Structure and Fabrication of Field-effect Transistor for Alleviating Short-channel Effects and/or Reducing Junction Capacitance

Abstract

An IGFET ( 40 or 42 ) has a channel zone ( 64 or 84 ) situated in body material ( 50 ). Short-channel threshold voltage roll-off and punchthrough are alleviated by arranging for the net dopant concentration in the channel zone to longitudinally reach a local surface minimum at a location between the IGFET's source/drain zones ( 60 and 62 or 80 and 82 ) and by arranging for the net dopant concentration in the body material to reach a local subsurface maximum more than 0.1 μm deep into the body material but not more than 0.1 μm deep into the body material. The source/drain zones ( 140 and 142 or 160 and 162 ) of a p-channel IGFET ( 120 or 122 ) are provided with graded-junction characteristics to reduce junction capacitance, thereby increasing switching speed.

1 - 94 . (canceled) 
     
     
         95 . A structure comprising a field-effect transistor comprising:
 a channel zone situated in body material of a semiconductor body having an upper surface;   first and second source/drain zones situated in the semiconductor body along its upper surface, laterally separated by the channel zone, and forming respective pn junctions with the body material, the first source/drain zone comprising a main portion and a more lightly doped lateral extension laterally continuous with the main portion, the channel zone being terminated by the lateral extension and the second source/drain zone along the body's upper surface, the body material having a net dopant concentration which (i) longitudinally reaches a local surface minimum along the body's upper surface at a location between the source/drain zones and (ii) vertically reaches three separate local subsurface maxima along an imaginary line that extends generally perpendicular to the body's upper surface through the channel zone and into underlying matter of the body material, each local subsurface maximum in the body material's net dopant concentration extending continuously laterally so as to underlie at least part of each source/drain zone;   a gate dielectric layer overlying the channel zone; and   a gate electrode overlying the gate dielectric layer above the channel zone.   
     
     
         96 . A structure as in  claim 95  wherein each local subsurface maximum in the body material's net dopant concentration underlies substantially all of each source/drain zone. 
     
     
         97 . A structure as in  claim 95  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward only the second of the source/drain zones. 
     
     
         98 . A structure as in  claim 95  wherein the second source/drain zone comprises a main portion and a more lightly doped lateral extension laterally continuous with the main portion of the second source/drain zone such that the channel zone is terminated by the lateral extensions along the body's upper surface. 
     
     
         99 . A structure as in  claim 98  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward each source/drain zone. 
     
     
         100 . A structure as in  claim 99  wherein the local surface minimum in the body material's net dopant concentration occurs approximately at a single point along an imaginary line extending longitudinally between the source/drain zones along the body's upper surface. 
     
     
         101 . A structure as in  claim 99  wherein the body material's net dopant concentration is approximately constant at the local surface minimum along a non-zero portion of an imaginary line extending longitudinally between the source/drain zones along the body's upper surface. 
     
     
         102 . A structure as in  claim 99  wherein the body material's net dopant concentration is of an average value that increases with decreasing length of the channel zone when a given amount of semiconductor dopant per unit width of the channel zone causes the body' material's net dopant concentration to vary along the body's upper surface. 
     
     
         103 . A structure as in  claim 98  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward only one of the source/drain zones. 
     
     
         104 . A structure as in  claim 95  wherein the three local subsurface maxima in the body material's net dopant concentration consist of;
 a primary local subsurface maximum whose depth at a primary position beneath the gate electrode is 0.1-0.4 μm below the body's upper surface; 
 an additional local subsurface maximum which occurs deeper below the body's upper surface than the primary local subsurface maximum and whose depth at an additional position beneath the gate electrode is 0.3-0.5 μm below the body's upper surface; and 
 a further local subsurface maximum which occurs deeper below the body's upper surface than the additional local subsurface maximum and whose depth at a further position beneath the gate electrode is 0.4-0.7 pin below the body's upper surface. 
 
     
     
         105 . A structure as in  claim 104  wherein:
 the body material's net dopant concentration at the primary and additional local subsurface maxima is 2×10 17 -8×10 17  atoms/cm 3 ; and 
 the body material's net dopant concentration at the further local subsurface maximum is 5×10 17 -1×10 18  atoms/cm 3 . 
 
     
     
         106 . A structure as in  claim 95  wherein the transistor is an n-channel transistor. 
     
     
         107 . A structure comprising a field-effect transistor comprising:
 a channel zone situated in body material of a semiconductor body having an upper surface, a channel surface depletion region extending along the body's upper surface into the channel zone so as to reach a maximum thickness at a location in the channel zone;   first and second source/drain zones situated in the semiconductor body along its upper surface, laterally separated by the channel zone, and forming respective pn junctions with the body material, the first source/drain zone comprising a main portion and a more lightly doped lateral extension laterally continuous with the main portion, the channel zone being terminated by the lateral extension and the second source/drain zone along the body's upper surface, the body material having a net dopant concentration which (i) longitudinally reaches a local surface minimum at a location along the body's upper surface between the source/drain zones and (ii) vertically reaches three separate local subsurface maxima along an imaginary line that extends generally perpendicular to the body's upper surface through the channel zone and into underlying matter of the body material, each local subsurface maximum in the body material's net dopant concentration occurring at a location below the location of the channel surface depletion region at its maximum thickness, each of the two deepest local subsurface maxima in the body material's net dopant concentration extending continuously laterally so as to underlie at least part of each source/drain zone;   a gate dielectric layer overlying the channel zone; and   a gate electrode overlying the gate dielectric layer above the channel zone.   
     
     
         108 . A structure as in  claim 107  wherein each of the two deepest local subsurface maxima in the body material's net dopant concentration underlies substantially all of each source/drain zone. 
     
     
         109 . A structure as in  claim 107  wherein the shallowest local subsurface maximum in the body material's net dopant concentration also extends continuously laterally so as to underlie at least part of each source/drain zone. 
     
     
         110 . A structure as in  claim 109  wherein each local subsurface maximum in the body material's net dopant concentration underlies substantially all of each source/drain zone. 
     
     
         111 . A structure as in  claim 107  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward only the second of the source/drain zones. 
     
     
         112 . A structure as in  claim 107  wherein the second source/drain zone comprises a main portion and a more lightly doped lateral extension laterally continuous with the main portion of the second source/drain zone such that the channel zone is terminated by the lateral extensions along the body's upper surface. 
     
     
         113 . A structure as in  claim 112  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward each source/drain zone. 
     
     
         114 . A structure as in  claim 113  wherein the local surface minimum in the body material's net dopant concentration occurs approximately at a single point along an imaginary line extending longitudinally between the source/drain zones along the body's upper surface. 
     
     
         115 . A structure as in  claim 113  wherein the body material's net dopant concentration is approximately constant at the local surface minimum along a non-zero portion of an imaginary line extending longitudinally between the source/drain zones along the body's upper surface. 
     
     
         116 . A structure as in  claim 113  wherein the body material's net dopant concentration is of an average value that increases with decreasing length of the channel zone when a given amount of semiconductor dopant per unit width of the channel zone causes the body' material's net dopant concentration to vary along the body's upper surface. 
     
     
         117 . A structure as in  claim 112  wherein the body material's net dopant concentration increases in moving along the body's upper surface from the location of the local surface minimum toward only one of the source/drain zones. 
     
     
         118 . A structure as in  claim 107  wherein the transistor is an n-channel transistor.