a number of known types of algorithms such as those that enhance the image, remove artifacts, perform uniformity corrections, and so forth. 
   Note in this example that the Detector Bias DAC  32 A can be programmed during operation, e.g., on a frame-by-frame basis, to provide the desired sensitivity to different wavelength bands. 
   Note as well that the output of the Detector Bias DAC  32 A is shown as being applied in common to all of the unit cells. It is, however, within the scope of these teachings to provide two or more Detector Bias DACs  32 A, and suitable selection and multiplexing circuitry, for applying different Detector Bias signals to different photodetectors  10 . In this manner, and by example, during a single frame certain ones of the photodetectors  10  are made responsive to a first wavelength band and certain others of the photodetectors  10  are made responsive to a second wavelength band that may or may not completely or partially overlap the first wavelength band. 
   In addition, and as was discussed above, a smaller AC signal can be superimposed on the DC bias voltage, where the magnitude of the AC signal determines the spectral width of the bandpass. By sweeping the DC bias voltage the spectral bandpass is swept across a range of the IR spectrum. By using a phase sensitive AC detection technique, such as a synchronous detection technique that is phase locked to the phase of the superimposed AC signal, the SNR of the detector  10  can be significantly improved. 
   While described in the context of exemplary semiconductor materials, dopants, dopant concentrations, layer thicknesses, compositional profiles, wavelengths, bias voltages, circuit embodiments, waveform levels and times, these are intended to be viewed in a non-limiting and exemplary sense, and are not intended to be construed as limiting the scope or practice of the teachings in accordance with this invention.