CN113725242A

CN113725242A - Integrated optical sensor with pinned photodiode

Info

Publication number: CN113725242A
Application number: CN202110570140.2A
Authority: CN
Inventors: D·迪塔特
Original assignee: STMicroelectronics Crolles 2 SAS
Current assignee: STMicroelectronics Crolles 2 SAS
Priority date: 2020-05-26
Filing date: 2021-05-25
Publication date: 2021-11-30
Also published as: CN218896638U

Abstract

Embodiments of the present disclosure relate to integrated optical sensors having pinned photodiodes. The integrated optical sensor is formed from a pinned photodiode. The semiconductor substrate includes a first semiconductor region having a first conductivity type, the first semiconductor region being located between a second semiconductor region having a second conductivity type opposite to the first conductivity type and a third semiconductor region having the second conductivity type. The third semiconductor region is thicker, less doped and deeper in the substrate than the second region. The third semiconductor region includes silicon and germanium. In one implementation, the germanium within the third semiconductor region has at least one concentration gradient. In another implementation, the germanium concentration in the third semiconductor region is substantially constant.

Description

Integrated optical sensor with pinned photodiode

Priority declaration

The present application claims priority from french patent application No.2005537, filed on 26/5/2020, the contents of which are incorporated herein by reference in their entirety to the maximum extent allowed by law.

Technical Field

Implementations and embodiments relate to integrated optical sensors, and in particular, integrated optical sensors including pinned photodiodes.

Background

In recent years, more and more applications (such as facial recognition, virtual reality, and active car safety) require high-performance, low-profile, and low-cost imaging systems.

In this regard, imaging systems based on the use of indirect time-of-flight (iToF) measurement techniques and benefiting from highly integrated structures and accurate and fast performance are particularly well meeting these desires.

More specifically, with periodically modulated excitation obtained from a laser, for example, the distance separating the object to be measured from an imaging system (referred to as an "iToF" imaging system) may be measured indirectly via phase shift measurement of the signal received after reflection on the object relative to the emitted radiation, and the data collection of the optical signal may be extended over several excitation and emission cycles in order to improve the accuracy of the measurement.

This type of detector is particularly suitable for applications using radiation whose wavelength is in the near infrared (e.g., 0.94 microns).

As such, there are increasing applications implemented particularly not only in time-of-flight sensors but also in CMOS imagers.

Typically, the sensors used are integrated silicon-based sensors.

However, silicon has low absorption power in the infrared even in the near infrared (e.g., 0.94 microns). For example, a silicon substrate having a thickness of 1 micron has an absorption of 1.7% at a wavelength of 0.94 microns.

This absorbed power increases with greater thickness, for example thicknesses on the order of 6 microns are typical of the pinned photodiodes of integrated optical sensors.

But silicon devices have low sensitivity in the near infrared. For example, a silicon substrate having a thickness of 6 microns has a quantum efficiency in the range of 7% to 8% at a wavelength of 0.94 microns.

Furthermore, at this thickness, the collection of minority carriers in the pinned photodiode is very slow, which is detrimental.

There is therefore a need to improve the performance of optical sensors implementing in particular one or more pinned photodiodes, in particular in terms of absorption and/or sensitivity and/or collection speed of minority carriers, most particularly in the near infrared range.

Disclosure of Invention

The aforementioned requirements are met by replacing silicon with a material having particularly better infrared absorption, while satisfying strict constraints such as, for example: compatibility with microelectronic components located on the front end of the substrate and compatibility with monocrystalline silicon; integratability in the active part of the semiconductor device (diode and pinned diode); sufficiently small generation of minority carriers; and as low a defect rate as possible.

The material is then advantageously temperature resistant, has good interface quality with silicon and with dielectric materials (such as silicon dioxide), and generally has good quality (in particular no or few dislocations, no or few contaminants).

The pinned photodiode includes within a semiconductor substrate a first semiconductor region, for example of the N-type, sandwiched between two semiconductor regions, for example a surface region of the P + -type and a thicker and deeper P-type region.

For example, it is proposed to incorporate a less significant amount of germanium in the P-type region in order to improve the performance of the optical sensor, particularly in terms of absorption and sensitivity, particularly in the near infrared range, while minimizing the risk of dislocation formation.

Furthermore, it is proposed to incorporate germanium, for example, in a P-type region having a positive concentration gradient from the bottom of the P-region to the N-region, in order to improve the performance of the optical sensor, in particular in terms of the collection speed of minority carriers, in particular in the near infrared range.

