Planar tensile strain germanium sensor
By designing a planar tensile structure and a multi-quantum well layer combination in a germanium sensor, the leakage current and compatibility problems of germanium sensor are solved, and the effects of low leakage current, low dark current and broad spectrum response are achieved, which promotes its Application in civilian field.
Patent Information
- Application Number
- CN202420716343.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-04-08
- Publication Date
- 2024-12-06
- Estimated Expiration
- 2034-04-08
AI Technical Summary
The existing germanium-based sensors have large leakage current due to the contact between the side walls of the ion-doped layer and the electrode, which affects the sensitivity and reliability, and are not compatible with the CMOS manufacturing process, limiting their application in the short-wave infrared band.
A planar tensile strained germanium sensor is designed, by stacking the oxide layer, the dielectric layer, the second doped germanium layer, the second intrinsic germanium layer, the intrinsic germanium/germanium multi-quantum well layer, the first book The germanium layer and the ion-doped mask layer are intermittently distributed to form non-ion-doped regions to avoid leakage current, and pass through the second intrinsic germanium layer and the intrinsic germanium/germanium multi-quantum well layer Expand the spectrum response range.
It effectively reduces the leakage current and dark current of germanium-based sensors, improves reliability, and makes them compatible with CMOS processes, expands the spectrum response range, reduces production costs, and promotes civilian applications.
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Figure CN222126553U_ABST
Abstract
Description
Technical Field
[0001] The utility model relates to the technical field of semiconductors, in particular to a planar tensile strain germanium sensor. Background Art
[0002] InGaAs short-wave infrared sensors are widely used in machine vision, laser communication, astronomy, life sciences, semiconductor detection, security, medical diagnosis, low-altitude technology and automotive assisted driving, but they have technical bottlenecks such as small wafer size, incompatibility with CMOS manufacturing process, low yield and inability to mass produce. As a result, germanium sensors that are more compatible with CMOS manufacturing process and also have photoelectric response in the short-wave infrared band are attracting more and more attention.
[0003] At present, germanium-on-insulator sensors are considered to be one of the most promising technology routes. However, almost all germanium-on-insulator sensors currently use a table structure, with exposed ion-doped layer sidewalls on the sidewalls of the pixels. The surface of the ion-doped layer is in contact with an electrode. For germanium sensors with small pixel spacing, the sidewalls of the ion-doped layer will contact another electrode, resulting in large leakage currents on the pixel surface and sidewalls, which will have a serious impact on the sensitivity, power consumption, reliability and other performance of the germanium sensors.
[0004] Therefore, how to reduce the leakage current of germanium-based sensors has become a problem that needs to be solved. Utility Model Content
[0005] Based on the above problems, the utility model provides a planar tensile strain germanium sensor, which can reduce the leakage current of the germanium sensor.
[0006] The utility model embodiment discloses the following technical solution:
[0007] The present invention provides a planar tensile strain germanium sensor, the planar tensile strain germanium sensor comprising:
[0008] An oxide layer, a dielectric layer, a second doped germanium layer, a second intrinsic germanium layer, an intrinsic germanium / germanium silicon multi-quantum well layer, a first intrinsic germanium layer and an ion-doped mask layer stacked in sequence;
[0009] The ion doping mask layer is intermittently distributed on a side of the first intrinsic germanium layer away from the oxide layer;
[0010] The area of the first intrinsic germanium layer that is not covered by the ion doping mask layer is an ion doping area; the ion doping type of the ion doping area is different from that of the second doped germanium layer;
[0011] The area of the first intrinsic germanium layer covered by the ion doping mask layer has a non-ion doped area; the non-ion doped area has a groove, and the bottom of the groove is the second doped germanium layer;
[0012] The side of the ion-doped region away from the oxide layer has a first electrode; the groove is filled with a second electrode;
[0013] The non-ion-doped region is provided between the second electrode and the ion-doped region.
[0014] Optionally, the intrinsic germanium / germanium silicon multi-quantum well layer includes a germanium layer and a germanium silicon layer;
[0015] The germanium layer is a barrier layer with a thickness ranging from 10 to 100 nm; the germanium silicon layer is a potential well layer with a thickness ranging from 10 to 100 nm; and the content of silicon in the germanium silicon layer is 0 to 30%.
