Abstract:
A field effect transistor includes a substrate having a doping of a first conductivity type, a drain area in the substrate having a doping of a second conductivity type oppposite the first conductivity type, a source area in the substrate being laterally spaced from the drain area and having a doping of the second conductivity type, and a channel area in the substrate that is arranged between the source area and the drain area. In a portion of the substrate bordering the drain area, an area having a doping of the second conductivity type, which is connected to the drain area, is arranged such that in the portion alternating regions having the first conductivity type and having the second conductivity type are arranged.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a field effect transistor with reduced capacitive coupling between drain and substrate. 
   2. Description of the Related Art 
   For numerous large signal applications, LDMOS transistors or LDMOS field effect transistors (LDMOS=lateral diffused metal oxide semiconductor) are used, such as for power amplifiers for base stations, hand sets, mobile telephones, etc. The output capacity of a LDMOS field effect transistor is dependent on the drain voltage or the voltage between the drain or the drain area on the one hand and the substrate often connected with a reference potential on the other. 
     FIG. 3  is a schematic illustration of a vertical section through a conventional LDMOS field effect transistor. A p-doped base substrate  10  comprises a first lower surface  12  and a second upper surface  14 . At the lower surface  12 , the base substrate  10  comprises a backside contact in the form of a metal coating  16 . On the other surface  14  of the base substrate  10  a p-doped epitaxial layer  20  is created by means of an epitaxial method, such as by means of CVD epitaxy (CVD=chemical vapor deposition). The base substrate  10  and the epitaxial layer  20  together form a device substrate  30  with a surface  32  that is at the same time a surface of the epitaxial layer  20  facing away from the base substrate  10 . 
   In or on the epitaxial layer  20 , a field effect transistor or its semiconductor function elements are arranged. A source area  40  is formed by an n + -doped area at or directly below the surface  32 . A p-doped enhance area  42  borders the side of the source area  40  facing away from the surface  32 . A p-doped body area  44 , which has, however, in contrast to the enhance area  42 , a greater expansion than the source area  40  in at least one direction and thus also laterally borders the source area  40  and the enhance area  42  as well as the surface  32 , borders a side of the enhance area  42  facing away from the source area  40  and the surface  32 . 
   A drain area, which is formed from free drain sub-areas  50 ,  52 ,  54  with differently high doping concentration in this embodiment, is arranged on the surface  32 , laterally spaced from the source area  40  but laterally bordering the body area  44 . A first drain sub-area  50  having the greatest distance to the source area  40  is n + -doped. In direction to the source area  40 , a second drain sub-area  52  whose doping concentration is lower than that of the first drain sub-area  50  borders the first drain sub-area  50 . A third drain sub-area  54  bordering the body area  44  and having a lower doping concentration than the second drain sub-area  52  borders the second drain sub-area  52 . The second drain sub-area  52  and the third drain sub-area  54  together are also called resurf area (resurf=reduced surface field). 
   A p + -doped area  60  on the surface  32  borders a side of the source area  40  facing away from the drain area  50 ,  52 ,  54 . Between the p+-doped area  60  and the base substrate  10  or its upper surface  14 , a p-doped sinker  62  extends that increases the electric conductivity between the p+-doped area  60  and the base substrate  10 . 
   At a side of the p+-doped area  60  and the sinker  62  facing away from the source area  40 , the enhance area  42 , and the body area  44 , further structures  40 ′,  42 ′,  44 ′ border laterally, which are for example a further source area, a further enhance area, and a further body area, or the source area  40 , enhance area  42 , and the body area  44  that are laterally guided around the p+-doped area  60  and the sinker  62  in the form of an open or closed arc or frame. 
   On the epitaxial layer  20 , electrically conductive structures from metals or other electric conductors are arranged embedded in a dielectric layer  66 . A source metallization  70  borders the source area  40  and the p+-doped area  60  and contacts them or is connected thereto in an electrically conductive manner. Throughhole conductors  72  connect the source metallization  70  to shielding conductors  74  overlapping laterally or being arranged partly vertically above the source metallization  70  and being part of an overlying metallization plane in an electrically conductive manner. 