In both cases mentioned above (less large, e.g. constant amount of germanium and positive concentration gradient), the germanium concentration profile preferably stops at the beginning of the depletion region of the P-region.

Thus, according to one aspect, an integrated optical sensor is proposed, comprising at least one detection module comprising a pinned photodiode.

The photodiode includes within a semiconductor substrate a first semiconductor region having a first conductivity type (e.g., N-type conductivity) between a second semiconductor region having a second conductivity type (e.g., P-type conductivity, opposite the first conductivity type) and a third semiconductor region also having the second conductivity type.

The third region is thicker than the second region.

The third region is less doped than the second region. Thus, the third region may be P-doped, while the second region may be P + doped.

Furthermore, the third region is located deeper in the substrate than the second region.

This third region comprises silicon and germanium, advantageously in a small quantity or atomic percentage, for example with an atomic percentage comprised between 3% and 6%, and preferably with at least a first concentration gradient.

The first concentration gradient is advantageously a positive gradient, the atomic percentage of germanium increasing towards the first region.

The presence of germanium allows the absorption coefficient of the semiconductor material to be improved.

Furthermore, the presence of a favorable positive concentration gradient allows to reduce the space between the valence and conduction bands of the semiconductor material and to cause a tilt of the conduction band towards the surface of the photodiode, which will cause an electric field to be obtained that will accelerate the movement of minority carriers from the third region to the first region.

Thus, the duration of collection of minority carriers is reduced.

The atomic percentage of germanium preferably increases from 0% to 6%.

In addition, limiting the atomic percentage of germanium to 6% allows limiting the risk of dislocations occurring and generally allows the detection module to remain compatible with other steps of manufacturing the optical sensor and other components of the integrated circuit incorporating the optical sensor.

Furthermore, the growth may occur in any possible way, e.g. linearly or in step(s).

The third region typically includes a depletion region and a non-depletion region (referred to as a neutral region), the depletion region being in contact with the first region and being located above the non-depletion region, and the first concentration gradient is preferably located in the non-depletion region.

In addition, it is also preferred that the germanium has a second negative concentration gradient in the depletion region.

This negative gradient in germanium concentration does create an electric field opposite to that caused by the conduction band tilt.

By placing a negative germanium gradient in the depletion region, it becomes negligible compared to the electric field created by the diode itself.

For example, the atomic percentage of germanium may be reduced from 6% to 0% in the depletion region.

Although in practice relieved by the interdiffusion phenomenon of germanium and silicon atoms, the reduction in atomic percentage of germanium according to the second gradient may be gradual or abrupt (second "infinite" gradient).

According to another aspect, an integrated optical sensor is provided, comprising at least one pinned photodiode including within a semiconductor substrate a first semiconductor region of a first conductivity type located between a second semiconductor region of a second conductivity type opposite to the first conductivity type and a third semiconductor region of the second conductivity type, the third semiconductor region being thicker, less doped and deeper located in the substrate than the second region, and the third semiconductor region including germanium having an atomic percentage less than or equal to 6% (e.g. comprised between 3% and 6%).

Indeed, as indicated above, the presence of a small amount of germanium, independently of the presence of a germanium concentration gradient, allows to improve the absorption coefficient while minimizing the risk of dislocations appearing, and generally allows the detection module to remain compatible with other manufacturing steps of the optical sensor and of other components of the integrated circuit incorporating the same.

According to one embodiment, the non-depletion region comprises germanium having a substantially constant atomic percentage.

The constant atomic percentage of germanium is for example comprised between 4% and 5%.

The third region may comprise a silicon germanium alloy, for example obtained by a ramped gradient in the concentration of the dopant (germanium) in the epitaxial reactor, or an alternation of silicon and silicon germanium layers.

As described above, the first conductive type may be N-type and the second conductive type may be P-type, but it is also possible that the second conductive type is N-type and the first conductive type is P-type.

According to one embodiment, the substrate may be a silicon-on-insulator type substrate comprising a Buried insulating layer known to the person skilled in the art by the acronym BOX ("Buried Oxide") which is capped by a semiconductor film containing said pinned photodiode.

The sensor may comprise several detection modules, for example arranged in a row or in a matrix.

According to another aspect, an imaging system, for example a camera, is proposed, comprising at least one sensor as defined above.

