PATENT DOCUMENT

Publication Number: US-12206032-B2
Application Number: US-202117385813-A
Country: US
Kind Code: B2

Title: Wideband back-illuminated electromagnetic radiation detectors

Abstract:
An electromagnetic radiation detector includes an InP substrate having a first surface opposite a second surface; a first InGaAs electromagnetic radiation absorber stacked on the first surface and configured to absorb a first set of electromagnetic radiation wavelengths; a set of one or more buffer layers stacked on the first InGaAs electromagnetic radiation absorber and configured to absorb at least some of the first set of electromagnetic radiation wavelengths; a second InGaAs electromagnetic radiation absorber stacked on the set of one or more buffer layers and configured to absorb a second set of electromagnetic radiation wavelengths; and an immersion condenser lens formed on the second surface and configured to direct electromagnetic radiation through the InP substrate and toward the first InGaAs electromagnetic radiation absorber and the second InGaAs electromagnetic radiation absorber.

Claims:
What is claimed is: 
     
       1. An electromagnetic radiation detector, comprising:
 an indium phosphide (InP) substrate having a first surface opposite a second surface; 
 a first indium gallium arsenide (InGaAs) electromagnetic radiation absorber stacked on the first surface and configured to absorb a first set of electromagnetic radiation wavelengths; 
 a set of one or more buffer layers stacked on the first InGaAs electromagnetic radiation absorber and configured to absorb at least some of the first set of electromagnetic radiation wavelengths; 
 a second InGaAs electromagnetic radiation absorber stacked on the set of one or more buffer layers and configured to absorb a second set of electromagnetic radiation wavelengths; and 
 an immersion condenser lens formed on the second surface and configured to direct electromagnetic radiation through the InP substrate and toward the first InGaAs electromagnetic radiation absorber and the second InGaAs electromagnetic radiation absorber; wherein, 
 the second set of electromagnetic radiation wavelengths includes at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths; and 
 a first responsivity of the first InGaAs electromagnetic radiation absorber and a second responsivity of the second InGaAs electromagnetic radiation absorber have a crossover point at an electromagnetic radiation wavelength within a water absorption band of about 1.85 μm to about 2.0 μm. 
 
     
     
       2. The electromagnetic radiation detector of  claim 1 , wherein:
 the set of one or more buffer layers is a first set of one or more buffer layers; and 
 the electromagnetic radiation detector further comprises a second set of one or more buffer layers disposed directly on the InP substrate, between the InP substrate and the first InGaAs electromagnetic radiation absorber. 
 
     
     
       3. The electromagnetic radiation detector of  claim 2 , wherein the second set of one or more buffer layers comprises one or more indium arsenide phosphide (InAsP) layers. 
     
     
       4. The electromagnetic radiation detector of  claim 1 , wherein:
 at least the InP substrate, the first InGaAs electromagnetic radiation absorber, the set of one or more buffer layers, the second InGaAs electromagnetic radiation absorber, and the immersion condenser lens define at least a first detector unit; and 
 the electromagnetic radiation detector comprises an array of detector units including the first detector unit, each detector unit in the array of detector units including a different portion of at least the InP substrate, the first InGaAs electromagnetic radiation absorber, the set of one or more buffer layers, and the second InGaAs electromagnetic radiation absorber. 
 
     
     
       5. The electromagnetic radiation detector of  claim 1 , wherein a first responsivity of the first InGaAs electromagnetic radiation absorber and a second responsivity of the second InGaAs electromagnetic radiation absorber have a crossover point at an electromagnetic radiation wavelength of about 1.9 micrometers. 
     
     
       6. The electromagnetic radiation detector of  claim 1 , wherein the first and second InGaAs electromagnetic radiation absorbers are configured as two photodetectors facing a same direction and are connected by a tunnel junction. 
     
     
       7. The electromagnetic radiation detector of  claim 1 , wherein the set of one or more buffer layers comprises one or more indium arsenide phosphide (InAsP) layers. 
     
     
       8. An electromagnetic radiation detection system, comprising:
 a substrate; 
 on a first surface of the substrate,
 a first indium gallium arsenide (InGaAs) electromagnetic radiation absorber; 
 a second InGaAs electromagnetic radiation absorber; and 
 a buffer positioned between the first and second InGaAs electromagnetic radiation absorbers; 
 
 a first electromagnetic radiation emitter; 
 a second electromagnetic radiation emitter; and 
 a detection circuit configured to operate the first and second electromagnetic radiation emitters, and to separately detect,
 first electromagnetic radiation emitted by the first electromagnetic radiation emitter by reading a first current generated by the first InGaAs electromagnetic radiation absorber; and 
 second electromagnetic radiation emitted by the second electromagnetic radiation emitter by reading a second current generated by the second InGaAs electromagnetic radiation absorber; wherein, 
 
 a first responsivity of the first InGaAs electromagnetic radiation absorber and a second responsivity of the second InGaAs electromagnetic radiation absorber have a crossover point at an electromagnetic radiation wavelength in a range of about 1.9 μm to about 2.0 μm. 
 
     
     
       9. The electromagnetic radiation detection system of  claim 8 , wherein the detection circuit is configured to operate the first and second electromagnetic radiation emitters at a same time, while separately detecting the first and second electromagnetic radiation emitted by the first and second electromagnetic radiation emitters. 
     
     
       10. The electromagnetic radiation detection system of  claim 8 , wherein the detection circuit is configured to operate the first and second electromagnetic radiation emitters at different times, while separately detecting the first and second electromagnetic radiation emitted by the first and second electromagnetic radiation emitters. 
     
     
       11. The electromagnetic radiation detection system of  claim 8 , wherein the buffer is a first buffer and the electromagnetic radiation detection system further comprises:
 on the first surface of the substrate,
 a second buffer positioned between the substrate and the first InGaAs electromagnetic radiation absorber. 
 
 
     
     
       12. The electromagnetic radiation detection system of  claim 8 , further comprising:
 on a second surface of the substrate, opposite the first surface, 
 an immersion condenser lens configured to direct electromagnetic radiation through the substrate and toward the first and second InGaAs electromagnetic radiation absorbers. 
 
     
     
       13. The electromagnetic radiation detection system of  claim 8 , wherein a first absorption range of the first InGaAs electromagnetic radiation absorber and a second absorption range of the second InGaAs electromagnetic radiation absorber are non-overlapping. 
     
     
       14. The electromagnetic radiation detection system of  claim 8 , wherein the buffer comprises one or more indium arsenide phosphide (InAsP) layers. 
     
     
       15. An electronic device, comprising:
 a housing; 
 an electromagnetic radiation emitter configured to emit electromagnetic radiation through the housing; and 
 an electromagnetic radiation detector configured to receive electromagnetic radiation returned from a target; wherein, 
 the electromagnetic radiation detector includes,
 a substrate having a first surface opposite a second surface; 
 a first buffer stacked directly on the first surface; 
 a first indium gallium arsenide (InGaAs) electromagnetic radiation absorber stacked on the first buffer and configured to absorb a first set of electromagnetic radiation wavelengths; 
 a second buffer stacked on the first InGaAs electromagnetic radiation absorber and configured to absorb at least some of the first set of electromagnetic radiation wavelengths; and 
 a second InGaAs electromagnetic radiation absorber stacked on the second buffer and configured to absorb a second set of electromagnetic radiation wavelengths; wherein, 
 
 the second set of electromagnetic radiation wavelengths includes at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths; and 
 a first responsivity of the first InGaAs electromagnetic radiation absorber and a second responsivity of the second InGaAs electromagnetic radiation absorber have a crossover point at an electromagnetic radiation wavelength within a water absorption band of about 1.85 μm to about 2.0 μm. 
 
     
     
       16. The electronic device of  claim 15 , wherein the substrate is an indium phosphide (InP) substrate. 
     
     
       17. The electronic device of  claim 15 , further comprising:
 a first electrical contact on the first buffer; 
 a second electrical contact on the second InGaAs electromagnetic radiation absorber; and 
 a third electrical contact on the second buffer. 
 
     
     
       18. The electronic device of  claim 15 , further comprising:
 a first electrical contact on the first buffer; and 
 a second electrical contact on the second InGaAs electromagnetic radiation absorber; wherein, 
 the first and second electrical contacts are forward biased or reverse biased to read a current generated by the first InGaAs electromagnetic radiation absorber or the second InGaAs electromagnetic radiation absorber. 
 
     
     
       19. The electronic device of  claim 15 , further comprising:
 a cap layer stacked on the second InGaAs electromagnetic radiation absorber; wherein, 
 the cap layer, the second InGaAs electromagnetic radiation absorber, the second buffer, and the first InGaAs electromagnetic radiation absorber are configured as a pnnp, nppn, or pnpn layer structure. 
 
     
     
       20. The electronic device of  claim 15 , wherein:
 the housing has a back configured to face skin of a user when the electronic device is worn on a body part of the user; and 
 the electronic device further comprises a band configured to attach the housing to the body part. 
 
     
     