   A drain metallization  80  borders the most highly doped first drain sub-area  50  and is connected thereto in an electrically conductive manner. 
   Above the portion of the body area  54  bordering the surface  32 , a gate  90  from a doped polysilicon layer  92  and a silicide layer  94  is arranged. The gate  90  or the polysilicon layer  92  thereof is spatially spaced and electrically insulated from the surface  32  or the body area  44  substantially opposite the gate  90  by a thin insulating layer  96  (gate oxide). 
   When applying a positive voltage to the gate  90 , a thin conductive layer, a so-called channel, forms in the body area  44  opposite gate  90  close to the surface  32 . The area in which the channel forms when applying the positive voltage is designated as channel area  98  in the following. 
   A pn-junction is present between the drain area  50 ,  52 ,  54  on the one hand and adjacent areas of the epitaxial layer  20  on the other. A space charge zone or a depletion zone forms there. The thickness or expansion of the space charge or depletion zone that is perpendicular to the pn-junction is dependent on the magnitude of the applied drain voltage or on a potential difference between the drain area  50 ,  52 ,  54  on the one hand and the substrate  10  on the other hand. The reverse-biased pn-junction between the drain area  50 ,  52 ,  54  and the substrate  10  at the same time forms a capacitor whose capacitance is dependent on the thickness of the mentioned space charge zone, and thus on the drain voltage. 
   As already mentioned above, the output capacitance, which is dependent on the drain voltage, or the capacitance between the drain area  50 ,  52 ,  54  and the substrate  10 , complicates the matching of a circuit therewith, which is connected to the field effect transistor. Previously, this output capacitance of the field effect transistor, which is dependent on the drain voltage, had to be tolerated. 
   SUMMARY OF THE INVENTION 
   It is the object of embodiments of the present invention to provide a field effect transistor having a capacitance that is substantially independent of the drain voltage between a drain area and a substrate. 
   In accordance with a first aspect, the present invention provides a field effect transistor having a substrate ( 30 ) having a doping of a first conductivity type; a drain area in the substrate having a doping of a second conductivity type opposite to the first conductivity type; a source area in the substrate being laterally spaced from the drain area and having a doping of the second conductivity type; a channel area in the substrate that is arranged between the source area and the drain area; and an area having a doping of the second conductivity type and connected to the drain area and arranged in a portion of the substrate adjacent to the drain area such that alternating regions having the first conductivity type and having the second conductivity type are disposed in the portion. 
   The present invention provides a field effect transistor with a substrate with a doping of a first conductivity type, a drain area in the substrate with a doping of a second conductivity type opposite to the first conductivity type, a source area and a substrate being laterally spaced from the drain area and having a doping of the second conductivity type, and a channel area in the substrate disposed between the source area and the drain area. To the drain area, an area with a doping of the second conductivity type is connected, which is disposed in a portion of the substrate bordering the drain area such that alternating regions with the first conductivity type and with the second conductivity type are arranged in the portion. 
   According to a preferred embodiment, the present invention provides a semiconductor chip with the inventive field effect transistor. 
   The present invention is based on the finding to provide an area below the drain area, which causes complete depletion within a layer, which is as thick as possible but independent of the drain voltage, already at low drain voltages due to its spatial structure, so that no more or no substantial change of the thickness of the depletion zone occurs at higher drain voltages. This is for example achieved by the area having one or more columns or lamellae or the form of one or more columns or lamellae with a doping whose charge carrier type equals that of the drain area and is opposite to that of the substrate. The thickness of the columns or lamellae and the dimensions of the areas of the oppositely doped substrate remaining therebetween are chosen so (small) that, already at a drain voltage as low as possible, space charge zones are created that completely fill the columns or lamellae and the gaps therebetween. 