[0016] Optionally, the intrinsic germanium / germanium silicon multi-quantum well layer includes 2 to 10 quantum wells.
[0017] Optionally, the thickness of the intrinsic germanium / germanium silicon multi-quantum well layer is in the range of 200 to 1000 nm.
[0018] Optionally, the thickness of the ion doping mask layer ranges from 100 to 500 nm.
[0019] Optionally, the thickness of the second intrinsic germanium layer is in the range of 100 to 3000 nm.
[0020] Optionally, the second doped germanium layer has a thickness ranging from 100 to 500 nm.
[0021] Optionally, a passivation layer is further provided inside the groove and on the surface of the ion-doping mask layer.
[0022] Optionally, a side of the oxide layer facing away from the dielectric layer further comprises a silicon substrate.
[0023] Optionally, the germanium-based sensor further includes a readout circuit, and the first electrode and the second electrode are electrically connected to the readout circuit.
[0024] Compared with the prior art, the utility model has the following beneficial effects:
[0025] The utility model provides a planar tensile strain germanium sensor, which comprises: an oxide layer, a dielectric layer, a second doped germanium layer, a second intrinsic germanium layer, an intrinsic germanium / germanium silicon multi-quantum well layer, a first intrinsic germanium layer and an ion-doped mask layer stacked in sequence; the ion-doped mask layer is discontinuously distributed on the side of the first intrinsic germanium layer away from the oxide layer; the area of the first intrinsic germanium layer not covered by the ion-doped mask layer is an ion-doped area; the ion-doping type of the ion-doped area is different from that of the second doped germanium layer; the area of the first intrinsic germanium layer covered by the ion-doped mask layer has a non-ion-doped area; the non-ion-doped area has a groove, and the bottom of the groove is the second doped germanium layer; the side of the ion-doped area away from the oxide layer has a first electrode; the groove is filled with a second electrode; the non-ion-doped area is between the second electrode and the ion-doped area. Thus, on the one hand, the first intrinsic germanium layer of the germanium sensor includes an ion-doped region and a non-ion-doped region, and the ion-doped region, the intrinsic germanium / germanium silicon multi-quantum well layer, the second intrinsic germanium layer and the second doped germanium layer constitute a planar PIN structure. For each pixel, there is a non-ion-doped region on the side wall but no ion-doped region, which avoids the ion-doped region from contacting both the first electrode and the second electrode, thereby reducing or even completely avoiding the leakage current problem on the surface and side wall of the pixel, and at the same time, it can also reduce the electrical crosstalk between adjacent pixels, and obtain a germanium sensor with small pixel spacing, low leakage current and low dark current; on the other hand, there are two photoelectric sensing layers, the second intrinsic germanium layer and the intrinsic germanium / germanium silicon multi-quantum well layer, wherein the second intrinsic germanium layer is a visible light and short-wave infrared sensing layer, and the intrinsic germanium / germanium silicon multi-quantum well layer is an infrared band sensing layer, and the combined structure of the two can significantly expand the spectral response range of the germanium sensor. In addition, the production cost required for the germanium sensor is low, and it can be made through a production process that is highly compatible with the CMOS process, which is conducive to promoting the application of germanium sensors in the civilian field. BRIEF DESCRIPTION OF THE DRAWINGS
[0026] In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.
[0027] Figure 1 A donor substrate structure diagram provided for an embodiment of the utility model;
[0028] Figure 2 A structural diagram of an acceptor substrate provided in an embodiment of the utility model;
[0029] Figure 3 A structural diagram of a planar tensile strain germanium sensor provided in an embodiment of the utility model. DETAILED DESCRIPTION
[0030] As described above, the germanium-on-insulator sensor is currently considered to be one of the most promising technology routes. However, almost all germanium-on-insulator sensors currently use a table structure, with exposed ion-doped layer sidewalls on the pixel sidewalls, and the surface of the ion-doped layer is in contact with an electrode. For germanium sensors with large pixel sizes and large spacing between pixels, the table structure germanium sensors can meet the needs of most application scenarios; but in order to meet more application scenarios, it is necessary to make the pixel spacing and pixel size of the germanium sensor smaller, so as to obtain a focal plane array germanium sensor with higher resolution, larger array specifications, higher fill factor and lower cost.