       21. The electronic device of  claim 15 , wherein each of the first buffer and the second buffer comprise one or more indium arsenide phosphide (InAsP) layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a nonprovisional of, and claims the benefit under 35 U.S.C. § 119(e) of, U.S. Provisional Patent Application No. 63/059,862, filed Jul. 31, 2020, the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     The described embodiments relate to the detection of electromagnetic radiation and, more particularly, to the detection of electromagnetic radiation over a wide band. 
     BACKGROUND 
     Sensors are included in many of today&#39;s electronic devices, including electronic devices such as smartphones, computers (e.g., tablet computers or laptop computers), wearable electronic devices (e.g., electronic watches, smart watches, or health monitors), game controllers, navigation systems (e.g., vehicle navigation systems or robot navigation systems), and so on. Sensors may variously sense the presence of objects, distances to objects, proximities of objects, movements of objects (e.g., whether objects are moving, or the speed, acceleration, or direction of movement of objects), compositions of objects, and so on. 
     One useful type of sensor is the electromagnetic radiation detector (or equivalently, an electromagnetic radiation sensor, an optical sensor, or an optical detector). Some electromagnetic radiation detectors may be configured to sense a wide band of electromagnetic radiation wavelengths, while others may be configured to sense a narrow band or multiple different bands of electromagnetic radiation wavelengths. At times, a new type of electromagnetic radiation detector needs to be developed to sense a particular band or bands of electromagnetic radiation wavelengths, or to sense a particular band or bands of electromagnetic radiation effectively (e.g., with a high enough signal-to-noise ratio (SNR)). 
     SUMMARY 
     Embodiments of the systems, devices, methods, and apparatus described in the present disclosure are directed to wideband back-illuminated electromagnetic radiation detectors. In some embodiments, wideband electromagnetic radiation detectors are described. For purposes of this description, a wideband electromagnetic radiation detector is a detector that is capable of detecting electromagnetic radiation wavelengths within a spectral wavelength range of 0.5 micrometers (μm) or greater. In some embodiments, short-wave infrared (SWIR) electromagnetic radiation detectors are described. For purposes of this description, SWIR electromagnetic radiation is considered to be electromagnetic radiation within a range of about 0.7 μm to about 3.0 μm. Despite many of the disclosed examples pertaining to wideband and/or SWIR electromagnetic radiation detectors, the systems, devices, methods, and apparatus described herein can be configured to detect various ranges of electromagnetic radiation wavelengths, including wavelengths within a narrow band and/or wavelengths outside of SWIR electromagnetic radiation. 
     In a first aspect, the present disclosure describes an electromagnetic radiation detector. The electromagnetic radiation detector may include an indium phosphide (InP) substrate having a first surface opposite a second surface; a first indium gallium arsenide (InGaAs) electromagnetic radiation absorber stacked on the first surface and configured to absorb a first set of electromagnetic radiation wavelengths; a set of one or more buffer layers stacked on the first InGaAs electromagnetic radiation absorber and configured to absorb at least some of the first set of electromagnetic radiation wavelengths; a second InGaAs electromagnetic radiation absorber stacked on the set of one or more buffer layers and configured to absorb a second set of electromagnetic radiation wavelengths; and an immersion condenser lens formed on the second surface and configured to direct electromagnetic radiation through the InP substrate and toward the first InGaAs electromagnetic radiation absorber and the second InGaAs electromagnetic radiation absorber. The second set of electromagnetic radiation wavelengths may include at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths. 
     In a second aspect, the present disclosure describes an electromagnetic radiation detection system. The system may include a substrate, a first electromagnetic radiation emitter, a second electromagnetic radiation emitter, and a detection circuit. On a first surface of the substrate, there may be a first electromagnetic radiation absorber, a second electromagnetic radiation absorber, and a buffer positioned between the first and second electromagnetic radiation absorbers. The detection circuit may be configured to operate the first and second electromagnetic radiation emitters, and to separately detect: first electromagnetic radiation emitted by the first electromagnetic radiation emitter by reading a first current generated by the first electromagnetic radiation absorber; and second electromagnetic radiation emitted by the second electromagnetic radiation emitter by reading a second current generated by the second electromagnetic radiation absorber. 
     In a third aspect, the present disclosure describes an electronic device. The electronic device may include a housing, an electromagnetic radiation emitter configured to emit electromagnetic radiation through the housing, and an electromagnetic radiation detector configured to receive electromagnetic radiation returned from a target. The electromagnetic radiation detector may also include a substrate having a first surface opposite a second surface; a first electromagnetic radiation absorber stacked on the first surface and configured to absorb a first set of electromagnetic radiation wavelengths; a buffer stacked on the first electromagnetic radiation absorber and configured to absorb at least some of the first set of electromagnetic radiation wavelengths; and a second electromagnetic radiation absorber stacked on the buffer and configured to absorb a second set of electromagnetic radiation wavelengths. The second set of electromagnetic radiation wavelengths may include at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG.  1    is a graph that illustrates the transmittance of various wavelengths of electromagnetic radiation in the atmosphere; 
         FIG.  2 A  shows a first example of an InGaAs detector; 
         FIG.  2 B  shows example responsivities of the absorbers of the InGaAs detector described with reference to  FIG.  2 A ; 
         FIG.  3 A  shows example responsivities for one particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  2 A ; 
         FIG.  3 B  shows example detectivities for the particular embodiment described with reference to  FIG.  3 A ; 
         FIG.  4 A  shows a second example of an InGaAs detector; 
         FIG.  4 B  shows example responsivities of the absorbers of the InGaAs detector described with reference to  FIG.  4 A ; 
         FIG.  5 A  shows example responsivities for one particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  4 A ; 
         FIG.  5 B  shows example detectivities for the particular embodiment described with reference to  FIG.  5 A ; 
         FIG.  6 A  shows example responsivities for another particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  4 A ; 
         FIG.  6 B  shows example detectivities for the particular embodiment described with reference to  FIG.  6 A ; 
         FIG.  7 A  shows an example use of a back-illuminated InGaAs detector having an immersion condenser lens; 
         FIG.  7 B  shows an example use of a back-illuminated InGaAs detector in conjunction with an emitter unit; 
         FIGS.  8 A- 11    illustrate various example contact arrangements for an InGaAs detector; 
         FIGS.  12 A and  12 B  show an example of a device that includes a set of sensors; 
         FIGS.  13 A and  13 B  shows another example of a device that includes a set of sensors; 
         FIG.  14    shows an example of an earbud that includes a set of sensors; 
         FIG.  15    shows an example elevation of a system of electromagnetic radiation emitters and detectors that may be included in an electronic device; and 
         FIG.  16    shows a sample electrical block diagram of an electronic device. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof), and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     Although the constructions and techniques described herein can be applied to electromagnetic radiation detectors that are tailored for the detection of various ranges of electromagnetic radiation wavelengths, the described techniques have particular applicability to, and are described primarily with reference to, wideband SWIR electromagnetic radiation detectors. Most of the electromagnetic radiation within this range of wavelengths is only minimally absorbed by atmospheric components such as water, oxygen, and carbon dioxide and, thus, this range is a good range for sensing other materials or components (e.g., particulate matter, skin, blood, and so on). 
     Some of the constructions and techniques described herein can also increase SNR. For example, some constructions and techniques can increase SNR by reducing dark current. 
     One type of electromagnetic radiation detector is the InGaAs detector. Some InGaAs detectors include a layer of InGaAs (an electromagnetic radiation absorber) that is epitaxially grown on a substrate (e.g., an InP substrate) in a lattice-matched configuration. Although such an electromagnetic radiation detector has low dark current and high SNR, its useful absorption range (or detection range) can be limited. 
     To extend the absorption range of an InGaAs detector to longer electromagnetic radiation wavelengths, a set of one or more buffer layers can be grown on a substrate, and an InGaAs layer (an electromagnetic radiation absorber) can be grown on the set of one or more buffer layers in a non-lattice-matched configuration. Such an InGaAs detector can be configured to receive electromagnetic radiation through its front side (i.e., in a front side illuminated (FSI) or front-illuminated configuration) or through its back side (i.e., in a back side illuminated (BSI) or back-illuminated configuration). An FSI configuration can eliminate lost light due to substrate or buffer absorption, and is easy to fabricate. In a BSI configuration, the InGaAs detector receives electromagnetic radiation through the detector&#39;s substrate before the electromagnetic radiation impinges on the detector&#39;s InGaAs layer. These kinds of InGaAs detector may have an absorption range that includes longer wavelengths than an InGaAs detector having a lattice-matched InGaAs layer. However, the detector&#39;s absorption of shorter wavelengths is sacrificed in a BSI configuration, as these wavelengths tend to be absorbed by the buffer layer(s) before they reach the InGaAs layer. The detector&#39;s ability to detect shorter wavelengths may also be more significantly impacted by dark current. 
     To extend the bandwidth of an electromagnetic radiation detector while maintaining a good SNR across much or all of its absorption range, two InGaAs electromagnetic radiation absorbers may be stacked on an InP substrate instead of one. To “tune” the detector and extend its bandwidth, one or more buffer layers may be grown between the InGaAs electromagnetic radiation absorbers. Because the buffer layer(s) may have high absorption in the same wavelength range as the InGaAs electromagnetic radiation absorber that is closest to the InP substrate, the detector may be used in a BSI configuration. In this manner, wavelengths that might otherwise be absorbed by the buffer layer(s) are first absorbed by the InGaAs electromagnetic radiation absorber closest to the InP substrate. By absorbing wavelengths in the absorption range of the InGaAs electromagnetic radiation absorber that is closest to the InP substrate, the buffer layer(s) enable the farther InGaAs electromagnetic radiation absorber to primarily absorb a range of wavelengths that does not overlap the range of wavelengths absorbed by the closer InGaAs electromagnetic radiation absorber and buffer layer(s). In some embodiments of this type of electromagnetic radiation detector, a second set of one or more buffer layers may be grown between the InP substrate and the closest InGaAs electromagnetic radiation absorber, thereby adjusting the range of wavelengths absorbed by the closest InGaAs electromagnetic radiation absorber. The absorption cut-on and cut-off of each InGaAs electromagnetic radiation absorber may be adjusted by changing the number or thickness of the buffer layer(s), or adjusting the materials or doping of the buffer layer(s) and/or InGaAs electromagnetic radiation absorbers, or in other ways. Different cut-ons and cut-offs may provide advantages in different sensing applications (e.g., different sensing ranges, different levels of dark current and SNR, different responsivities or detectivities, and so on). An additional advantage that is enabled by a BSI-configured InGaAs detector having two InGaAs electromagnetic radiation absorbers is a simplified InGaAs electromagnetic radiation absorber construction, which is sometimes helpful for improving signal absorption. A BSI-configured InGaAs detector also has a higher electrical connection density, which can be useful for InGaAs detectors that are used in detector arrays. 
     In some embodiments, the detector described in the preceding paragraph may be used in an FSI configuration. 
     In some embodiments, an immersion condenser lens may be formed on the back side of a BSI-configured InGaAs detector (i.e., on the surface of the detector that does not carry the InGaAs electromagnetic radiation absorbers. This can enable more electromagnetic radiation to be received and condensed onto a detector having a smaller footprint (i.e., smaller surface area). An immersion condenser lens may in some cases provide an electromagnetic radiation collection efficiency with improvement equal to the square of the refractive index of the detector substrate (e.g., a collection efficiency increase of about 10× for a detector with an InP substrate). 
     In some cases, the detectors described herein may be used to simultaneously or sequentially detect different wavelengths of electromagnetic radiation emitted by the same or different electromagnetic radiation emitters. 
