Abstract:
The invention relates to a vertical field-effect transistor in source-down structure, in which the active zones ( 10, 7, 11 ) are introduced from trenches ( 5, 8, 9 ) into a semiconductor body ( 1 ), a source electrode ( 18 ) being connected via the filling ( 6 ) of a body trench ( 5 ) to a highly doped substrate ( 2 ) via a conductive connection ( 15 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to a vertical field-effect transistor (FET) in source-down structure. The term “source-down structure” is to be understood as meaning that in the vertical FET, drain and gate terminals are located on one side (top side) of a semiconductor body, while the source terminal is located on the other side (underside) of the semiconductor body. 
   BACKGROUND 
   A high packing density has long been the aim in very general terms in semiconductor technology and therefore also for vertical FETs. Furthermore, there are applications in which it is desirable for the source terminal of an FET to be located on the underside of a semiconductor body. This applies, for example, to semiconductor components, the source terminal of which is at reference potential and which require good cooling. One possible application area is FETs for the automotive industry. 
   SUMMARY 
   It is an object of the present invention to provide a vertical FET with a high packing density in a source-down structure. 
   These objects are achieved by embodiments of the invention. 
   The vertical field-effect transistor in source-down structure according to the invention therefore comprises in particular: 
   a semiconductor body comprising a semiconductor substrate of a first conduction type, on which a semiconductor layer of the other conduction type, opposite to the first conduction type, is provided, which semiconductor layer for its part forms a main surface of the semiconductor body. The first conduction type is in this context preferably the p-conduction type. 
   At least one first trench extends from one main surface to the semiconductor substrate. This at least one first trench is at least partially surrounded by a zone of the first conduction type and is filled with polycrystalline semiconductor material which is doped with a dopant of the first conduction type. It is preferable for the zone of the first conduction type to completely surround the at least one trench, and in particular polycrystalline silicon is selected for the polycrystalline semiconductor material. 
   Furthermore, at least one second trench and at least one third trench extend from one main surface into the semiconductor layer without reaching the semiconductor substrate. These second and third trenches, in a first direction on the main surface, in each case lie on one side or the other of the first trench and are each at least partially surrounded by a zone of the other conduction type. Like the first trench, the second and third trenches are also filled with polycrystalline semiconductor material, in particular polycrystalline silicon, which is doped with a dopant of the other conduction type. It is preferable for the second and third trenches also each to be completely surrounded by a zone of the other conduction type. 
   A fourth trench likewise extends from one main surface into the semiconductor layer and is lined with an insulating layer on its walls and base. This fourth trench is filled with conductive material. On the main surface, in the first direction it is located at a distance from the first trench and in a second direction, substantially perpendicular to the first direction, it is located at a distance from the second and third trenches. The at least one fourth trench may but does not have to extend as far as the semiconductor substrate. Doped polycrystalline semiconductor material, in particular polycrystalline silicon, or a metal can be selected for the conductive material. 
   On one main surface, there is preferably a conductive connection, for which, for example, doped polycrystalline semiconductor material, in particular silicon, can likewise be used, between the polycrystalline semiconductor material of the second trench and the polycrystalline semiconductor material of the first trench. 
   Finally, there is also a source electrode, which is electrically connected to the semiconductor substrate, i.e. the underside of the semiconductor body, a drain electrode, which is electrically connected to the polycrystalline semiconductor material of the fourth trench on the one main surface, and a gate electrode, which is electrically connected to the conductive material of the at least one fourth trench, likewise on the one main surface. 
   The vertical FET in source-down structure according to the invention is distinguished by a very high packing density, since all the zones are contact-connected via trenches. Moreover, in this FET, on account of the conductive connection between the polycrystalline semiconductor material of the second trench and the polycrystalline semiconductor material of the first trench, the source terminal lies on the underside of the semiconductor body, which is on the opposite side from the one main surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The text which follows provides a more detailed explanation of an exemplary embodiment of the invention with reference to the drawings, in which: 
       FIG. 1  shows a perspective illustration of the vertical FET according to the invention, 
       FIG. 2  shows a section II-II through the FET shown in  FIG. 1 , and 
       FIG. 3  shows an equivalent circuit diagram for the FET according to the invention. 