[0031] For germanium sensors with smaller pixel pitch and pixel size, the sidewall of the ion-doped layer will contact another electrode, resulting in large leakage current on the pixel surface and sidewall, which will have a serious impact on the sensitivity, power consumption, reliability and other performance of the germanium sensor. In addition, the electrical crosstalk between adjacent small pixels of the mesa-structured germanium sensor is more serious, resulting in higher dark current, which is not conducive to improving the performance of the focal plane array.
[0032] In order to solve the above problems, the utility model provides a planar tensile strain germanium sensor, which comprises: an oxide layer, a dielectric layer, a second doped germanium layer, a second intrinsic germanium layer, an intrinsic germanium / germanium silicon multi-quantum well layer, a first intrinsic germanium layer and an ion-doped mask layer stacked in sequence; the ion-doped mask layer is discontinuously distributed on the side of the first intrinsic germanium layer away from the oxide layer; the area of the first intrinsic germanium layer not covered by the ion-doped mask layer is an ion-doped area; the ion doping type of the ion-doped area is different from that of the second doped germanium layer; the area of the first intrinsic germanium layer covered by the ion-doped mask layer has a non-ion-doped area; the non-ion-doped area has a groove, and the bottom of the groove is the second doped germanium layer; the side of the ion-doped area away from the oxide layer has a first electrode; the groove is filled with a second electrode; and a non-ion-doped area is provided between the second electrode and the ion-doped area.
[0033] Therefore, the first intrinsic germanium layer of the planar tensile strain germanium sensor includes an ion-doped region and a non-ion-doped region, and the ion-doped region, the intrinsic germanium / germanium silicon multi-quantum well layer, the second intrinsic germanium layer and the second doped germanium layer constitute a planar PIN structure. For each pixel, there is a non-ion-doped region on the side wall but no ion-doped region, which avoids the ion-doped region contacting both the first electrode and the second electrode, thereby reducing or even completely avoiding the leakage current problem on the pixel surface and side wall, and also reducing the electrical crosstalk between adjacent pixels, thereby obtaining a germanium sensor with small pixel spacing, low leakage current and low dark current.
[0034] In order to enable those skilled in the art to better understand the solution of the utility model, the technical solution in the embodiment of the utility model will be clearly and completely described below in conjunction with the drawings in the embodiment of the utility model. Obviously, the described embodiment is only a part of the embodiment of the utility model, not all of the embodiments. Based on the embodiment of the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.
[0035] See also Figure 1 , which is a structural diagram of a donor substrate provided by an embodiment of the utility model, and the donor substrate comprises:
[0036] A first silicon substrate 101 , a first intrinsic germanium layer 102 , an intrinsic germanium / silicon germanium multi-quantum well layer 103 , a second intrinsic germanium layer 104 , a second doped germanium layer 105 and a dielectric layer 106 are sequentially stacked.
[0037] Among them, the first intrinsic germanium layer 102, the intrinsic germanium / germanium silicon multi-quantum well layer 103 and the second intrinsic germanium layer 104 can be formed by sequentially performing epitaxy on one side of the first silicon substrate 101, and the first silicon substrate 101 can be a large-size silicon substrate with a diameter greater than or equal to 8 inches.
[0038] Optionally, the dielectric layer 107 is an Al 2 O 3 layer, so that the donor substrate and the acceptor substrate can be more easily directly wafer-bonded.
[0039] See also Figure 2 , this figure is a structural diagram of an acceptor substrate provided by an embodiment of the utility model, and the acceptor substrate includes: a second silicon substrate 201 and an oxide layer 202 stacked in sequence.
[0040] Specifically, the second silicon substrate 201 can be a silicon substrate, a sapphire substrate or a glass substrate; the oxide layer 202 can be a layer of SiO2 or a stack of multiple layers of SiO2 formed on a silicon substrate using, for example, a thermal oxidation process or a chemical vapor deposition process, and the thickness of the oxide layer 202 can range from 100 to 1000 nm.