     These and other techniques are described with reference to  FIGS.  1 - 16   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
     Directional terminology, such as “top”, “bottom”, “upper”, “lower”, “front”, “back”, “over”, “under”, “above”, “below”, “left”, “right”, etc. is used with reference to the orientation of some of the components in some of the figures described below. Because components in various embodiments can be positioned in a number of different orientations, directional terminology is used for purposes of illustration only and is in no way limiting. The directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude components being oriented in different ways. Also, as used herein, the phrase “at least one of” preceding a series of items, with the term “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list. The phrase “at least one of” does not require selection of at least one of each item listed; rather, the phrase allows a meaning that includes at a minimum one of any of the items, and/or at a minimum one of any combination of the items, and/or at a minimum one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or one or more of each of A, B, and C. Similarly, it may be appreciated that an order of elements presented for a conjunctive or disjunctive list provided herein should not be construed as limiting the disclosure to only that order provided. 
     As used herein, a “substrate” refers to a block or mass of common material. As used herein, a “layer” refers to one or more materials that are typically, but not necessarily, parallel to the top surface and/or bottom surface of a substrate or another layer. As used herein, a component, material, or layer that is “stacked” on another component, material, or layer may be formed directly on the other component, material, or layer, or may be attached to and separated from the other component, material, or layer by one or more other components, materials, or layers in a common “stack.” The terms “on” and “directly on” are used interchangeably herein. 
       FIG.  1    is a graph  100  that illustrates the transmittance of various wavelengths of electromagnetic radiation in the atmosphere. Electromagnetic radiation wavelengths (in μm) are shown along the horizontal axis of the graph  100 , and transmittance (in percent (%)) is shown along the vertical axis of the graph  100 . A transmittance of less than 100% for a particular wavelength means the atmosphere absorbs at least some portion of the electromagnetic radiation having the particular wavelength. 
     Also shown in  FIG.  1    is a block representation of the atmosphere  102 , and some of the molecules in the atmosphere  102  that absorb electromagnetic radiation at particular wavelengths (in some cases, interfering with or inhibiting an intended sensing operation). The molecules shown in  FIG.  1    include water (H 2 O), oxygen (O 2 ), and carbon dioxide (CO 2 ). 
     In some cases, it may be useful to detect a wide range of SWIR wavelengths. For example, it may be useful to detect a range (or multiple ranges) of SWIR wavelengths that include SWIR wavelengths between about 1.4-1.5 μm and about 2.5 μm. This range includes two windows in which SWIR wavelengths are not absorbed (or conversely, are transmitted). A first window, or range of transmitted SWIR wavelengths, extends from about 1.4-1.5 μm to about 1.85 μm. A second window, or range of transmitted SWIR wavelengths, extends from about 2.0 μm to about 2.5 μm. These windows/ranges are illustrated in  FIG.  1   . SWIR wavelengths between these two windows (i.e., SWIR wavelengths of about 1.85 μm to about 2.0 μm) are absorbed by water. 
     InGaAs electromagnetic radiation detectors (hereafter simply referred to as InGaAs detectors, or simply detectors) can be used to detect SWIR wavelengths in the range of about 1.4-2.5 μm but, at least conventionally, a single-absorber BSI-configured InGaAs detector has been unable to detect SWIR wavelengths at the low end of this range with high SNR. To detect electromagnetic radiation within a wider range of SWIR wavelengths using a single detector, or to detect different ranges of SWIR wavelengths using a single detector, a new type of SWIR electromagnetic radiation detector is needed. Although an FSI-configured InGaAs detector can detect a larger range of SWIR wavelengths, and does not suffer from buffer absorption, an immersion condenser lens cannot be formed on an FSI-configured InGaAs detector. An InGaAs detector with an immersion condenser lens will typically be able to achieve a higher SNR by concentrating received wavelengths of electromagnetic radiation into a smaller area than would otherwise be possible. 
     In some cases, the techniques described herein may be used to construct electromagnetic radiation detectors that can detect electromagnetic radiation wavelengths in other spectral ranges. These other ranges may include ranges that include, overlap, or are disjoint from SWIR wavelengths. 
       FIG.  2 A  shows a first example of an InGaAs detector  200 . The detector  200  includes an InP substrate  202  having a first surface  204  opposite a second surface  206 . A number of InGaAs electromagnetic radiation absorbers  208 ,  210  and other structures (e.g., a set of one or more buffer layers  212 ) may be formed on the first surface  204 , and electromagnetic radiation  214  may be received into the detector  200  by means of electromagnetic radiation  214  impinging on and passing through the second surface  206 . The reception of electromagnetic radiation  214  through the second surface  206 , and subsequent propagation of electromagnetic radiation  214  through the InP substrate  202  before it impinges on the InGaAs electromagnetic radiation absorbers  208 ,  210  stacked on the first surface  204 , makes the detector  200  a back-illuminated InGaAs detector. 
     A first InGaAs electromagnetic radiation absorber  208  may be stacked on the first surface  204  of the InP substrate  202 , and more particularly may be epitaxially grown directly on the first surface  204  as a lattice-matched InGaAs layer (i.e., a layer that is lattice-matched to the InP substrate). A p-n junction may be formed in the first InGaAs electromagnetic radiation absorber  208  via implantation and/or doping during epitaxial growth. 
     A set of one or more buffer layers  212  may be stacked on the first InGaAs electromagnetic radiation absorber  208 . More particularly, and in some embodiments, each buffer layer may be grown directly on the first InGaAs electromagnetic radiation absorber  208 , or on another one of the buffer layers  212 . Each buffer layer may be a layer of InAsP for example, and each successive buffer layer may be grown in a graded configuration to transition a lattice constant from a first value of the first InGaAs electromagnetic radiation absorber  208  toward a second value of the second InGaAs electromagnetic radiation absorber  210 . 
     A second InGaAs electromagnetic radiation absorber  210  may be stacked on the set of one or more buffer layers  212 , and more particularly may be epitaxially grown directly on one of the buffer layers. The second InGaAs electromagnetic radiation absorber  210  may be grown on a buffer layer as a non-latticed matched InGaAs layer (i.e., as a layer that is non-lattice-matched to the InP substrate  202  (e.g., an extended or strained InGaAs layer)), but may have a lattice constant that is substantially matched to that of the buffer layer on which it is grown. Thus, the second InGaAs electromagnetic radiation absorber  210  may experience a lattice-constant environment (and hence the performance) that it would normally have as an absorber in isolation. A p-n junction may be formed in the second InGaAs electromagnetic radiation absorber  210  via implantation, with doping during epitaxial growth, and/or with diffusion doping. A p-n junction may also be formed in a cap layer of InAsP instead of in the InGaAs electromagnetic radiation absorber  210  itself. 
     The second InGaAs electromagnetic radiation absorber  210  may have a smaller band gap than the set of one or more buffer layers  212 . 
     When electromagnetic radiation  214  impinges on the second surface  206  of the InP substrate  202 , some or all of the electromagnetic radiation  214  may pass through the InP substrate  202  and be absorbed by the first InGaAs electromagnetic radiation absorber  208 . For example, a first set of electromagnetic radiation wavelengths (e.g., a first range of electromagnetic radiation wavelengths) may pass through the InP substrate  202  and be absorbed by the first InGaAs electromagnetic radiation absorber  208 . Some of the electromagnetic radiation  214  that impinges on the first InGaAs electromagnetic radiation absorber  208  may pass through the first InGaAs electromagnetic radiation absorber  208  without being absorbed, or with minimal absorption, and be absorbed by the set of one or more buffer layers  212 . The electromagnetic radiation absorbed by the set of one or more buffer layers  212  may include at least some of the first set of electromagnetic radiation wavelengths. The electromagnetic radiation absorbed by the set of one or more buffer layers  212  may also include other electromagnetic radiation wavelengths. Some of the electromagnetic radiation that impinges on the first InGaAs electromagnetic radiation absorber  208  may also pass through the set of one or more buffer layers  212  and impinge on the second InGaAs electromagnetic radiation absorber  210 . At least some of this electromagnetic radiation (e.g., a second set of electromagnetic radiation wavelengths, such as a second range of electromagnetic radiation wavelengths) may be absorbed by the second InGaAs electromagnetic radiation absorber  210 . The second set of electromagnetic radiation wavelengths may include at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths and, in some cases, may only include electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths. The second set of electromagnetic radiation wavelengths generally includes longer wavelengths than the first set of wavelengths. 
     In some embodiments, the first InGaAs electromagnetic radiation absorber  208  may be configured to absorb electromagnetic radiation wavelengths in the range of about 1.4 μm to about 1.7 μm, and the second InGaAs electromagnetic radiation absorber  210  may be configured to absorb electromagnetic radiation wavelengths in the range of about 1.7 μm to about 2.5 μm. For purposes of this description, a material or layer, such as an electromagnetic radiation absorber, is considered to “absorb” an electromagnetic radiation wavelength if it has a responsivity of 0.5 Amperes/Watt (A/W) or greater for the electromagnetic radiation wavelength (and preferably, a responsivity of 0.6 A/W or 0.7 A/W or greater); or in the case of a buffer or buffer layer, if the buffer or buffer layer has an equivalent absorption in some arbitrary units (a.u.). Also for purposes of this description, “about” a particular electromagnetic radiation wavelength is considered to be within +/−50 nanometers (nm) of the particular electromagnetic radiation wavelength. 
       FIG.  2 B  shows example responsivities  220  of the absorbers of the InGaAs detector described with reference to  FIG.  2 A . 
     The first InGaAs electromagnetic radiation absorber  208  has a first responsivity  222  and may absorb a particular range of electromagnetic radiation wavelengths about a first electromagnetic radiation wavelength  224 . 
     The second InGaAs electromagnetic radiation absorber  210  has a second responsivity  226  and may absorb a particular range of electromagnetic radiation wavelengths about a second electromagnetic radiation wavelength  228 . As shown, the second InGaAs electromagnetic radiation absorber  210  may generally absorb longer electromagnetic radiation wavelengths than the first InGaAs electromagnetic radiation absorber  208 . Because electromagnetic radiation impinges on the first InGaAs electromagnetic radiation absorber  208  first, and only impinges on the second InGaAs electromagnetic radiation absorber  210  after passing through the first InGaAs electromagnetic radiation absorber  208 , fewer (or in some cases none) of the electromagnetic radiation wavelengths absorbed by the first InGaAs electromagnetic radiation absorber  208  may impinge on (and be absorbed by) the second InGaAs electromagnetic radiation absorber  210 . In other embodiments, the absorption ranges of the first and second InGaAs electromagnetic radiation absorbers  208 ,  210  may not overlap (e.g., they may be disjoint). 
     The set of one or more buffer layers  212  is associated with an absorption (or buffer loss)  230 . The absorption range of the one or more buffer layers  212  may overlap some or all of the absorption range of the first InGaAs electromagnetic radiation absorber  208 , and in some cases may overlap some (but not all) of the absorption range of the second InGaAs electromagnetic radiation absorber  210 . Because electromagnetic radiation impinges on the first InGaAs electromagnetic radiation absorber  208  first, and only impinges on the set of one or more buffer layers  212  after impinging on the first InGaAs electromagnetic radiation absorber  208 , electromagnetic radiation within the absorption range of the first InGaAs electromagnetic radiation absorber  208  may be absorbed by the first InGaAs electromagnetic radiation absorber  208  before it is absorbed by the set of one or more buffer layers  212 , thereby avoiding the impact of buffer absorption loss on the first InGaAs electromagnetic radiation absorber  208 . Also, the set of one or more buffer layers  212  may absorb some or all of the electromagnetic radiation that could be absorbed by the first InGaAs electromagnetic radiation absorber  208 , but is not absorbed by the first InGaAs electromagnetic radiation absorber  208 , so that the responsivity of the second InGaAs electromagnetic radiation absorber  210  is more specifically tuned to an absorption range that does not overlap the absorption range of the first InGaAs electromagnetic radiation absorber (though some overlap in the electromagnetic radiation wavelengths absorbed by the first and second InGaAs electromagnetic radiation absorbers  208 ,  210  is possible and even likely. 