According to another aspect, an electronic device is proposed, for example of the tablet computer or mobile cellular phone type, comprising at least one imaging system as defined above.

According to another aspect, there is provided a method for manufacturing an integrated pinned photodiode, for example in the context of manufacturing an integrated optical sensor incorporating the photodiode, the method comprising an embodiment of a first semiconductor region, a second semiconductor region and a third semiconductor region within a semiconductor substrate, the first semiconductor region having a first conductivity type (e.g. N-type conductivity) and being located between the second semiconductor region and the third semiconductor region, the second semiconductor region having a second conductivity type (e.g. P-type conductivity, opposite to the first conductivity type), the third semiconductor region also having the second conductivity type, the third semiconductor region being thicker, less doped than the second region and being located deeper in the substrate than the second region.

According to a variant of this aspect, the production of the third region comprises forming a material comprising silicon and germanium, advantageously in a small quantity or atomic percentage, for example with an atomic percentage comprised between 3% and 6%, and preferably with at least a first concentration gradient.

According to another variant of this aspect, the production of the third region comprises forming a material comprising silicon and germanium, the germanium having an atomic percentage less than or equal to 6%, for example comprised between 3% and 6%.

Regardless of the variations, the formation of the material may include silicon germanium epitaxy or a continuous epitaxy of silicon and a continuous epitaxy of silicon germanium.

Drawings

Other advantages and features of the invention will become apparent by examining the detailed description of embodiments and implementations and the accompanying drawings, in which:

FIG. 1 illustrates a cross-section of a pinned photodiode located within a semiconductor substrate, and also illustrates dopant concentration information;

FIG. 2 illustrates a cross-section of a pinned photodiode located within a semiconductor substrate, and also illustrates the alternation of doped layers;

FIG. 3 schematically illustrates an integrated optical sensor;

FIG. 4 schematically illustrates an imaging system including an integrated optical sensor; and

fig. 5 schematically illustrates an electronic device comprising an integrated optical sensor.

Detailed Description

On the right part of fig. 1, reference sign MD generally designates an inspection module comprising a pinned photodiode PPD located within a semiconductor substrate, here a silicon-on-insulator (SOI) type substrate.

The module MD is integrated within the integrated circuit IC.

The SOI type substrate comprises a semiconductor film 1, the semiconductor film 1 being located above a buried insulating layer BX, which in turn is located above a carrier substrate 3.

The thickness Ep of the semiconductor film 1 may be comprised between about 3 and 6 micrometers, and in this example is of the order of 6 micrometers.

The photodiode PPD is a buried photodiode formed by a double junction (here, a double P + NP junction).

Here, the photodiode PPD is electrically insulated from the remaining portion of the semiconductor film 1 by the deep insulation trench 4.

More specifically, the photodiode PPD includes a first semiconductor region RG1 having N-type conductivity within the semiconductor film 1, the first semiconductor region RG1 being located between a second semiconductor region RG2 (or pinning implant) having P-type conductivity and a third semiconductor region RG3 having P-type conductivity.

The first region RG1 may advantageously be formed in the middle of the module and not extend to the deep isolation trench 4.

The second region RG2 has a thickness e2 of the order of 0.07 micrometer.

The second region RG2 is at a concentration of about 10²⁰atoms/cm³P + doped with a dopant.

This pinning implant allows to greatly reduce the dark current of the detection module MD.

The first region RG1 has a thickness e1 of the order of 0.3 micrometer.

The first region RG1 is at a concentration of about 2 x 10¹⁷atoms/cm³Is doped N.

The third region RG3 has a thickness e3 of the order of 6 micrometers.

The third region RG3 is less doped and is located deeper in the substrate than the second region RG 2.

It is P-doped, with a dopant concentration of between 10¹⁴-10¹⁵atoms/cm³In the range of (1).

This third region RG3 may be very lightly doped over 90% of its thickness, but may comprise a more heavily doped P-type P-layer (10) at its base¹⁷To 10¹⁸atoms/cm³) That is to say here adjacent to or on the buried insulating layer BX.

The third region RG3 includes a depletion region Z30 in contact with the first region RG1 and a non-depletion region Z31 below the depletion region Z30.

The depletion zone Z30 has a thickness e30 of the order of 2 to 3 microns, the doping of this depletion zone Z30 corresponding to the P doping (at 10) of the non-depletion zone Z31¹⁴To 10¹⁵Atoms/cm 3) overlap with the N-doped "tail" of the second region RG 2.

