# US 8,772,717 B2 · Radiation detector having a bandgap engineered absorber > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,772,717 B2 - Document ID: US8772717B2 - Status: Granted - Kind: B2 - Application number: 12/853,174 - Filed: 2010-08-09 - Published: 2011-02-10 - Granted: 2014-07-08 - Earliest claimed: 2010-08-09 - Assignees: DRS RSTA, Inc. - Applicants: Pradip Mitra; Jeffrey D. Beck; Mark R. Skokan - Inventors: Pradip Mitra; Jeffrey D. Beck; Mark R. Skokan - Web page: https://patentgrep.com/patent/US8772717B2 ## CPC - G01J 1/44: Electric circuits {(for command of an exposure part G03B7/02)} - G01J 5/024: {Special manufacturing steps or sacrificial layers or layer structures} - G01J 5/046: {Materials; Selection of thermal materials} - G01J 5/0853: {having infrared absorbers other than the usual absorber layers deposited on infrared detectors like bolometers, wherein the heat propagation between the absorber and the detecting element occurs within a solid} - H10F 30/21: the devices being sensitive to infrared, visible or ultraviolet radiation - H10F 30/2212: {the devices comprising active layers made of only Group II-VI materials, e.g. HgCdTe infrared photodiodes} - H10F 30/225: the potential barrier working in avalanche mode, e.g. avalanche photodiodes - H10F 39/107: {having multiple elements covered by H10F30/00 in a repetitive configuration, e.g. radiation detectors comprising photodiode arrays} - H10F 39/1847: {Multispectral infrared image sensors having a stacked structure, e.g. NPN, NPNPN or multiple quantum well [MQW] structures} - H10F 71/1253: {comprising at least three elements, e.g. HgCdTe} - H10F 77/1237: {having at least three elements, e.g. HgCdTe} - H10F 77/306: {for devices having potential barriers} - H10F 77/40: Optical elements or arrangements (surface textures H10F77/70) - H10F 77/60: Arrangements for cooling, heating, ventilating or compensating for temperature fluctuations - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) - H10F: INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION ## Abstract A radiation detector is provided that includes a photodiode having a radiation absorber with a graded multilayer structure. Each layer of the absorber is formed from a semiconductor material, such as HgCdTe. A first of the layers is formed to have a first predetermined wavelength cutoff. A second of the layers is disposed over the first layer and beneath the first surface of the absorber through which radiation is received. The second layer has a graded composition structure of the semiconductor material such that the wavelength cutoff of the second layer varies from a second predetermined wavelength cutoff to the first predetermined wavelength cutoff such that the second layer has a progressively smaller bandgap than the first bandgap of the first layer. The graded multilayer radiation absorber structure enables carriers to flow toward a conductor that is used for measuring the radiation being sensed by the radiation absorber. ## Claims ### Claim 1 A radiation detector, comprising: a photodiode including a radiation absorber having a first surface upon which radiation is received into the radiation absorber, the radiation absorber having a plurality of layers each formed from a semiconductor material, a first of the layers is formed to have a first predetermined wavelength cutoff, a second of the layers is disposed over the first layer and beneath the first surface of the radiation absorber, the second layer having a graded composition structure of the semiconductor material such that the second layer has a wavelength cutoff that varies from a second predetermined wavelength cutoff to the first predetermined wavelength cutoff, wherein the first of the layers is electrically coupled to the second of the layers, and a metal layer is in contact with each of the plurality of layers. ### Claim 2 (depends on 1) The radiation detector according to claim 1, wherein the semiconductor material is HgCdTe. ### Claim 3 (depends on 2) The radiation detector according to claim 2, wherein a third of the absorber layers is disposed on the second layer and defines the first surface of the radiation absorber, the third layer is formed of a homogenous composition of HgCdTe to have the second predetermined wavelength cutoff. ### Claim 4 (depends on 3) The radiation detector according to claim 3, wherein the first thickness of the first layer is minimized to a predetermined value that reflects a tradeoff between dark current at a desired temperature of operation and functionality of the photodiode. ### Claim 5 (depends on 3) The radiation detector according to claim 3, wherein a first thickness of the first layer is 0.2 microns or less. ### Claim 6 (depends on 5) The radiation detector according to claim 5, wherein a second thickness of the second layer is 1.8 microns or greater. ### Claim 7 (depends on 1) The radiation detector according to claim 1, wherein the first layer has a first bandgap and the second layer has a progressively larger bandgap than the first bandgap of the first layer. ### Claim 8 (depends on 1) The radiation detector according to claim 1, wherein a third of the absorber layers is disposed on the second layer, the third layer is formed of a homogenous composition of HgCdTe to have the second predetermined wavelength, and the third layer has a third bandgap that is equal to or greater than the largest bandgap of the second layer. ### Claim 9 (depends on 1) The radiation detector according to claim 1, wherein the radiation absorber is a p-type region of the photodiode and the photodiode has an n-type region that has the same plurality of layers as the p-type region. ### Claim 10 (depends on 9) The radiation detector according to claim 9, wherein the photodiode further includes: a via extending through the first and second layers, wherein the via is characterized by walls, and wherein the metal layer coats the walls of the via and is in contact with the n-type region; and an input preamp disposed below the first layer with respect to the second layer and part of a readout integrated circuit, the metal layer being in electrical communication with the input preamp. ### Claim 11 A radiation detector comprising: an anti-reflection coating operable to receive radiation; a passivation layer disposed adjacent the anti-reflection coating; and a photodiode including: a first layer of a first semiconductor material adjacent the passivation layer and characterized by a first predetermined wavelength cutoff; a second layer of a second semiconductor material adjacent the first layer and having a first surface adjacent the first layer and a second surface opposing the first surface, wherein the second semiconductor material has a graded composition characterized by the first predetermined wavelength cutoff at the first surface and a second predetermined wavelength cutoff at the second surface, the second predetermined wavelength cutoff being greater than the first predetermined wavelength cutoff; a third layer of a third semiconductor material adjacent the second layer and characterized by the second predetermined wavelength cutoff; and a metal layer in contact with the first layer, the second layer, and the third layer. ### Claim 12 (depends on 11) The radiation detector according to claim 11, wherein the first semiconductor material comprises a homogeneous layer of Hgx-1CdxTe and the third semiconductor material comprises a homogeneous layer of Hgy-1CdyTe. ### Claim 13 (depends on 11) The radiation detector according to claim 11, wherein a thickness of the third layer is 0.2 μm or less. ### Claim 14 (depends on 13) The radiation detector according to claim 13, wherein a thickness of the second layer is 1.8 μm or greater. ### Claim 15 (depends on 11) The radiation detector according to claim 11, wherein: the first layer is characterized by a first bandgap; the second layer is characterized by the first bandgap at the first surface and a second bandgap at the second surface, the second bandgap being less than the first bandgap; and the third layer is characterized by the second bandgap. ### Claim 16 (depends on 11) The radiation detector according to claim 11, wherein the first layer and the second layer are electrically coupled. ### Claim 17 (depends on 16) The radiation detector according to claim 16, wherein the second layer and the third layer are electrically coupled.