       FIG.  3 A  shows example responsivities  300  for one particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  2 A .  FIG.  3 B  shows example detectivities  310  for the particular embodiment. As shown in  FIG.  3 A , the first InGaAs electromagnetic radiation absorber  208  has a responsivity  302  with an absorption cut-off at about 1.7 μm, and may have an absorption range that extends to below 1.2 μm (and in some cases to the beginning of the optical spectrum). The second InGaAs electromagnetic radiation absorber  210  has a responsivity  304  that extends from about 1.4 μm to about 2.5 μm, with an absorption cut-on of about 1.7 μm. The responsivities  302 ,  304  of the first and second InGaAs electromagnetic radiation absorbers  208 ,  210  therefore intersect (i.e., have a crossover point) at about 1.7 μm. 
     The set of one or more buffer layers  212  is associated with an absorption (or buffer loss)  306  that overlaps a portion of the first InGaAs electromagnetic radiation absorber&#39;s responsivity  302 . 
     As shown in  FIG.  3 B , the detectivity  312  of the first InGaAs electromagnetic radiation absorber  208  (in Jones) is higher (as between the first and second InGaAs electromagnetic radiation absorbers  208 ,  210 ) below about 1.7 μm, and the detectivity  314  of the second InGaAs electromagnetic radiation absorber  210  is higher between about 1.7 μm and 2.5 μm. The InGaAs detector described with reference to  FIGS.  3 A and  3 B  can be advantageous in that its first InGaAs electromagnetic radiation absorber is lattice-matched to its InP substrate, and there is no buffer layer that might interfere with the first InGaAs electromagnetic radiation absorber by, for example, increasing the dark current that may interfere with a readout of the current it generates in response to its absorption of electromagnetic radiation within its absorption range. Also, shorter-wavelength (i.e., larger bandgap) semiconductors generally have low dark current densities. 
     In some cases, it may be desirable to adjust the range of electromagnetic radiation wavelengths to which an absorber is responsive (i.e., the range of electromagnetic radiation wavelengths that the absorber absorbs). In some cases, the responsivity of an InGaAs electromagnetic radiation absorber that is farther from an InP substrate may be adjusted by adjusting the number or thickness of the one or more buffer layers that separate it from an InGaAs electromagnetic radiation absorber that is positioned earlier in an electromagnetic radiation propagation path. In some cases, the responsivity of an InGaAs electromagnetic radiation absorber that is positioned closest to an InP substrate may be adjusted by separating it from the InP substrate by an additional set of one or more buffer layers, as described below with reference to  FIGS.  4 A and  4 B  for example. In some cases, the responsivity of an InGaAs electromagnetic radiation absorber may be adjusted by altering its composition or growth method, and/or by adjusting the number, thickness, or type of buffer layers that separate it from an InP substrate or other InGaAs electromagnetic radiation absorber. 
       FIG.  4 A  shows a second example of an InGaAs detector  400 . The detector  400  includes an InP substrate  402  having a first surface  404  opposite a second surface  406 . A number of InGaAs electromagnetic radiation absorbers  408 ,  410  and other structures (e.g., one or more sets of buffer layers  412 ,  414 ) may be formed on the first surface  404 , and electromagnetic radiation  416  may be received into the detector  400  by means of electromagnetic radiation  416  impinging on and passing through the second surface  406 . The reception of electromagnetic radiation  416  through the second surface  406 , and subsequent propagation of electromagnetic radiation  416  through the InP substrate  402  before it impinges on the InGaAs electromagnetic radiation absorbers  408 ,  410  stacked on the first surface  404 , makes the detector  400  a back-illuminated InGaAs detector. 
     A first set of one or more buffer layers  412  may be stacked on the first surface  404  of the InP substrate  402 . More particularly, and in some embodiments, each buffer layer in the first set may be epitaxially grown directly on the InP substrate  402 , or on another one of the buffer layers  412 . Each buffer layer may be a layer of InAsP for example, and each successive buffer layer may be grown in a graded configuration to transition a lattice constant from a first value of the InP substrate  402  toward a second value of the first InGaAs electromagnetic radiation absorber  408 . 
     A first InGaAs electromagnetic radiation absorber  408  may be stacked on the first set of one or more buffer layers  412 , with the buffer layer(s)  412  disposed between the InP substrate  402  and the first InGaAs electromagnetic radiation absorber  408 . More particularly, and in some embodiments, the first InGaAs electromagnetic radiation absorber  408  may be epitaxially grown directly on a buffer layer in the first set of one or more buffer layers  412  as a short lattice-mismatched InGaAs layer (i.e., as a layer that is lattice-mismatched to the InP substrate), but may have a lattice constant that is substantially matched to that of the buffer layer on which it is grown. Thus, the first InGaAs electromagnetic radiation absorber  408  may experience a lattice-constant environment (and hence the performance) that it would normally have as an absorber in isolation. A p-n junction may be formed in the first InGaAs electromagnetic radiation absorber  408  via implantation, with doping during epitaxial growth, and/or with diffusion doping. A p-n junction may also be formed in a cap layer of InAsP instead of in the InGaAs electromagnetic radiation absorber  410  itself. 
     A second set of one or more buffer layers  414  may be stacked on the first InGaAs electromagnetic radiation absorber  408 . More particularly, and in some embodiments, each buffer layer in the set may be epitaxially grown directly on the first InGaAs electromagnetic radiation absorber  408 , or on another one of the buffer layers  414 . Each buffer layer may be a layer of InAsP for example, and each successive buffer layer may be grown in a graded configuration to transition a lattice constant from a first value of the first InGaAs electromagnetic radiation absorber  408  toward a second value of the second InGaAs electromagnetic radiation absorber  410 . 
     A second InGaAs electromagnetic radiation absorber  410  may be stacked on the second set of one or more buffer layers  414 , with the buffer layer(s)  414  disposed between the first InGaAs electromagnetic radiation absorber  408  and the second InGaAs electromagnetic radiation absorber  410 . More particularly, and in some embodiments, the second InGaAs electromagnetic radiation absorber  410  may be epitaxially grown on a buffer layer in the second set of one or more buffer layers  414  as a long or extended latticed mismatched InGaAs layer (i.e., as a layer that is non-lattice-matched to the InP substrate  402  (e.g., an extended or strained InGaAs layer)), but may have a lattice constant that is substantially matched to that of the buffer layer on which it is grown. Thus, the second InGaAs electromagnetic radiation absorber  410  may experience a lattice-constant environment (and hence the performance) that it would normally have as an absorber in isolation. A p-n junction may be formed in the second InGaAs electromagnetic radiation absorber  410  via implantation, with doping during epitaxial growth, and/or with diffusion doping. A p-n junction may also be formed in a cap layer of InAsP instead of in the InGaAs electromagnetic radiation absorber  410  itself. 
     The first InGaAs electromagnetic radiation absorber  408  may have a larger band gap than the first set of one or more buffer layers  412 . Similarly, the second InGaAs electromagnetic radiation absorber  410  may have a larger band gap than the second set of one or more buffer layers  414 . This enables each InGaAs electromagnetic radiation absorber  408 ,  410  to absorb longer electromagnetic radiation wavelengths that are absorbed by layers that are closer to the InP substrate  402 . 
     When electromagnetic radiation  416  impinges on the second surface  406  of the InP substrate  402 , some or all of the electromagnetic radiation  416  may pass through the InP substrate  402  and be absorbed by the first InGaAs electromagnetic radiation absorber  408 . For example, a first set of electromagnetic radiation wavelengths (e.g., a first range of electromagnetic radiation wavelengths) may pass through the InP substrate  402  and be absorbed by the first InGaAs electromagnetic radiation absorber  408 . The set of electromagnetic radiation wavelengths that is absorbed by the first InGaAs electromagnetic radiation absorber  408  may be influenced by the absorption range of the first set of one or more buffer layers  412 . For example, electromagnetic radiation wavelengths that are absorbed by the first set of buffer layers  412  will not impinge on the first InGaAs electromagnetic radiation absorber  408 . In some cases, the first set of one or more buffer layers  412  may be used to adjust (or tune) one or both bounds of a range of electromagnetic radiation wavelengths absorbed by the first InGaAs electromagnetic radiation absorber  408 . 
     Some of the electromagnetic radiation  416  that impinges on the first InGaAs electromagnetic radiation absorber  408  may pass through the first InGaAs electromagnetic radiation absorber  408  without being absorbed, or with minimal absorption, and be absorbed by the second set of one or more buffer layers  414 . The electromagnetic radiation absorbed by the second set of one or more buffer layers  414  may include at least some of the first set of electromagnetic radiation wavelengths. The electromagnetic radiation absorbed by the second set of one or more buffer layers  414  may also include other electromagnetic radiation wavelengths. Some of the electromagnetic radiation that impinges on the first InGaAs electromagnetic radiation absorber  408  may also pass through the second set of one or more buffer layers  414  and impinge on the second InGaAs electromagnetic radiation absorber  410 . At least some of this electromagnetic radiation (e.g., a second set of electromagnetic radiation wavelengths, such as a second range of electromagnetic radiation wavelengths) may be absorbed by the second InGaAs electromagnetic radiation absorber  410 . The second set of electromagnetic radiation wavelengths may include at least some electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths and, in some cases, may only include electromagnetic radiation wavelengths that are not in the first set of electromagnetic radiation wavelengths. The second set of electromagnetic radiation wavelengths generally includes longer wavelengths than the first set of wavelengths. 
     In some embodiments, the first InGaAs electromagnetic radiation absorber  408  may be configured to absorb electromagnetic radiation wavelengths in the range of about 1.4 μm to about 1.9 μm or 2.0 μm, and the second InGaAs electromagnetic radiation absorber  410  may be configured to absorb electromagnetic radiation wavelengths in the range of about 1.9 μm or 2.0 μm to about 2.5 μm. Configuring the cut-off of the absorption range of the first InGaAs electromagnetic radiation detector within a range of about 1.85 μm to about 2.0 μm (i.e., a band in which electromagnetic radiation is largely absorbed by water, as discussed with reference to  FIG.  1   ), and configuring the cut-on of the absorption range of the second InGaAs electromagnetic radiation detector within the same range, places the crossover between absorber responsivities within a range of electromagnetic radiation wavelengths where little useful sensing can be performed (at least in some applications). 
       FIG.  4 B  shows example responsivities  420  of the absorbers of the InGaAs detector described with reference to  FIG.  4 A . 
     The first InGaAs electromagnetic radiation absorber  408  has a first responsivity  422  and may absorb a particular range of electromagnetic radiation wavelengths about a first electromagnetic radiation wavelength  424 . 
     The second InGaAs electromagnetic radiation absorber  410  has a second responsivity  426  and may absorb a particular range of electromagnetic radiation wavelengths about a second electromagnetic radiation wavelength  428 . As shown, the second InGaAs electromagnetic radiation absorber  410  may generally absorb longer electromagnetic radiation wavelengths than the first InGaAs electromagnetic radiation absorber  408 . Because electromagnetic radiation impinges on the first InGaAs electromagnetic radiation absorber  408  first, and only impinges on the second InGaAs electromagnetic radiation absorber  410  after passing through the first InGaAs electromagnetic radiation absorber  408 , fewer (or in some cases none) of the electromagnetic radiation wavelengths absorbed by the first InGaAs electromagnetic radiation absorber  408  may impinge on (and be absorbed by) the second InGaAs electromagnetic radiation absorber  410 . In other embodiments, the absorption ranges of the first and second InGaAs electromagnetic radiation absorbers  408 ,  410  may not overlap (e.g., they may be disjoint). 
     The first set of one or more buffer layers  412  is associated with an absorption (or buffer loss)  430 . The absorption range of the first set of one or more buffer layers  412  may be negligible, and may be generally outside the absorption ranges of the first and second InGaAs electromagnetic radiation absorbers  408 ,  410 . 