The non-depletion region Z31 has a thickness e31 on the order of 3 to 4 microns and, as indicated above, has a thickness of 10¹⁴To 10¹⁵atoms/cm³Dopant concentration of the order of magnitude.

Furthermore, the module MD comprises a portion 2 above the upper surface FS Of the semiconductor film 1, which portion 2 comprises the processing circuits (Of conventional construction and known per se) Of the photodiode, in particular the collection allowing accumulation Of minority carriers in the first region N, and an interconnection region known to the person skilled in the art by the acronym BEOL ("Back End Of Line").

As illustrated in the left part of fig. 1, the third region RG3 includes silicon and germanium, which may have different concentration profiles.

Thus, according to the first embodiment, preferably in the non-depletion region Z31, the germanium concentration may have a profile PRF corresponding to a substantially constant germanium concentration. Substantially constant in this context means within +/-1 to 3% of the target dopant concentration level throughout the region of interest (i.e., region Z31).

As an example, the atomic percentage of germanium is of the order of 4% to 5% throughout the thickness of the non-depletion region Z31, then abruptly drops to 0 at the depletion region Z30.

The presence of this small amount of germanium (typically having an atomic percentage of germanium of less than 6% in the third region RG 3) allows in particular to improve the absorption coefficient while minimizing the risk of dislocations appearing.

However, the third region RG3 may include silicon and germanium, which advantageously have a first concentration gradient GR1 (or GR10) and a second concentration gradient GR2 (or GR 20).

Gradients GR1, GR2, GR10 and GR20 are possible but non-limiting examples.

The first concentration gradient GR1 or GR10 is a positive gradient, meaning that the atomic percentage of germanium increases from the buried oxide layer BX towards the first region RG 1.

With respect to the gradient GR1, the atomic percent of germanium increased from 0% to 6% from the bottom of the non-depleted zone Z31 to the limit of the depleted zone Z30.

With respect to the gradient GR10, the atomic percentage of germanium increased, for example, from 3% to 6% from the bottom of the non-depletion region Z31 to the limit of the depletion region Z30.

The second germanium concentration gradient GR2 or GR20 located in depletion zone Z30 is a negative gradient, meaning that the atomic percent of germanium decreases from the limit of depletion zone Z30 towards first region RG 1.

More specifically, for each of the second germanium concentration gradients GR2 or GR20, the atomic percent of germanium decreases from 6% to 0% in depletion region Z30.

Alternatively, the second gradient may be "infinite", which corresponds to an abrupt transition, here from 6% to 0% (similar to what is illustrated with respect to the change in the distribution PRF).

In practice, however, the phenomenon of interdiffusion of germanium and silicon atoms smoothes this abrupt transition.

The presence of the concentration gradient GR1 or GR10 allows to reduce the space between the valence and conduction bands of the semiconductor material and to cause a tilt of the conduction band towards the surface of the photodiode, which will result in obtaining an electric field that will accelerate the movement of minority carriers from the third region to the first region and thus reduce the duration of the collection of the minority carriers.

However, the second germanium concentration gradient creates an electric field opposite to that of the first gradient.

By placing this second negative gradient of germanium GR2 or GR20 in depletion region Z30, it may become negligible compared to the electric field created by the diode itself.

Thus, the duration of the collection of minority carriers will not be negatively affected.

Furthermore, here again, the presence of a small amount of germanium (for example with an average atomic percentage between 3% and 6%) allows to improve the absorption coefficient while minimizing the risk of dislocations appearing.

The embodiment of fig. 1, which provides for the presence of a germanium concentration gradient, thus allows for a fast detection modulus with an improved absorption coefficient and a reduced risk of dislocations.

Although in the embodiment of fig. 1 the third region RG3 is formed of a homogeneous silicon germanium alloy, the third region RG3 may be formed of alternating layers of silicon 11 and silicon germanium 10 as illustrated in fig. 2.

The volume percentage of germanium for each of these layers 11 is chosen such that the final atomic percentage of germanium in the region RG3 follows the gradient considered.

An embodiment of a method for manufacturing the module MD of fig. 1 will now be described.

In the case of a SOI type substrate, epitaxy is performed above the buried layer BX (and where appropriate a thin layer of silicon is more heavily doped to form the substrate of the third region RG 3) so as to form a third region RG3, which third region RG3 comprises silicon and germanium.

