Single photon avalanche diode for CMOS circuits

A single photon avalanche diode SPAD, the SPAD comprising:

• a SPAD multiplication junction;
• a first region of a first conductivity type arranged on or in a second region comprising an epi-layer or substrate of a second conductivity type;
• wherein a bottom of said first region is in contact with the second region to define said SPAD multiplication junction;
• wherein the first region comprises a first well having a first depth and a second well below the first well having a second depth extending to said bottom and the location of the SPAD multiplication junction;
• a guard ring surrounding and in contact with the first region and formed by a region of the substrate or epi-layer of the second conductivity type;
• a first contact connected to the first region via a conductive pathway of the first conductivity type formed in the first region;
• and
• a second contact connected to the second region via a conductive pathway of the second conductivity type formed in the epi-layer or substrate;
• wherein the conductive pathway of the second conductivity type comprises a third well having a third depth less than the second depth of the SPAD multiplication junction;
• and
• wherein the guard ring has a retrograde doping concentration which increases with depth towards the SPAD multiplication junction so that breakdown voltage is smaller at the SPAD multiplication junction between the bottom of the first region and the second region than at sides of the first region in contact with the guard ring.

A single photon avalanche diode SPAD, the SPAD comprising:

• a first region comprising a first well of a first conductivity type;
• a second region comprising a second well of the first conductivity type formed below the first well;
• a third region of a second conductivity type, the first and second regions formed within and above the third region;
• an enhancement implant of the second conductivity type formed within the third region below the second well and in contact with a bottom of the second well at a SPAD multiplication junction;
• a first contact connected to the first region;
• and
• a second contact connected to the third region via a conductive pathway of the second conductivity type;
• wherein the doping in the vicinity of the second region is controlled such that the breakdown voltage is smaller at the SPAD multiplication junction than at sides of the second region, and
• wherein SPAD multiplication occurs at the SPAD multiplication junction when an appropriate bias voltage is applied between the first and second contacts.

A single photon avalanche diode SPAD, the SPAD comprising:

• a SPAD multiplication junction;
• a first well of a first conductivity type;
• a substrate region of a second conductivity type, the first well located within the substrate region, wherein the substrate region is in contact with a bottom of the first well to provide said SPAD multiplication junction;
• a guard ring surrounding an upper portion of the first well, wherein a depth of the bottom of the second well at the SPAD multiplication junction is lower than a lowest part of the guard ring and wherein the guard ring is formed by a region of the first conductivity type which is more lightly doped that the first well;
• a first contact connected to the first region;
• and
• a second contact connected to the third region via a conductive pathway of the second conductivity type.