     The second set of one or more buffer layers  414  is associated with an absorption (or buffer loss)  432 . The absorption range of the second set of one or more buffer layers  414  may overlap some or all of the absorption range of the first InGaAs electromagnetic radiation absorber  408 , and in some cases may overlap some (but not all) of the absorption range of the second InGaAs electromagnetic radiation absorber  410 . Because electromagnetic radiation impinges on the first InGaAs electromagnetic radiation absorber  408  first, and only impinges on the second set of one or more buffer layers  414  after impinging on the first InGaAs electromagnetic radiation absorber  408 , electromagnetic radiation within the absorption range of the first InGaAs electromagnetic radiation absorber  408  may be absorbed by the first InGaAs electromagnetic radiation absorber  408  before it is absorbed by the second set of one or more buffer layers  414 , thereby avoiding the impact of buffer absorption loss on the first InGaAs electromagnetic radiation absorber  408 . Also, the second set of one or more buffer layers  414  may absorb some or all of the electromagnetic radiation that could be absorbed by the first InGaAs electromagnetic radiation absorber  408 , but is not absorbed by the first InGaAs electromagnetic radiation absorber  408 , so that the responsivity of the second InGaAs electromagnetic radiation absorber  410  is more specifically tuned to an absorption range that does not overlap the absorption range of the first InGaAs electromagnetic radiation absorber (though some overlap in the electromagnetic radiation wavelengths absorbed by the first and second InGaAs electromagnetic radiation absorbers  408 ,  410  is possible and even likely. 
       FIG.  5 A  shows example responsivities  500  for one particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  4 A .  FIG.  5 B  shows example detectivities  510  for the particular embodiment. 
     As shown in  FIG.  5 A , the first InGaAs electromagnetic radiation absorber  408  has a responsivity  502  with an absorption cut-off at about 1.9 μm, and may have an absorption range that extends to about 1.2 μm. The second InGaAs electromagnetic radiation absorber  410  has a responsivity  504  that extends from about 1.4 μm to about 2.5 μm, with an absorption cut-on of about 1.9 μm. The responsivities  502 ,  504  of the first and second InGaAs electromagnetic radiation absorbers  408 ,  410  therefore intersect (i.e., have a crossover point) at about 1.9 μm. 
     The first set of one or more buffer layers  412  is associated with an absorption (or buffer loss)  506  that is generally outside the responsivity of the first and second InGaAs electromagnetic radiation detectors  408 ,  410 , and the second set of one or more buffer layers  414  is associated with an absorption (or buffer loss)  508  that overlaps portions of the first and second InGaAs electromagnetic radiation absorber responsivities  502 ,  504 . 
     As shown in  FIG.  5 B , the detectivity  512  of the first InGaAs electromagnetic radiation absorber  408  (in Jones) is higher (as between the first and second InGaAs electromagnetic radiation absorbers  408 ,  410 ) between about 1.2 μm and 1.9 μm, and the detectivity  514  of the second InGaAs electromagnetic radiation absorber  410  is higher between about 1.9 μm and 2.5 μm. 
     The InGaAs detector described with reference to  FIGS.  5 A and  5 B  can be advantageous in that the crossover point between its first and second InGaAs electromagnetic radiation absorbers is within a water absorption band (e.g., the water absorption band between about 1.85 μm and about 2.0 μm, as described with reference to  FIG.  1   ). In other words, the crossover point is at an electromagnetic radiation wavelength (or wavelength range) where little useful sensing may be done. 
       FIG.  6 A  shows example responsivities  600  for another particular embodiment of an InGaAs detector that is constructed as described with reference to  FIG.  4 A .  FIG.  6 B  shows example detectivities  610  for the particular embodiment. 
     As shown in  FIG.  6 A , the first InGaAs electromagnetic radiation absorber  408  has a responsivity  602  with an absorption cut-off at about 2.1 μm, and may have an absorption range that extends to about 1.4 μm. The second InGaAs electromagnetic radiation absorber  410  has a responsivity  604  that extends from about 1.6 μm to about 2.5 μm, with an absorption cut-on of about 2.1 μm. The responsivities  602 ,  604  of the first and second InGaAs electromagnetic radiation absorbers  408 ,  410  therefore intersect (i.e., have a crossover point) at about 2.1 μm. 
     The first set of one or more buffer layers  412  is associated with an absorption (or buffer loss)  606  that overlaps a portion of the responsivity of the first InGaAs electromagnetic radiation detector  400 , and the second set of one or more buffer layers  414  is associated with an absorption (or buffer loss)  608  that overlaps portions of the first and second InGaAs electromagnetic radiation absorber responsivities  602 ,  604 . 
     As shown in  FIG.  6 B , the detectivity  612  of the first InGaAs electromagnetic radiation absorber  408  (in Jones) is higher (as between the first and second InGaAs electromagnetic radiation absorbers  408 ,  410 ) between about 1.4 μm and 2.1 μm, and the detectivity  614  of the second InGaAs electromagnetic radiation absorber  410  is higher between about 2.1 μm and 2.5 μm. 
     The InGaAs detector described with reference to  FIGS.  6 A and  6 B  can be advantageous in that its first InGaAs electromagnetic radiation absorber has a cut-off at a longer electromagnetic radiation wavelength than the other InGaAs detectors described herein. When the buffer layer(s) through which electromagnetic radiation passes, e.g., to reach the first InGaAs electromagnetic radiation absorber, do not absorb electromagnetic radiation wavelengths in a range of interest (e.g., when the buffer layer(s) only absorb electromagnetic radiation wavelengths below 1.4 μm), there may be less dark current that interferes with a readout of the current it generates in response to its absorption of electromagnetic radiation within its absorption range. The above limitation on the absorption range of the buffer layer(s) closer to the InP substrate may put a constraint on the composition of the buffer layer(s) (e.g., an InAsP composition), and hence on the lattice constant that can be attained, and hence on the composition of the first electromagnetic radiation absorber (e.g., an InGaAs composition), and hence on the long-wavelength cut-off of the first electromagnetic radiation absorber. This design ensures that no absorber&#39;s responsivity is degraded by buffer absorption, at any wavelength of interest. 
     In some embodiments, an InGaAs detector constructed as described with reference to  FIG.  2 A or  4 A  may include one or more additional InGaAs electromagnetic radiation absorbers, stacked on one or more additional sets of one or more buffer layers. For example, an InGaAs detector may include a third InGaAs electromagnetic radiation absorber stacked on a third set of one or more buffer layers, which third set of one or more buffer layers may be stacked on the detector&#39;s second InGaAs electromagnetic radiation absorber. 
     In some embodiments, a detector may include a substrate, buffer layer(s), and/or electromagnetic radiation absorbers that include other materials. For example, the substrate may be formed of Gallium Arsenide (GaAs), Cadmium Telluride (CdTe), or Silicon (Si). In the case of GaAs, CdTe, or Si substrates, the electromagnetic radiation absorbers may in some cases be formed of Mercury Cadmium Telluride (HgCdTe). 
       FIG.  7 A  shows an example use of a back-illuminated InGaAs detector  700  having an immersion condenser lens  702 . In some examples, the InGaAs detector  700  may include the InGaAs electromagnetic radiation absorbers and buffer layer(s) described with reference to one or more of  FIGS.  2 A- 6 B . 
     The InGaAs detector  700  may have multiple InGaAs electromagnetic radiation absorbers (i.e., two or more) and one or more sets of one or more buffer layers, collectively labeled  704 , stacked (e.g., grown) on a first surface  706  of an InP substrate  708 . The InP substrate  708  may have a second surface  710  on which the immersion condenser lens  702  is formed (e.g., etched), such that the InGaAs electromagnetic radiation absorbers and buffer layer(s)  704  are immersed in a continuous high-refractive-index medium (i.e., the InP substrate  708 ). The immersion condenser lens  702  can improve the transfer of electromagnetic radiation  712  through the InP substrate  708  of the InGaAs detector  700 . 
     The immersion condenser lens  702  may receive electromagnetic radiation  712  through the second surface  710  of the InP substrate  708 , which may be a convex surface, and focus the electromagnetic radiation onto the InGaAs electromagnetic radiation absorbers and buffer layers  704 . 
     In some cases, the InGaAs detector  700  may be used as a singular (e.g., stand-alone) detector unit. In other cases, the InGaAs detector  700  may be one detector unit in an array (e.g., a one-dimensional or two-dimensional array) of InGaAs detectors  700 ,  714  or detector units. The different InGaAs detectors  700 ,  714  may share a common InP substrate  708  (as shown) and/or other components, or may be separately fabricated (or jointly fabricated and then diced) and mounted on a carrier substrate, or within a housing, for example. 
     In some cases, and as shown in  FIG.  7 B , the InGaAs detector  700  may be used as a detector unit and positioned near an emitter unit. The emitter unit may include an electromagnetic radiation emitter  716  that is stacked on, or positioned near, a surface  718  of a lens  720  and configured to emit electromagnetic radiation through the lens  720 . By way of example, the lens  720  may collimate, spread, or focus the electromagnetic radiation emitted by the electromagnetic radiation emitter  716 . The electromagnetic radiation emitter  716  may emit electromagnetic radiation within, throughout, or including the range of electromagnetic radiation that can be absorbed by the InGaAs detector  700 . In some embodiments, the lens  720  may not be provided. 
       FIGS.  8 A- 11    illustrate various example contact arrangements for an InGaAs detector. By way of example, the contact arrangements are shown for an InGaAs detector similar to the InGaAs detector described with reference to  FIG.  4   . However, the various contact arrangements may be used for any of the InGaAs detectors described herein. 
       FIG.  8 A  shows a plan view of a 3-contact InGaAs detector  800 , and  FIG.  8 B  shows a cross-sectional elevation of the InGaAs detector  800 . The InGaAs detector  800  includes an InP substrate  802  on which an optional first set of one or more buffer layers (collectively referred to as a first buffer  804 ) is stacked. A first InGaAs electromagnetic radiation absorber  806  is stacked on the first buffer  804  (i.e., with the first buffer  804  disposed between the InP substrate  802  and the first InGaAs electromagnetic radiation absorber  806 ), or on the InP substrate  802  when the first buffer  804  is not provided. A second set of one or more buffer layers (collectively referred to as a second buffer  808 ) is stacked on the first InGaAs electromagnetic radiation absorber  806 , and a second InGaAs electromagnetic radiation absorber  810  is stacked on the second buffer  808  (i.e., with the second buffer  808  disposed between the first InGaAs electromagnetic radiation absorber  806  and the second InGaAs electromagnetic radiation absorber  810 ). 
     The first InGaAs electromagnetic radiation absorber may be electrically disposed between a first electrical contact  812  and a second electrical contact  814 , with the first and second electrical contacts  812 ,  814  being used to electrically bias the first InGaAs electromagnetic radiation absorber  806  and sense a first current generated by the first InGaAs electromagnetic radiation absorber  806 . In some embodiments, the first electrical contact  812  may be deposited on the first buffer  804 , and the second electrical contact  814  may be deposited on the second buffer  808 . In other embodiments, the first electrical contact  812  may be deposited on the InP substrate  802  for example, or the second electrical contact  814  may be deposited on the first InGaAs electromagnetic radiation absorber  806 . 
     The second InGaAs electromagnetic radiation absorber may be electrically disposed between the second electrical contact  814  and a third electrical contact  816 , with the second and third electrical contacts  814 ,  816  being used to electrically bias the second InGaAs electromagnetic radiation absorber  810  and sense a second current generated by the second InGaAs electromagnetic radiation absorber  810 . In some embodiments, the third electrical contact  816  may be deposited on the second InGaAs electromagnetic radiation absorber  810 . 
     As shown in  FIG.  8 A , the first buffer  804  may extend over the entirety of the InP substrate  802 , or over a greater portion of the InP substrate  802  than each of the first and second InGaAs electromagnetic radiation absorbers  806 ,  810  and second buffer  808 . The first electrical contact  812  is shown to extend along three sides of the first InGaAs electromagnetic radiation absorber  806 , but in other embodiments may be positioned adjacent only one side of the first InGaAs electromagnetic radiation absorber  806 , or may surround the first InGaAs electromagnetic radiation absorber  806 , or have other configurations. Although the perimeters of all substrate, buffer, and absorber components are shown to be rectangular or square in  FIG.  8 A , the perimeters of these elements could alternatively have any shape. 
       FIG.  9 A  shows a plan view of a 2-contact InGaAs detector  900 , and  FIG.  9 B  shows a cross-sectional elevation of the InGaAs detector  900 . The InGaAs detector  900  includes an InP substrate  902  on which an optional first set of one or more buffer layers (collectively referred to as a first buffer  904 ) is stacked. A first InGaAs electromagnetic radiation absorber  906  is stacked on the first buffer  904  (i.e., with the first buffer  904  disposed between the InP substrate  902  and the first InGaAs electromagnetic radiation absorber  906 ), or on the InP substrate  902  when the first buffer  904  is not provided. A second set of one or more buffer layers (collectively referred to as a second buffer  908 ) is stacked on the first InGaAs electromagnetic radiation absorber  906 , and a second InGaAs electromagnetic radiation absorber  910  is stacked on the second buffer  908  (i.e., with the second buffer  908  disposed between the first InGaAs electromagnetic radiation absorber  906  and the second InGaAs electromagnetic radiation absorber  910 ). 