[0041] See also Figure 3 , which is a structural diagram of a planar tensile strain germanium sensor provided by an embodiment of the utility model, and the planar tensile strain germanium sensor comprises:
[0042] The oxide layer 202 , the dielectric layer 106 , the second doped germanium layer 105 , the second intrinsic germanium layer 104 , the intrinsic germanium / silicon germanium multi-quantum well layer 103 , the first intrinsic germanium layer 102 and the ion doping mask layer 107 are stacked in sequence.
[0043] The ion doping mask layer 107 is intermittently distributed on a side of the first intrinsic germanium layer 102 away from the oxide layer 202 .
[0044] As an example, the donor substrate and the acceptor substrate can be bonded by direct wafer bonding process to form a second silicon substrate 201, an oxide layer 202, a dielectric layer 106, a second doped germanium layer 105, a second intrinsic germanium layer 104, an intrinsic germanium / germanium silicon multi-quantum well layer 103, a first intrinsic germanium layer 102 and a first silicon substrate 101 which are stacked in sequence; and then mechanical grinding and chemical mechanical polishing are performed. The first silicon substrate 101 is removed by a process such as CMP to obtain a second silicon substrate 201, an oxide layer 202, a dielectric layer 106, a second doped germanium layer 105, a second intrinsic germanium layer 104, an intrinsic germanium / germanium silicon multi-quantum well layer 103 and a first intrinsic germanium layer 102 which are stacked in sequence; then an ion doping mask layer is formed on the side of the first intrinsic germanium layer 102 away from the oxide layer 202 by a process such as chemical vapor deposition, and the ion doping mask layer can be a film layer with good ion blocking effect such as a silicon oxide layer or a silicon nitride layer, and the thickness ranges from 100 to 500 nm; finally, the ion doping mask layer 107 is etched to open holes by a process such as reactive ion etching (RIE), so as to form an ion doping mask layer 107 which is discontinuously distributed on the side of the first intrinsic germanium layer 102 away from the oxide layer 202.
[0045] The area of the first intrinsic germanium layer 102 not covered by the ion doping mask layer 107 is an ion doping area 1021; the ion doping type of the ion doping area 1021 is different from that of the second doped germanium layer 105, so that the ion doping area 1021, the intrinsic germanium / silicon germanium multi-quantum well layer 103, the second intrinsic germanium layer 104 and the second doped germanium layer 105 form a planar PIN structure. The area of the first intrinsic germanium layer 102 covered by the ion doping mask layer 107 has a non-ion doping area 1022; the non-ion doping area 1022 has a groove, and the bottom of the groove is the second doped germanium layer 105.
[0046] Specifically, the first intrinsic germanium layer 102 may be ion-doped under the mask of the ion-doping mask layer 107 by an ion-doping process such as ion implantation or diffusion, thereby forming an ion-doped region 1021 with ion doping and a non-ion-doped region 1022 without ion doping. It is understood that the range of the ion-doped region 1021 will be slightly larger than the region of the first intrinsic germanium layer 102 not covered by the ion-doping mask layer 107, and the range of the non-ion-doped region 1022 will be slightly smaller than the region of the first intrinsic germanium layer 102 covered by the ion-doping mask layer 107. The ion doping type of the ion-doped region 1021 is different from that of the second doped germanium layer 105. For example, if the second doped germanium layer 105 is N-type ion doped, the ion-doped region 1021 is P-type ion doped.
[0047] For example, the center of the non-ion doped region 1022 can be etched by processes such as dry etching, and the etching depth reaches the second doped germanium layer 105, so as to form a groove with the second doped germanium layer 105 at the bottom in the non-ion doped region 1022, and the non-ion doped region 1022 is larger than the formation area of the groove. Thus, a plurality of pixels are formed, and the ion doped region 1021 of each pixel is wrapped by the non-ion doped region 1022 in the vertical direction of the film layer stacking direction.