   

   DETAILED DESCRIPTION 
     FIGS. 1 and 2  show a semiconductor body  1  comprising a p + -conducting semiconductor substrate  2  and an n-conducting semiconductor layer  3  which has been applied to it by epitaxy. The semiconductor substrate  2  and the semiconductor layer  3  preferably consist of silicon. However, it is also possible to select other semiconductor materials, such as for example SiC, compound semiconductors A III B v , etc. It is also possible for the conduction types indicated to be reversed, i.e. the p-type conductivity may be replaced by the n-type conductivity if the p-type conductivity is selected instead of the n-type conductivity. 
   First trenches  5 , which extend as far as the semiconductor substrate  2 , are filled with p + -doped polycrystalline silicon  6  and have been formed in particular by etching, are provided in the semiconductor layer  3  leading from the main surface  4  thereof on the opposite side from the semiconductor substrate. p-Type dopant, for example boron, has been introduced from this polycrystalline silicon  6  into the region of the semiconductor layer  3  surrounding the trenches  5 , so as to form a continuous p-doped zone  7  which preferably completely surrounds the trenches  5 . The zone  7  has therefore been produced by outdiffusion from the p + -doped polycrystalline silicon  6  from trenches  5 . The trenches  5  number at least one. If appropriate, it is, however, also possible to provide a plurality of trenches, as shown in the exemplary embodiment illustrated in  FIGS. 1 and 2 . 
   Furthermore, at least one second trench  8  and at least one third trench  9  are provided, likewise extending from the first main surface  4  into the semiconductor layer  3 , but without reaching the semiconductor substrate  2 . The trenches  8  and  9  have likewise been produced by etching, in each case run in a first direction (sectional plane shown in  FIG. 2 ) and lie on one side or the other side, respectively, of the direction of extent of the first trenches  5 . Like the first trenches  5 , the trenches  8  and  9  are filled with polycrystalline semiconductor material, in particular polycrystalline silicon  18  or  19 , which in this case, however, is n + -conducting in the trenches  8  and  9 . n-Conducting dopant, for example phosphorus or arsenic, has diffused out of the trenches  8  and  9  into the region of the semiconductor layer  3  surrounding the trenches  8  and  9 , so as to in each case form n + -conducting zones  10  and  11 . 
   Finally, fourth trenches  12  have likewise been introduced by etching from the main surface  4  into the semiconductor layer  3 ; these trenches may extend all the way to the semiconductor substrate  2 , but may also end in the semiconductor layer  3  before they reach the semiconductor substrate  2 . These fourth trenches  12  are lined with an insulating layer  13  of, for example, silicon dioxide and/or silicon nitride on their walls and their base and are otherwise filled with conductive material  14 , such as in particular doped polycrystalline silicon and/or metal. 
   The polycrystalline semiconductor material  6  of the trench  5  is connected to the n + -doped polycrystalline semiconductor material  18  in the trench  8  via a conductive connection  15 , for example likewise comprising doped polycrystalline silicon. This conductive connection  15  is preferably formed on the main surface  4  before a drain metallization layer (not shown) is applied over the whole of this surface. Of course, this conductive connection  15  is electrically isolated from this drain metallization by an insulation layer, for example of silicon dioxide. 
   Therefore, the trench  9  with the polycrystalline semiconductor material  19  which has been introduced into it forms a drain trench, while the trench  8  with the doped polycrystalline semiconductor material  18  which has been introduced into it forms a source trench. As a result of the conductive connection  15 , the polycrystalline semiconductor material  18  of the source trench is directly electrically connected, via the polycrystalline semiconductor material  6  of the trench  5 , to the semiconductor substrate  2 , resulting in a source-down structure. 
   Whereas the trenches  5 ,  8  and  9  extend in the first direction, the trenches  12  which form gate trenches extend in a second direction, which is perpendicular to the first direction. 
   The semiconductor substrate  2  is connected to a source electrode S, which is preferably earthed (cf.  FIG. 3 ), while the polycrystalline semiconductor material  19  of the drain trench  9  is connected to a drain electrode D. Furthermore, the conductive material  14  of the trenches  12  is connected to a gate electrode G. 
   The vertical FET in source-down structure according to the invention is distinguished by a very high packing density, since all the zones have been introduced via trenches.