     The first and second InGaAs electromagnetic radiation absorbers  906 ,  910  may be electrically disposed between a first electrical contact  912  and a second electrical contact  914 , with the first and second electrical contacts  912 ,  914  being used to electrically bias the first InGaAs electromagnetic radiation absorber  906  in a forward direction and sense a current corresponding to a total current generated by the first InGaAs electromagnetic radiation absorber  906 . Alternatively, the first and second electrical contacts  912 ,  914  may be used to electrically bias the second InGaAs electromagnetic radiation absorber  910  in a reverse direction and sense a current corresponding to a total current generated by the second InGaAs electromagnetic radiation absorber  910 . In other embodiments, and depending on the implantation or doping of the first and second InGaAs electromagnetic radiation absorbers  906 ,  910 , the electrical contacts  912 ,  914  may be used to bias and read the first and second InGaAs electromagnetic radiation absorbers  906 ,  910  in opposite directions. In some embodiments, the first electrical contact  912  may be deposited on the first buffer  904 , and the second electrical contact  914  may be deposited on the second InGaAs electromagnetic radiation absorber  910 . 
     As shown in  FIG.  9 A , the first buffer  904  may extend over the entirety of the InP substrate  902 , or over a greater portion of the InP substrate  902  than each of the first and second InGaAs electromagnetic radiation absorbers  906 ,  910  and second buffer  908 . The first electrical contact  912  is shown to extend along three sides of the first InGaAs electromagnetic radiation absorber  906 , but in other embodiments may be positioned adjacent only one side of the first InGaAs electromagnetic radiation absorber  906 , or may surround the first InGaAs electromagnetic radiation absorber  906 , or have other configurations. Although the perimeters of all substrate, buffer, and absorber components are shown to be rectangular or square in  FIG.  9 A , the perimeters of these elements could alternatively have any shape. 
       FIG.  10    shows an elevation of an InGaAs detector  1000  in which first and second InGaAs electromagnetic radiation absorbers  1006 ,  1010  are configured as back-to-back photodiodes. The InGaAs detector  1000  includes an InP substrate  1002  on which a first set of one or more buffer layers (collectively referred to as a first buffer  1004 ) is stacked. A first InGaAs electromagnetic radiation absorber  1006  is stacked on the first buffer  1004  (i.e., with the first buffer  1004  disposed between the InP substrate  1002  and the first InGaAs electromagnetic radiation absorber  1006 ). A second set of one or more buffer layers (collectively referred to as a second buffer  1008 ) is stacked on the first InGaAs electromagnetic radiation absorber  1006 , and a second InGaAs electromagnetic radiation absorber  1010  is stacked on the second buffer  1008  (i.e., with the second buffer  1008  disposed between the first InGaAs electromagnetic radiation absorber  1006  and the second InGaAs electromagnetic radiation absorber  1010 ). A cap layer  1012  is stacked on the second InGaAs electromagnetic radiation absorber  1010  (i.e., with the second InGaAs electromagnetic radiation absorber  1010  disposed between the second buffer  1008  and the cap layer  1012 . 
     Each of the first and second InGaAs electromagnetic radiation absorbers  1006 ,  1010  may be n-doped (or n-type) InGaAs electromagnetic radiation absorbers. Each of the first and second buffers  1004 ,  1008  may include one or more layers of indium arsenide phosphide (InAsP), with the first buffer  1004  including one or more p-doped (or p-type) InAsP layers, and with the second buffer  1008  including one or more n-doped (or n-type) InAsP layers. The InGaAs electromagnetic radiation absorbers  1006 ,  1010 , buffers  1004 ,  1008 , and cap layer  1012  therefore form a pnnp layer structure including back-to-back photodiodes. A forward bias (or alternatively, a reverse bias) applied to the pnnp layer structure enables a readout of the first InGaAs electromagnetic radiation absorber  1006  (or the second InGaAs electromagnetic radiation absorber  1010 ). 
     In alternative embodiments, the InGaAs electromagnetic radiation absorbers  1006 ,  1010 , buffers  1004 ,  1008 , and cap layer  1012  may be implanted or doped to form a nppn, nBn, or pBp layer structure including back-to-back photodiodes. 
       FIG.  11    shows an elevation of an InGaAs detector  1100  in which first and second InGaAs electromagnetic radiation absorbers  1106 ,  1110  are configured as two photodetectors facing the same direction and connected by a tunnel junction. The InGaAs detector  1100  includes an InP substrate  1102  on which a first set of one or more buffer layers (collectively referred to as a first buffer  1104 ) is stacked. A first InGaAs electromagnetic radiation absorber  1106  is stacked on the first buffer  1104  (i.e., with the first buffer  1104  disposed between the InP substrate  1102  and the first InGaAs electromagnetic radiation absorber  1106 ). A second set of one or more buffer layers (collectively referred to as a second buffer  1108 ) is stacked on the first InGaAs electromagnetic radiation absorber  1106 , and a second InGaAs electromagnetic radiation absorber  1110  is stacked on the second buffer  1108  (i.e., with the second buffer  1108  disposed between the first InGaAs electromagnetic radiation absorber  1106  and the second InGaAs electromagnetic radiation absorber  1110 ). A cap layer  1112  is stacked on the second InGaAs electromagnetic radiation absorber  1110  (i.e., with the second InGaAs electromagnetic radiation absorber  1110  disposed between the second buffer  1108  and the cap layer  1112 ). 
     Each of the first and second InGaAs electromagnetic radiation absorbers  1106 ,  1110  may be n-doped (or n-type) InGaAs electromagnetic radiation absorbers. Each of the first and second buffers  1104 ,  1108  may include one or more layers of indium arsenide phosphide (InAsP), with the first buffer  1104  including one or more n-doped (or n-type) InAsP layers, and with the second buffer  1108  including one or more n-doped (or n-type) InAsP layers and one or more p-doped (or p-type) InAsP layers. The InGaAs electromagnetic radiation absorbers  1106 ,  1110 , buffers  1104 ,  1108 , and cap layer  1112  therefore form a pnpn layer structure including stacked photodiodes with a tunnel junction. The tunnel junction allows photocurrent to be read out from both InGaAs electromagnetic radiation absorbers  1106 ,  1110  with the same bias polarity, unlike the InGaAs detector described with reference to  FIG.  10   , which requires different bias voltages to extract photocurrents from different InGaAs electromagnetic radiation absorbers. 
       FIGS.  12 A and  12 B  show an example of a device  1200  (an electronic device) that includes a set of sensors. The sensors may be used, for example, to acquire biological information from the wearer or user of the device  1200  (e.g., a heart rate, respiration rate, blood pressure, blood flow rate, blood oxygenation, blood glucose level, and so on), or to determine a status of the device  1200  (e.g., whether the device  1200  is being worn or a tightness of the device  1200 ). The device&#39;s dimensions and form factor, and inclusion of a band  1204  (e.g., a wrist band), suggest that the device  1200  is an electronic watch, fitness monitor, or health diagnostic device. However, the device  1200  could alternatively be any type of wearable device.  FIG.  12 A  shows a front isometric view of the device  1200 , and  FIG.  12 B  shows a back isometric view of the device  1200 . 
     The device  1200  may include a body  1202  (e.g., a watch body) and a band  1204 . The body  1202  may include an input or selection device, such as a crown  1218  or a button  1220 . The band  1204  may be attached to a housing  1206  of the body  1202 , and may be used to attach the body  1202  to a body part (e.g., an arm, wrist, leg, ankle, or waist) of a user. The body  1202  may include a housing  1206  that at least partially surrounds a display  1208 . In some embodiments, the housing  1206  may include a sidewall  1210 , which sidewall  1210  may support a front cover  1212  ( FIG.  12 A ) and/or a back cover  1214  ( FIG.  12 B ). The front cover  1212  may be positioned over the display  1208 , and may provide a window through which the display  1208  may be viewed. In some embodiments, the display  1208  may be attached to (or abut) the sidewall  1210  and/or the front cover  1212 . In alternative embodiments of the device  1200 , the display  1208  may not be included and/or the housing  1206  may have an alternative configuration. 
     The display  1208  may include one or more light-emitting elements including, for example, light-emitting elements that define a light-emitting diode (LED) display, organic LED (OLED) display, liquid crystal display (LCD), electroluminescent (EL) display, or other type of display. In some embodiments, the display  1208  may include, or be associated with, one or more touch and/or force sensors that are configured to detect a touch and/or a force applied to a surface of the front cover  1212 . 
     In some embodiments, the sidewall  1210  of the housing  1206  may be formed using one or more metals (e.g., aluminum or stainless steel), polymers (e.g., plastics), ceramics, or composites (e.g., carbon fiber). The front cover  1212  may be formed, for example, using one or more of glass, a crystal (e.g., sapphire), or a transparent polymer (e.g., plastic) that enables a user to view the display  1208  through the front cover  1212 . In some cases, a portion of the front cover  1212  (e.g., a perimeter portion of the front cover  1212 ) may be coated with an opaque ink to obscure components included within the housing  1206 . In some cases, all of the exterior components of the housing  1206  may be formed from a transparent material, and components within the device  1200  may or may not be obscured by an opaque ink or opaque structure within the housing  1206 . 
     The back cover  1214  may be formed using the same material(s) that are used to form the sidewall  1210  or the front cover  1212 . In some cases, the back cover  1214  may be part of a monolithic element that also forms the sidewall  1210 . In other cases, and as shown, the back cover  1214  may be a multi-part back cover, such as a back cover having a first back cover portion  1214 - 1  attached to the sidewall  1210  and a second back cover portion  1214 - 2  attached to the first back cover portion  1214 - 1 . The second back cover portion  1214 - 2  may in some cases have a circular perimeter and an arcuate exterior surface  1216  (i.e., an exterior surface  1216  having an arcuate profile). 
     The front cover  1212 , back cover  1214 , or first back cover portion  1214 - 1  may be mounted to the sidewall  1210  using fasteners, adhesives, seals, gaskets, or other components. The second back cover portion  1214 - 2 , when present, may be mounted to the first back cover portion  1214 - 1  using fasteners, adhesives, seals, gaskets, or other components. 
     A display stack or device stack (hereafter referred to as a “stack”) including the display  1208  may be attached (or abutted) to an interior surface of the front cover  1212  and extend into an interior volume of the device  1200 . In some cases, the stack may include a touch sensor (e.g., a grid of capacitive, resistive, strain-based, ultrasonic, or other type of touch sensing elements), or other layers of optical, mechanical, electrical, or other types of components. In some cases, the touch sensor (or part of a touch sensor system) may be configured to detect a touch applied to an outer surface of the front cover  1212  (e.g., to a display surface of the device  1200 ). 
     In some cases, a force sensor (or part of a force sensor system) may be positioned within the interior volume below and/or to the side of the display  1208  (and in some cases within the device stack). The force sensor (or force sensor system) may be triggered in response to the touch sensor detecting one or more touches on the front cover  1212  (or a location or locations of one or more touches on the front cover  1212 ), and may determine an amount of force associated with each touch, or an amount of force associated with the collection of touches as a whole. The force sensor (or force sensor system) may alternatively trigger operation of the touch sensor (or touch sensor system), or may be used independently of the touch sensor (or touch sensor system). 
     The device  1200  may include various sensors. In some embodiments, the device  1200  may have a port  1222  (or set of ports) on a side of the housing  1206  (or elsewhere), and an ambient pressure sensor, ambient temperature sensor, internal/external differential pressure sensor, gas sensor, particulate matter concentration sensor, or air quality sensor may be positioned in or near the port(s)  1222 . 
     In some cases, one or more skin-facing sensors  1226  may be included within the device  1200 . The skin-facing sensor(s) may emit or transmit signals through the housing  1206  (or back cover  1214 ) and/or receive signals or sense conditions through the housing  1206  (or back cover  1214 ). For example, in some embodiments, one or more such sensors may include a number of electromagnetic radiation emitters (e.g., visible light and/or IR emitters) and/or a number of electromagnetic radiation detectors (e.g., visible light and/or IR detectors, such as any of the InGaAs detectors described herein). The sensors may be used, for example, to acquire biological information from the wearer or user of the device  1200  (e.g., a heart rate, respiration rate, blood pressure, blood flow rate, blood oxygenation, blood glucose level, and so on), or to determine a status of the device  1200  (e.g., whether the device  1200  is being worn or a tightness of the device  1200 ). 
     The device  1200  may include circuitry  1224  (e.g., a processor and/or other components) configured to determine or extract, at least partly in response to signals received directly or indirectly from one or more of the device&#39;s sensors, biological parameters of the device&#39;s user and/or a status of the device  1200 , for example. In some embodiments, the circuitry  1224  may be configured to convey the determined or extracted parameters or statuses via an output device of the device  1200 . For example, the circuitry  1224  may cause the indication(s) to be displayed on the display  1208 , indicated via audio or haptic outputs, transmitted via a wireless communications interface or other communications interface, and so on. The circuitry  1224  may also or alternatively maintain or alter one or more settings, functions, or aspects of the device  1200 , including, in some cases, what is displayed on the display  1208 . 