[0048] The side of the ion-doped region 1021 facing away from the oxide layer 202 has a first electrode 301 ; the groove is filled with a second electrode 302 . A non-ion-doped region 1022 is provided between the second electrode 302 and the ion-doped region 1021 .
[0049] Therefore, on the one hand, the first intrinsic germanium layer 102 of the germanium sensor includes an ion-doped region 1021 and a non-ion-doped region 1022, and the ion-doped region 1021, the intrinsic germanium / germanium silicon multi-quantum well layer 103, the second intrinsic germanium layer 104 and the second doped germanium layer 105 constitute a planar PIN structure. For each pixel, there is a non-ion-doped region 1022 on the side wall but no ion-doped region 1021. The ion-doped region 1021 of each pixel is wrapped by the non-ion-doped region 1022 in the vertical direction of the film stacking direction, forming a planar structured germanium sensor. There is no exposed ion-doped region 1021 on the side wall of the groove, avoiding The ion doping region 1021 is not required to contact both the first electrode 301 and the second electrode 302, thereby reducing or even completely avoiding the leakage current problem on the surface and sidewall of the pixel, and at the same time, the electrical crosstalk between adjacent pixels can be reduced, so that a germanium sensor with small pixel spacing, low leakage current and low dark current can be obtained; on the other hand, there are two photoelectric sensing layers, the second intrinsic germanium layer 104 and the intrinsic germanium / germanium silicon multi-quantum well layer 103, wherein the second intrinsic germanium layer 104 is a visible light and short-wave infrared sensing layer, and the intrinsic germanium / germanium silicon multi-quantum well layer 103 is an infrared band sensing layer, and the combined structure of the two can significantly expand the spectral response range of the germanium sensor. In addition, the production cost required for the germanium sensor is low, and it can be manufactured through a production process that is highly compatible with the CMOS process, which is conducive to promoting the application of germanium sensors in the civilian field.
[0050] Optionally, the thickness of the first intrinsic germanium layer 102 is in the range of 100-500 nm.
[0051] Optionally, the intrinsic germanium / germanium silicon multi-quantum well layer 103 includes a germanium layer and a germanium silicon layer, wherein the germanium layer is a barrier layer with a thickness ranging from 10 to 100 nm; the germanium silicon layer is a potential well layer with a thickness ranging from 10 to 100 nm, the silicon content in the germanium silicon layer is 0 to 30%, and the germanium content is not less than 70%; the total thickness of the intrinsic germanium / germanium silicon multi-quantum well layer 103 ranges from 200 to 1000 nm, and may contain 2 to 10 quantum wells.
[0052] Optionally, the second intrinsic germanium layer 104 has a thickness ranging from 100 to 3000 nm, including a reserved electrode layer of 100 to 500 nm, and the rest is a photoelectric sensing layer; the second doped germanium layer 105 has a thickness ranging from 100 to 500 nm. As an example, a germanium layer can be first formed by epitaxy, and then part of the germanium layer is N-type or P-type doped by, for example, ion implantation or diffusion, so that the germanium layer is divided into a second intrinsic germanium layer 104 without ion doping and a second doped germanium layer 105 with ion doping.
[0053] In some other embodiments provided by the utility model, the external light signal will enter the planar tensile strain germanium sensor from the side of the oxide layer 202 close to the second silicon substrate 201. When the external light signal enters the photoelectric sensing layer of the germanium sensor, it will not only cause the transition of the inter-band carriers in the second intrinsic germanium layer 104, but also cause the transition of the inter-band carriers in the intrinsic germanium / germanium silicon multi-quantum well layer 103, that is, there are two photoelectric sensing layers, the second intrinsic germanium layer 104 and the intrinsic germanium / germanium silicon multi-quantum well layer 103, wherein the second intrinsic germanium layer 104 is a visible light and short-wave infrared sensing layer, and the intrinsic germanium / germanium silicon multi-quantum well layer 103 is an infrared band sensing layer. Therefore, the combined structure of the second intrinsic germanium layer 104 and the intrinsic germanium / germanium silicon multi-quantum well layer 103 is used as the sensing layer, which significantly expands the spectral response range of the germanium sensor.