Silicon germanium epitaxy is a well known step to those skilled in the art.

As an example, SiGe epitaxy can be performed by Chemical Vapor Deposition (CVD) using dichlorosilane + germanium + hydrogen chemistry at 900 to 950 ℃ and reduced pressure (10-60 Torr).

Diborane (B) may also be added₂H₆) To obtain P doping.

In order to obtain the desired concentration gradient for germanium, the amount of germanium may be adjusted in the epitaxial reactor such that they follow a ramp.

The SiGe epitaxy is then continued by another epitaxy, this time only a silicon epitaxy, conventional and known per se, so as to form a first region RG1 after the local ion implantation of N-type dopants and then a second P + doped region RG2 after the local implantation of P-type dopants.

By way of example, the conditions for this other epitaxy are substantially the same as those for SiGe epitaxy, optionally with the temperature increasing from 50 ℃ to 100 ℃.

It should be noted that these two epitaxy, which are often performed at the same step, may be performed in the same epitaxy operation and thus may be performed in the same epitaxy reactor with the same recipe, and thus often no cooling of the wafer between the two types of deposition is required.

With regard to the production of the third region RG3 as illustrated in fig. 2, in this case a successive epitaxy of silicon and silicon germanium is performed in order to obtain a stack of

layers

11 and 10.

The volume percentage of germanium used for each of these epitaxy is chosen so that the average final volume percentage of germanium is comprised between 3% and 6% and, where appropriate, follows the concentration gradient considered.

Then, regardless of the variant used, production continues by conventional formation of deep and shallow trenches and other components of the detection module.

Instead of performing these complete wafer epitaxy, it would also be possible to perform a first complete wafer silicon epitaxy to form the silicon film 1, then to etch the semiconductor film (corresponding to the pixels) locally in the region where the detection module is produced, then to proceed with the SiGe and Si epitaxy mentioned above, then to produce the various electronic components of the insulation trench and module MD.

Moreover, everything that has just been described for SOI type substrates applies to solid (i.e. bulk) substrates.

As illustrated in fig. 3, the integrated optical sensor SNS may comprise several detection modules MD1-MDn, for example arranged in a row or a matrix.

As illustrated in fig. 4, the sensor SNS may be incorporated within an imaging system CM, for example a camera, which in turn may be incorporated within an electronic device APP (fig. 5) of the type for example a tablet computer or a cellular mobile phone.

Claims

1. An integrated optical sensor comprising a pinned photodiode, the integrated optical sensor comprising:

a semiconductor substrate comprising:

a first semiconductor region having a first conductivity type;

a second semiconductor region having a second conductivity type opposite to the first conductivity type; and

a third semiconductor region having the second conductivity type;

wherein the first semiconductor region is located between the second semiconductor region and the third semiconductor region;

wherein the third semiconductor region is thicker, less doped and deeper in the semiconductor substrate than the second semiconductor region; and

wherein the third semiconductor region comprises silicon and germanium, wherein the germanium has a first concentration gradient.

2. The sensor of claim 1, wherein the first concentration gradient is a positive gradient in which an atomic percentage of germanium in the third semiconductor region increases from a bottom of the third semiconductor region toward the first semiconductor region.

3. The sensor of claim 2, wherein an atomic percentage of the germanium for the first concentration gradient increases from 0% to 6%.

4. The sensor of claim 2, wherein an atomic percentage of the germanium for the first concentration gradient increases from 3% to 6%.

5. The sensor of claim 1, wherein the third semiconductor region includes a depletion region and a non-depletion region, the depletion region being in contact with the first semiconductor region and being located above the non-depletion region, and wherein the first concentration gradient is located in the non-depletion region.

6. The sensor of claim 5, wherein the first concentration gradient is a positive gradient in which the atomic percent of germanium in the non-depleted region increases from the bottom of the non-depleted region toward the depleted region.

7. The sensor of claim 6, wherein an atomic percentage of the germanium for the first concentration gradient increases from 0% to 6%.

8. The sensor of claim 6, wherein an atomic percentage of the germanium for the first concentration gradient increases from 3% to 6%.

9. The sensor of claim 5, wherein the third semiconductor region further comprises silicon and germanium in the depletion region, wherein the germanium has a second concentration gradient, wherein the second concentration gradient is a negative concentration gradient, wherein the atomic percent of germanium in the depletion region decreases from the non-depletion region toward the first semiconductor region.