   A substantial advantage of the present invention is that from a predetermined minimum drain voltage on, at which the space charge zones, as mentioned, completely fill both the columns or lamellae of the area and the substrate material in their surroundings, a spatial expansion and in particular the thickness of this depletion zone is substantially only dependent on the geometry of these columns or lamellae and no longer on the drain voltage. The capacitance between the drain area and the substrate is then largely independent of the drain voltage. This enables simple, inexpensive, and efficient high-frequency matching of a circuit in which the inventive field effect transistor is used to the field effect transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a schematic sectional illustration of a field effect transistor according to a first embodiment of the present invention; 
       FIG. 2  shows a schematic sectional illustration of a field effect transistor according to a second embodiment of the present invention; and 
       FIG. 3  shows a schematic sectional illustration of a conventional field effect transistor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic illustration of a vertical section through a field effect transistor according to a first embodiment of the present invention. This field effect transistor differs from the conventional field effect transistor described above on the basis of  FIG. 3  in that, below the drain area  50 ,  52 ,  54  and in particular below the two more highly doped drain sub-areas  50 ,  52 , an area from a plurality of columns  102  is disposed, which is n-doped like the drain area  50 ,  52 ,  54 . The n-doped columns  102  are disposed perpendicularly to the surface  32  of the epitaxial layer  20  and immediately border the drain area  50 ,  52 ,  54  so that they are connected thereto in an electrically conductive manner. The columns  102  have a diameter as small as possible and a mutual or lateral distance as small as possible or gaps  104  as small as possible. Thereby the space charge zones originating from the border areas between the columns  102  and the surrounding material at the epitaxial layer are enabled to completely fill the columns  102  and the gaps  104  between the columns  102  as quickly as possible or at a drain voltage as low as possible when applying a drain voltage and thus when applying a voltage between the n-doped columns  102  and the p-doped material of the epitaxial layer  20  surrounding them in reverse direction. 
   The length of the columns  102  is preferably chosen so that they have a small vertical distance from the upper surface  14  of the base substrate  10 , which has approximately the same size as the distance between the columns  102  and the diameter of the columns  102 . When applying the above described minimum drain voltage, the epitaxial layer  20  is thus completely depleted below the most highly doped drain sub-areas  50 ,  52 . If the drain voltage is further increased starting from the minimum drain voltage, the depletion zone only grows minimally in vertical direction. Growth of the depletion zone dependent on the drain voltage is further strongly restricted if the base substrate  10  has a high doping concentration or at least a substantially higher doping concentration than the epitaxial layer  20 . In the embodiment shown in  FIG. 1  of the inventive field effect transistor, the capacity between the drain area  50 ,  52 ,  54  and the substrate  10  is thus approximately the capacity of a corresponding capacitor with a plate distance that is largely constant independently of the drain voltage and corresponds to the thickness of the epitaxial layer  20  minus the thickness or the vertical dimension of the drain area  50 ,  52 ,  54 . The capacity between the drain area  50 ,  52 ,  54  and the substrate  10  is thus small and approximately constant. 
   The present invention thus causes leveling of the output capacity in the area of the restricted layer and in particular in the area of the restricted layer forming between drain and substrate. 
   According to a variant of the first embodiment of the present invention, instead of the columns  102 , lamellae or plates are disposed below the drain area  50 ,  52 ,  54 , which border it and extend approximately to the upper surface  14  of the base substrate  10  in vertical direction.  FIG. 1  may also be interpreted so that the visible structures  102  are cross-sectional areas of these lamellae or plates. Instead of several lamellae or plates, alternatively only one lamella is provided that laterally has the form of a spiral. 