       FIGS.  13 A and  13 B  show another example of a device  1300  (an electronic device) that includes a set of sensors. The sensors may be used, for example, to acquire biological information from the user of the device  1300 , to determine parameters of an environment of the device  1300  (e.g., air quality), or to determine a distance to or composition of a target or object. The device&#39;s dimensions and form factor, including the ratio of the length of its long sides to the length of its short sides, suggest that the device  1300  is a mobile phone (e.g., a smartphone). However, the device&#39;s dimensions and form factor are arbitrarily chosen, and the device  1300  could alternatively be any portable electronic device including, for example a mobile phone, tablet computer, portable computer, portable music player, portable terminal, vehicle navigation system, robot navigation system, or other portable or mobile device. The device  1300  could also be a device that is semi-permanently located (or installed) at a single location (e.g., a door lock, thermostat, refrigerator, or other appliance).  FIG.  13 A  shows a front isometric view of the device  1300 , and  FIG.  13 B  shows a rear isometric view of the device  1300 . The device  1300  may include a housing  1302  that at least partially surrounds a display  1304 . The housing  1302  may include or support a front cover  1306  or a rear cover  1308 . The front cover  1306  may be positioned over the display  1304 , and may provide a window through which the display  1304  (including images displayed thereon) may be viewed by a user. In some embodiments, the display  1304  may be attached to (or abut) the housing  1302  and/or the front cover  1306 . 
     The display  1304  may include one or more light-emitting elements or pixels, and in some cases may be an LED display, an OLED display, an LCD, an EL display, a laser projector, or another type of electronic display. In some embodiments, the display  1304  may include, or be associated with, one or more touch and/or force sensors that are configured to detect a touch and/or a force applied to a surface of the front cover  1306 . 
     The various components of the housing  1302  may be formed from the same or different materials. For example, a sidewall  1318  of the housing  1302  may be formed using one or more metals (e.g., stainless steel), polymers (e.g., plastics), ceramics, or composites (e.g., carbon fiber). In some cases, the sidewall  1318  may be a multi-segment sidewall including a set of antennas. The antennas may form structural components of the sidewall  1318 . The antennas may be structurally coupled (to one another or to other components) and electrically isolated (from each other or from other components) by one or more non-conductive segments of the sidewall  1318 . The front cover  1306  may be formed, for example, using one or more of glass, a crystal (e.g., sapphire), or a transparent polymer (e.g., plastic) that enables a user to view the display  1304  through the front cover  1306 . In some cases, a portion of the front cover  1306  (e.g., a perimeter portion of the front cover  1306 ) may be coated with an opaque ink to obscure components included within the housing  1302 . The rear cover  1308  may be formed using the same material(s) that are used to form the sidewall  1318  or the front cover  1306 , or may be formed using a different material or materials. In some cases, the rear cover  1308  may be part of a monolithic element that also forms the sidewall  1318  (or in cases where the sidewall  1318  is a multi-segment sidewall, those portions of the sidewall  1318  that are non-conductive). In still other embodiments, all of the exterior components of the housing  1302  may be formed from a transparent material, and components within the device  1300  may or may not be obscured by an opaque ink or opaque structure within the housing  1302 . 
     The front cover  1306  may be mounted to the sidewall  1318  to cover an opening defined by the sidewall  1318  (i.e., an opening into an interior volume in which various electronic components of the device  1300 , including the display  1304 , may be positioned). The front cover  1306  may be mounted to the sidewall  1318  using fasteners, adhesives, seals, gaskets, or other components. 
     A display stack or device stack (hereafter referred to as a “stack”) including the display  1304  (and in some cases the front cover  1306 ) may be attached (or abutted) to an interior surface of the front cover  1306  and extend into the interior volume of the device  1300 . In some cases, the stack may also include a touch sensor (e.g., a grid of capacitive, resistive, strain-based, ultrasonic, or other type of touch sensing elements), or other layers of optical, mechanical, electrical, or other types of components. In some cases, the touch sensor (or part of a touch sensor system) may be configured to detect a touch applied to an outer surface of the front cover  1306  (e.g., to a display surface of the device  1300 ). 
     The stack may also include one or an array of sensors  1316 , with the sensors positioned in front of or behind, or interspersed with, the light-emitting elements of the display  1304 . In some cases, an array of sensors  1316  may extend across an area equal in size to the area of the display  1304 . Alternatively, the array of photodetectors  1316  may extend across an area that is smaller than or greater than the area of the display  1304 , or may be positioned entirely adjacent the display  1304 . Although the array of sensors  1316  is shown to have a rectangular boundary, the array could alternatively have a boundary with a different shape, including, for example, an irregular shape. The array of sensors  1316  may be variously configured as an ambient light sensor, a light-emitting element (e.g., OLED) health sensor (e.g., age sensor), a touch sensor, a proximity sensor, a health sensor, a biometric sensor (e.g., a fingerprint sensor or facial recognition sensor), a camera, a depth sensor, and so on. The array of sensors  1316  may also or alternatively function as a proximity sensor, for determining whether an object (e.g., a finger, face, or stylus) is proximate to the front cover  1306 . In some embodiments, the array of sensors  1316  may provide the touch sensing capability (i.e., touch sensor) of the stack. 
     In some cases, a force sensor (or part of a force sensor system) may be positioned within the interior volume below and/or to the side of the display  1304  (and in some cases within the stack). The force sensor (or force sensor system) may be triggered in response to the touch sensor detecting one or more touches on the front cover  1306  (or indicating a location or locations of one or more touches on the front cover  1306 ), and may determine an amount of force associated with each touch, or an amount of force associated with the collection of touches as a whole. 
     As shown primarily in  FIG.  13 A , the device  1300  may include various other components. For example, the front of the device  1300  may include one or more front-facing cameras  1310  (including one or more image sensors), speakers  1312 , microphones, or other components  1314  (e.g., audio, imaging, and/or sensing components) that are configured to transmit or receive signals to/from the device  1300 . In some cases, a front-facing camera  1310 , alone or in combination with other sensors, may be configured to operate as a bio-authentication or facial recognition sensor. Additionally or alternatively, the array of sensors  1316  may be configured to operate as a front-facing camera  1310 , a bio-authentication sensor, or a facial recognition sensor. 
     The device  1300  may also include buttons or other input devices positioned along the sidewall  1318  and/or on a rear surface of the device  1300 . For example, a volume button or multipurpose button  1320  may be positioned along the sidewall  1318 , and in some cases may extend through an aperture in the sidewall  1318 . The sidewall  1318  may include one or more ports  1322  that allow air, but not liquids, to flow into and out of the device  1300 . In some embodiments, one or more sensors may be positioned in or near the port(s)  1322 . For example, an ambient pressure sensor, ambient temperature sensor, internal/external differential pressure sensor, gas sensor, particulate matter concentration sensor, or air quality sensor may be positioned in or near a port  1322 . 
     In some embodiments, the rear surface of the device  1300  may include a rear-facing camera  1324 . A flash or light source  1326  may also be positioned along the rear of the device  1300  (e.g., near the rear-facing camera). In some cases, the rear surface of the device  1300  may include multiple rear-facing cameras. 
     In some cases, the sensor(s)  1316 , the front-facing camera  1310 , the rear-facing camera  1324 , and/or other sensors positioned on the front, back, or sides of the device  1300  may emit or transmit signals through the housing  1302  (including the front cover  1306 , back cover  1308 , or sidewall  1318 ) and/or receive signals or sense conditions through the housing  1302 . For example, in some embodiments, one or more such sensors may include a number of electromagnetic radiation emitters (e.g., visible light and/or IR emitters) and/or a number of electromagnetic radiation detectors (e.g., visible light and/or IR detectors, such as any of the InGaAs detectors described herein). 
     The device  1300  may include circuitry  1328  (e.g., a processor and/or other components) configured to determine or extract, at least partly in response to signals received directly or indirectly from one or more of the device&#39;s sensors, biological parameters of the device&#39;s user, a status of the device  1300 , parameters of an environment of the device  1300  (e.g., air quality), or a composition of a target or object, for example. In some embodiments, the circuitry  1328  may be configured to convey the determined or extracted parameters or statuses via an output device of the device  1300 . For example, the circuitry  1328  may cause the indication(s) to be displayed on the display  1304 , indicated via audio or haptic outputs, transmitted via a wireless communications interface or other communications interface, and so on. The circuitry  1328  may also or alternatively maintain or alter one or more settings, functions, or aspects of the device  1300 , including, in some cases, what is displayed on the display  1304 . 
       FIG.  14    shows an example of an earbud  1400  (an electronic device) that includes a set of sensors  1408 . The earbud  1400  may include a housing  1402 . The housing  1402  may hold a speaker  1410  that can be inserted into a user&#39;s ear, an optional microphone  1404 , and circuitry  1406  that can be used to acquire audio from the microphone  1404 , transmit audio to the speaker  1402 , and communicate audio between the speaker  1402 , the microphone  1404 , and one or more remote devices. The circuitry  1406  may communicate with a remote device wirelessly (e.g., via a wireless communications interface, using a Wi-Fi, BLUETOOTH®, or cellular radio communications protocol, for example) or via one or more wires (e.g., via a wired communications interface, such as a Universal Serial Bus (USB) communications interface). In addition to communicating audio, the circuitry  1406  may transmit or receive instructions and so on. 
     The sensors  1408  may be used, for example, to determine a proximity of a user to the earbud  1400  or speaker  1410 , or to receive input from a user. In some cases, a sensor may be used to identify a gesture of a user (e.g., a swipe gesture or a press gesture) made on a surface of the earbud  1400  or in free space in proximity to the earbud  1400 . The sensors  1408  may include skin-facing and/or non-skin-facing sensors. In some embodiments, one or more such sensors may include a number of electromagnetic radiation emitters (e.g., visible light and/or IR emitters) and/or a number of electromagnetic radiation detectors (e.g., visible light and/or IR detectors, such as any of the InGaAs detectors described herein). 
     The circuitry  1406  may include a processor and/or other components that are configured to determine or extract, at least partly in response to signals received directly or indirectly from one or more of the sensors  1408 , information related to a proximity of a user, an input of a user, and so on. In some embodiments, the circuitry  1406  may be configured to convey the determined or extracted parameters or statuses via an output device of the earbud  1400 . For example, the circuitry  1406  may cause the indication(s) to be output via the speaker  1410  or a haptic device, transmitted via a wireless communications interface or other communications interface, and so on. The circuitry  1406  may also or alternatively maintain or alter one or more settings, functions, or aspects of the earbud  1400 , including, in some cases, what is output via the speaker  1410 . 
       FIG.  15    shows an example elevation of a system  1500  of electromagnetic radiation emitters and detectors that may be included in an electronic device. In some cases, the system  1500  may be included in a wearable device, such as the wearable device described with reference to  FIG.  12 A- 12 B,  13 A- 13 B , or  14 . 
     By way of example, two electromagnetic radiation emitters  1502 ,  1504  and one electromagnetic radiation detector  1506  are shown in  FIG.  15   . When incorporated into the wearable device described with reference to  FIGS.  12 A and  12 B , the emitters  1502 ,  1504  may be positioned and/or oriented to emit electromagnetic radiation  1512  toward a target (e.g., a wrist or other body part  1508  of the user of the wearable device). Similarly, the detector  1506  may be positioned and/or oriented to receive electromagnetic radiation  1514  returned (e.g., reflected or scattered) from the hair, skin, or internal structures of the wrist or other body part  1508 . 