10. The sensor of claim 9, wherein the atomic percent of germanium decreases from 6% to 0% in the depletion region.

11. The sensor of claim 5, wherein the depletion region in the third semiconductor region does not include germanium.

12. The sensor of claim 1, wherein the third semiconductor region comprises a silicon germanium alloy.

13. The sensor of claim 1, wherein the third semiconductor region is formed of alternating layers of silicon and silicon germanium.

14. The sensor of claim 1, wherein the first conductivity type is N-type and the second conductivity type is P-type.

15. The sensor of claim 1, wherein the semiconductor substrate is a silicon-on-insulator type substrate comprising a buried insulating layer capped by a semiconductor film, and wherein the pinned photodiode is contained within the semiconductor film.

16. The sensor of claim 1, comprising at least one detection module comprising the pinned photodiode.

17. The sensor of claim 1, wherein the pinned photodiode is a component of an imaging system.

18. The sensor of claim 17, wherein the imaging system is a time-of-flight (ToF) system.

19. The sensor of claim 17, wherein the imaging system is a component of an electronic device.

20. The sensor of claim 19, wherein the electronic device is selected from the group consisting of: tablet computers and cellular mobile phones.

21. An integrated optical sensor comprising a pinned photodiode, the integrated optical sensor comprising:

a semiconductor substrate comprising:

a first semiconductor region having a first conductivity type;

a third semiconductor region having the second conductivity type;

wherein the third semiconductor region comprises silicon and germanium, the germanium having a substantially constant atomic percentage of germanium less than or equal to 6%.

22. The sensor of claim 21, wherein the third semiconductor region includes a depletion region and a non-depletion region, the depletion region being in contact with and above the first semiconductor region, and the non-depletion region including germanium having the substantially constant atomic percentage.

23. The sensor of claim 22, wherein the constant atomic percentage of germanium is between 4% and 5% in the undepleted region.

24. The sensor of claim 23, wherein the depletion region does not include germanium.

25. The sensor of claim 21, wherein the constant atomic percentage of germanium is between 4% and 5%.

26. The sensor of claim 21, wherein the third semiconductor region comprises a silicon germanium alloy.

27. The sensor of claim 21, wherein the third semiconductor region is formed of alternating layers of silicon and silicon germanium.

28. The sensor of claim 21, wherein the first conductivity type is N-type and the second conductivity type is P-type.

29. The sensor of claim 21, wherein the semiconductor substrate is a silicon-on-insulator type substrate comprising a buried insulating layer capped by a semiconductor film, and wherein the pinned photodiode is contained within the semiconductor film.

30. The sensor of claim 21, comprising at least one detection module comprising the pinned photodiode.

31. The sensor of claim 21, wherein the pinned photodiode is a component of an imaging system.

32. The sensor of claim 31, wherein the imaging system is a time-of-flight (ToF) system.

33. The sensor of claim 31, wherein the imaging system is a component of an electronic device.

34. The sensor of claim 33, wherein the electronic device is selected from the group consisting of: tablet computers and cellular mobile phones.

35. An integrated optical sensor comprising a pinned photodiode, the integrated optical sensor comprising:

a semiconductor substrate comprising:

a first semiconductor region having a first conductivity type;

a third semiconductor region having the second conductivity type;

wherein the third semiconductor region is thicker, less doped and deeper in the semiconductor substrate than the second semiconductor region;

wherein the third semiconductor region comprises a depletion region and a non-depletion region, the depletion region being in contact with the first semiconductor region and located above the non-depletion region; and

wherein the non-depletion region of the third semiconductor region comprises silicon and germanium, wherein the germanium has a positive concentration gradient in which the atomic percent of germanium increases from the bottom of the non-depletion region toward the depletion region.

36. The sensor of claim 35, wherein the atomic percentage of germanium for the positive concentration gradient increases from 0% to 6%.

37. The sensor of claim 35, wherein the atomic percentage of germanium for the positive concentration gradient increases from 3% to 6%.

38. The sensor of claim 35, wherein the depletion region comprises silicon and germanium, wherein the germanium has a negative concentration gradient, wherein the atomic percent of germanium in the depletion region decreases from the non-depletion region toward the first semiconductor region.

39. The sensor of claim 38, wherein the atomic percent of germanium decreases from 6% to 0% in the depletion region.

40. The sensor of claim 35, wherein the depletion region in the third semiconductor region does not include germanium.