     FIG. 2  is a schematic illustration of a vertical section through a field effect transistor according to a second embodiment of the present invention. The second embodiment differs from the first embodiment illustrated on the basis of  FIG. 1  in that, instead of the vertical columns or lamellae or plates  102 , n-doped columns or rods that are horizontal or arranged in parallel to the surface  32  of the epitaxial layer  20 , or plates or lamellae  106  are provided that are connected to a drain area  50 ,  52 ,  54  in a geometrical and electrically conductive manner via a further n-doped, but vertically-aligned, rod, column, plate, or lamella-shaped connection area  108 . The rods or plates  106  of the second embodiment as well as gaps  110  therebetween are preferably similarly or equally dimensioned as the columns or lamellae  102  of the first embodiment and have the same function. 
   The embodiments from  FIGS. 1 and 2  have in common that the area  102 ,  106 ,  108  formed from the columns, rods, lamellae or plates has a comb-shaped cross section at least along one sectional plane. With the vertical orientation of the columns or lamellae  102 , as the first embodiment illustrated on the basis of  FIG. 1  comprises them, a plurality or a multiplicity of columns or lamellae  102  or a single laterally spiral-shaped lamella  102  is preferably provided, so that the created depletion zone has a lateral expansion as great as possible that preferably corresponds approximately to the lateral expansion of at least the more highly doped drain sub-areas  50 ,  52 . In the case of the horizontally-aligned structures of the second embodiment illustrated on the basis of  FIG. 2 , a plate  106  with corresponding lateral expansion is sufficient to realize the above-described advantages of the present invention. A plurality of parallel plates  106 , however, is advantageous, since it causes a correspondingly thicker depletion zone. A single plate  106  that is horizontal or is parallel to the surface  32  does not have a comb-shaped cross section. But the described embodiments and their variants have in common that they create an alternating arrangement of areas or alternating areas with opposing conductivity types. 
   A field effect transistor according to the present invention is preferably manufactured by a method whose procedural steps partly correspond to a conventional manufacturing method. In particular, at first the base substrate, for example a single-crystal silicon substrate, is created by for example a corresponding slice being cut from a drawn single-crystal of silicon and their surfaces being polished. The epitaxial layer  20  is grown onto the upper surface  14  of the base substrate  10 . The vertically orientated columns or lamellae  102  of the first embodiment are preferably created by holes or trenches being etched in the finished epitaxial layer  20 , which are filled with silicon whose doping has a conductivity type that is opposite to the conductivity type of the substrate  10  and in particular the epitaxial layer  20 . Alternatively, at first only a sub-layer of the epitaxial layer  20  is created, which includes the area of the future columns or lamellae  102 . After creating the columns or lamellae  102 , a further sub-layer of the epitaxial layer  20  is deposited, in which the drain area  50 ,  52 ,  54  will be disposed later. 
   Alternatively, the columns or lamellae  102  are created after creating the epitaxial layer  20  by implantation of dopant atoms through a corresponding mask. 
   Alternatively, the epitaxial layer  20  is created in several sub-layers in which sub-pieces of the columns or lamellae  102  are each created by implantation, wherein these sub-pieces are laterally aligned and together form the columns or lamellae  102 . 
   Horizontal structures, as they are present in the second embodiment illustrated on the basis of  FIG. 2 , are preferably created by the epitaxial layer  20  being deposited in several sub-layers, wherein the horizontal rods or beams or plates  106  are created by implantation of dopant atoms or by etching corresponding trenches or recesses and filling them with doped silicon. 
   The creation of the drain area  50 ,  52 ,  54 , the source area  40 , the enhance area  42 , the body area  44 , the p+-doped area  60 , and the sinker  62  preferably takes place, as well as the creation of the conductor structures  70 ,  72 ,  74 ,  80  and the gate  90 , in a similar manner as in conventional field effect transistors. 
   The present invention has been described for a LDMOS field effect transistor with n-doped source and drain areas  40 ,  50 ,  52 ,  54  and a p-doped body area  44  in a p-doped epitaxial layer  20  on a p-doped base substrate  10 . The present invention, however, may be realized for all kinds of field effect transistors, in particular lateral field effect transistors in all kinds of semiconductor substrates with and without epitaxial layer. 
   While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.