[0054] In some other embodiments provided by the present invention, the germanium-based sensor further includes a passivation layer 303. The passivation layer 303 is located inside the groove and on the surface of the ion-doped mask layer 107.
[0055] Specifically, after forming the ion-doped region 1021 and the non-ion-doped region 1022 in the first intrinsic germanium layer 102, the ion-doped mask layer 107 and the exposed surface of the first intrinsic germanium layer 102 may be passivated to form a first passivation layer 3031; then the passivation layer on the surface of the ion-doped region 1021 may be opened to facilitate the formation of the first electrode 301 in contact with the ion-doped region 1021. After forming a groove located in the non-ion-doped region 1022, the inner surface of the groove may be passivated to form a second passivation layer 1032; then the passivation layer at the bottom of the groove may be opened to facilitate the formation of the second electrode 302 in contact with the second doped germanium layer 105.
[0056] In some other embodiments provided by the present invention, the planar tensile strain germanium sensor further includes a readout circuit ROIC, and the first electrode 301 and the second electrode 302 can be electrically connected to the readout circuit by bump bonding.
[0057] In some other embodiments provided by the present invention, in the planar tensile strain germanium sensor, the side of the oxide layer 202 away from the dielectric layer 106 may also have a portion of the second silicon substrate 201. Specifically, after the first electrode 301 and the second electrode 302 are electrically connected to the readout circuit, a portion of the second silicon substrate 201 may be removed, or the second silicon substrate 201 may be completely removed to expose the oxide layer 202.
[0058] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.
[0059] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A planar tensile strain germanium sensor, characterized in that: The germanium sensor comprises: An oxide layer, a dielectric layer, a second doped germanium layer, a second intrinsic germanium layer, an intrinsic germanium / germanium silicon multi-quantum well layer, a first intrinsic germanium layer and an ion-doped mask layer stacked in sequence; The ion doping mask layer is intermittently distributed on a side of the first intrinsic germanium layer away from the oxide layer; The area of the first intrinsic germanium layer that is not covered by the ion doping mask layer is an ion doping area; the ion doping type of the ion doping area is different from that of the second doped germanium layer; The area of the first intrinsic germanium layer covered by the ion doping mask layer has a non-ion doped area; the non-ion doped area has a groove, and the bottom of the groove is the second doped germanium layer; The side of the ion-doped region away from the oxide layer has a first electrode; the groove is filled with a second electrode; The non-ion-doped region is provided between the second electrode and the ion-doped region.
2. The planar tensile strain germanium sensor according to claim 1, characterized in that: The intrinsic germanium / germanium silicon multi-quantum well layer comprises a germanium layer and a germanium silicon layer; The germanium layer is a barrier layer with a thickness ranging from 10 to 100 nm; the germanium silicon layer is a potential well layer with a thickness ranging from 10 to 100 nm; and the content of silicon in the germanium silicon layer is 0 to 30%.
3. The planar tensile strain germanium sensor according to claim 1, characterized in that: The intrinsic germanium / germanium silicon multi-quantum well layer includes 2 to 10 quantum wells.
4. The planar tensile strain germanium sensor according to claim 1, characterized in that: The thickness of the intrinsic germanium / germanium silicon multi-quantum well layer is in the range of 200 to 1000 nm.
5. The planar tensile strain germanium sensor according to claim 1, characterized in that: The thickness of the ion doping mask layer ranges from 100 to 500 nm.
6. The planar tensile strain germanium sensor according to claim 1, characterized in that: The thickness of the second intrinsic germanium layer is in the range of 100 to 3000 nm.
7. The planar tensile strain germanium sensor according to claim 1, characterized in that: The thickness of the second doped germanium layer is in the range of 100-500 nm.
8. The planar tensile strain germanium sensor according to claim 1, characterized in that: A passivation layer is also provided inside the groove and on the surface of the ion-doping mask layer.
9. The planar tensile strain germanium sensor according to claim 1, characterized in that: The side of the oxide layer facing away from the dielectric layer also has a silicon substrate.
10. The planar tensile strain germanium sensor according to claim 1, characterized in that: The germanium-based sensor further includes a readout circuit, and the first electrode and the second electrode are electrically connected to the readout circuit.