     The detector  1506  may be configured similarly to any of the detectors (e.g., InGaAs detectors) described herein, and may include two or more electromagnetic radiation absorbers. 
     The emitters  1502 ,  1504  may emit the same or different electromagnetic radiation wavelengths, and may have the same or different constructions (e.g., both may be lasers, one may be a laser and one may be an LED, and so on). In some embodiments, the emitters  1502 ,  1504  may be configured to emit different electromagnetic radiation wavelengths, and the detector  1506  may be configured to detect the different electromagnetic radiation wavelengths. If the detector&#39;s different electromagnetic radiation absorbers are configured to detect different and non-overlapping ranges of electromagnetic radiation wavelengths, the emitters  1502 ,  1504  may be activated to emit their different electromagnetic radiation wavelengths at the same time, and the detector  1506  may separately receive and quantify the electromagnetic radiation received from each of the emitters  1502 ,  1504 . In this manner, the detector  1506  may be operated as a spectrometer with two or more resolvable spots. 
     A detection circuit  1510  may be configured to operate the emitters  1502 ,  1504  at the same or different times, and to read the current generated by the first and second electromagnetic radiation absorbers after simultaneous emissions from the emitters  1502 ,  1504 , overlapping emissions from the emitters  1502 ,  1504 , or disjoint (i.e., spaced in time) emissions from the emitters  1502 ,  1504 . 
       FIG.  16    shows a sample electrical block diagram of an electronic device  1600 , which electronic device may in some cases be implemented as the device described with reference to  FIG.  12 A- 12 B,  13 A- 13 B , or  14 . The electronic device  1600  may include an electronic display  1602  (e.g., a light-emitting display), a processor  1604 , a power source  1606 , a memory  1608  or storage device, a sensor system  1610 , or an input/output (I/O) mechanism  1612  (e.g., an input/output device, input/output port, or haptic input/output interface). The processor  1604  may control some or all of the operations of the electronic device  1600 . The processor  1604  may communicate, either directly or indirectly, with some or all of the other components of the electronic device  1600 . For example, a system bus or other communication mechanism  1614  can provide communication between the electronic display  1602 , the processor  1604 , the power source  1606 , the memory  1608 , the sensor system  1610 , and the I/O mechanism  1612 . 
     The processor  1604  may be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions, whether such data or instructions is in the form of software or firmware or otherwise encoded. For example, the processor  1604  may include a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), a controller, or a combination of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. In some cases, the processor  1604  may provide part or all of the circuitry described with reference to  FIGS.  12 A- 15   . 
     It should be noted that the components of the electronic device  1600  can be controlled by multiple processors. For example, select components of the electronic device  1600  (e.g., the sensor system  1610 ) may be controlled by a first processor and other components of the electronic device  1600  (e.g., the electronic display  1602 ) may be controlled by a second processor, where the first and second processors may or may not be in communication with each other. 
     The power source  1606  can be implemented with any device capable of providing energy to the electronic device  1600 . For example, the power source  1606  may include one or more batteries or rechargeable batteries. Additionally or alternatively, the power source  1606  may include a power connector or power cord that connects the electronic device  1600  to another power source, such as a wall outlet. 
     The memory  1608  may store electronic data that can be used by the electronic device  1600 . For example, the memory  1608  may store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, and data structures or databases. The memory  1608  may include any type of memory. By way of example only, the memory  1608  may include random access memory, read-only memory, Flash memory, removable memory, other types of storage elements, or combinations of such memory types. 
     The electronic device  1600  may also include a sensor system  1610 , including sensors positioned almost anywhere on the electronic device  1600 . In some cases, the sensor system  1610  may include one or more electromagnetic radiation emitters and detectors, positioned and/or configured as described with reference to any of  FIGS.  2 A- 15   . The sensor system  1610  may be configured to sense one or more type of parameters, such as but not limited to, vibration; light; touch; force; heat; movement; relative motion; biometric data (e.g., biological parameters) of a user; air quality; proximity; position; connectedness; matter type; and so on. By way of example, the sensor system  1610  may include one or more of (or multiple of) a heat sensor, a position sensor, a proximity sensor, a light or optical sensor (e.g., an electromagnetic radiation emitter and/or detector), an accelerometer, a pressure transducer, a gyroscope, a magnetometer, a health monitoring sensor, and an air quality sensor, and so on. Additionally, the sensor system  1610  may utilize any suitable sensing technology, including, but not limited to, interferometric, magnetic, pressure, capacitive, ultrasonic, resistive, optical, acoustic, piezoelectric, or thermal technologies. 
     The I/O mechanism  1612  may transmit or receive data from a user or another electronic device. The I/O mechanism  1612  may include the electronic display  1602 , a touch sensing input surface, a crown, one or more buttons (e.g., a graphical user interface “home” button), one or more cameras (including an under-display camera), one or more microphones or speakers, one or more ports such as a microphone port, and/or a keyboard. Additionally or alternatively, the I/O mechanism  1612  may transmit electronic signals via a communications interface, such as a wireless, wired, and/or optical communications interface. Examples of wireless and wired communications interfaces include, but are not limited to, cellular and Wi-Fi communications interfaces. 
     The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art, after reading this description, that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art, after reading this description, that many modifications and variations are possible in view of the above teachings. 
     As described above, one aspect of the present technology may be the gathering and use of data available from various sources. The present disclosure contemplates that, in some instances, this gathered data may include personal information data (e.g., biological information) that uniquely identifies or can be used to identify, locate, contact, or diagnose a specific person. Such personal information data can include demographic data, location-based data, telephone numbers, email addresses, home addresses, data or records relating to a user&#39;s health or level of fitness (e.g., vital signs measurements, medication information, exercise information), date of birth, or any other identifying or personal information. 
     The present disclosure recognizes that the use of such personal information data, in the present technology, can be used to the benefit of users. For example, the personal information data can be used to activate or deactivate various functions of the user&#39;s device, or gather performance metrics for the user&#39;s device or the user. Further, other uses for personal information data that benefit the user are also contemplated by the present disclosure. For instance, health and fitness data may be used to provide insights into a user&#39;s general wellness, or may be used as positive feedback to individuals using technology to pursue wellness goals. 
     The present disclosure contemplates that the entities responsible for the collection, analysis, disclosure, transfer, storage, or other use of such personal information data will comply with well-established privacy policies and/or privacy practices. In particular, such entities should implement and consistently use privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining personal information data private and secure. Such policies should be easily accessible by users, and should be updated as the collection and/or use of data changes. Personal information from users should be collected for legitimate and reasonable uses of the entity and not shared or sold outside of those legitimate uses. Further, such collection/sharing should occur after receiving the informed consent of the users. Additionally, such entities should consider taking any needed steps for safeguarding and securing access to such personal information data and ensuring that others with access to the personal information data adhere to their privacy policies and procedures. Further, such entities can subject themselves to evaluation by third parties to certify their adherence to widely accepted privacy policies and practices. In addition, policies and practices should be adapted for the particular types of personal information data being collected and/or accessed and adapted to applicable laws and standards, including jurisdiction-specific considerations. For instance, in the United States (US), collection of or access to certain health data may be governed by federal and/or state laws, such as the Health Insurance Portability and Accountability Act (HIPAA); whereas health data in other countries may be subject to other regulations and policies and should be handled accordingly. Hence different privacy practices should be maintained for different personal data types in each country. 
     Despite the foregoing, the present disclosure also contemplates embodiments in which users may selectively block the use of, or access to, personal information data. That is, the present disclosure contemplates that hardware and/or software elements can be provided to prevent or block access to such personal information data. For example, in the case of advertisement delivery services, the present technology can be configured to allow users to select to “opt in” or “opt out” of participation in the collection of personal information data during registration for services or anytime thereafter. In another example, users can select not to provide mood-associated data for targeted content delivery services. In yet another example, users can select to limit the length of time mood-associated data is maintained or entirely prohibit the development of a baseline mood profile. In addition to providing “opt in” and “opt out” options, the present disclosure contemplates providing notifications relating to the access or use of personal information. For instance, a user may be notified upon downloading an app that their personal information data will be accessed and then reminded again just before personal information data is accessed by the app. 
     Moreover, it is the intent of the present disclosure that personal information data should be managed and handled in a way to minimize risks of unintentional or unauthorized access or use. Risk can be minimized by limiting the collection of data and deleting data once it is no longer needed. In addition, and when applicable, including in certain health related applications, data de-identification can be used to protect a user&#39;s privacy. De-identification may be facilitated, when appropriate, by removing specific identifiers (e.g., date of birth, etc.), controlling the amount or specificity of data stored (e.g., collecting location data at a city level rather than at an address level), controlling how data is stored (e.g., aggregating data across users), and/or other methods. 
     Therefore, although the present disclosure broadly covers use of personal information data to implement one or more various disclosed embodiments, the present disclosure also contemplates that the various embodiments can also be implemented without the need for accessing such personal information data. That is, the various embodiments of the present technology are not rendered inoperable due to the lack of all or a portion of such personal information data. For example, content can be selected and delivered to users by inferring preferences based on non-personal information data or a bare minimum amount of personal information, such as the content being requested by the device associated with a user, other non-personal information available to the content delivery services, or publicly available information.

Metadata:
Filing Date: 20210726
Publication Date: 20250121
Grant Date: 20250121
Priority Date: 20200731
Inventors: ARBORE, Mark Alan
MOREA, Matthew T.
KANGAS, Miikka M.
CHEVALLIER, Romain F.
SARMIENTO, Tomas
Assignee: APPLE INC
CPC Classifications: [{"code": "H10F77/413", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/50", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/107", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F30/288", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/1248", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/407", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/021", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/806", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/804", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/80", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J2001/0257", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01J3/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0259", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0208", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/4228", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/0411", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/0271", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/1248", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J1/0209", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L31/02327", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L31/0203", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/1446", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L31/03046", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 80004583