Patent Publication Number: US-2023155038-A1

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-185539, filed on Nov. 15, 2021; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments relate to a semiconductor device. 
     BACKGROUND 
     It is desirable for FRDs (Fast Recovery Diodes) used in power converters to be highly resistant to a recovery current that flows while transitioning from the on-state to the off-state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view showing a semiconductor device according to an embodiment; 
         FIG.  2    is a schematic plan view showing the semiconductor device according to the embodiment; 
         FIG.  3    is a graph showing a characteristic of the semiconductor device according to the embodiment; and 
         FIGS.  4 A and  4 B  are schematic plan views showing semiconductor devices according to modifications of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a semiconductor device includes a first insulating film, a semiconductor part, and a second insulating film. The semiconductor part is provided on the first insulating film. The semiconductor part includes a bottom surface and an upper surface. The bottom surface contacts the first insulating film. The upper surface is at a side opposite to the bottom surface. The second insulating film surrounds the semiconductor part. The second insulating film fills a trench provided around the semiconductor part. The trench extends from the upper surface of the semiconductor part toward the first insulating film and reaches the first insulating film. The semiconductor part includes first to fourth semiconductor layers and first to third contact regions. The first and fourth semiconductor layers are of a first conductivity type. The second and third semiconductor layers are of a second conductivity type. The first and second contact regions are of the second conductivity type. The third contact region is of the first conductivity type. The first semiconductor layer extends along the first insulating film and contacting the second insulating film. The second to fourth semiconductor layers are provided on the first semiconductor layer and arranged in a first direction along the upper surface of the semiconductor part. The fourth semiconductor layer is provided between the second semiconductor layer and the third semiconductor layer. The second and third semiconductor layers each contact the second insulating film. The first semiconductor layer extends between the second semiconductor layer and the fourth semiconductor layer and between the third semiconductor layer and the fourth semiconductor layer. The fourth semiconductor layer is apart from the second and third semiconductor layers with the first semiconductor layer interposed at the upper surface of the semiconductor part. The first contact region is provided on the second semiconductor layer. The second contact region is provided on the third semiconductor layer. The third contact region is provided on the fourth semiconductor layer. The first and second contact regions each include a second-conductivity-type impurity with a higher concentration than a concentration of a second-conductivity-type impurity in the second semiconductor layer and a concentration of a second-conductivity-type impurity in the third semiconductor layers. The fourth semiconductor layer includes a first-conductivity-type impurity with a higher concentration than a concentration of a first-conductivity-type impurity in the first semiconductor layer. The third contact region includes a first-conductivity-type impurity concentration with a higher concentration than the concentration of the first-conductivity-type impurity in the fourth semiconductor layer. The first to third contact regions extend in a second direction along the upper surface of the semiconductor part, the second direction crossing the first direction. The first to third contact regions are apart from the second insulating film. The first and second contact regions being provided with first distances in the second direction to the second insulating film. The first distances are less than a second distance in the second direction from the third contact region to the second insulating film. 
     Embodiments will now be described with reference to the drawings. The same portions inside the drawings are marked with the same numerals; a detailed description is omitted as appropriate; and the different portions are described. The drawings are schematic or conceptual; and the relationships between the thicknesses and widths of portions, the proportions of sizes between portions, etc., are not necessarily the same as the actual values thereof. The dimensions and/or the proportions may be illustrated differently between the drawings, even in the case where the same portion is illustrated. 
     There are cases where the dispositions of the components are described using the directions of XYZ axes shown in the drawings. The X-axis, the Y-axis, and the Z-axis are orthogonal to each other. Hereinbelow, the directions of the X-axis, the Y-axis, and the Z-axis are described as an X-direction, a Y-direction, and a Z-direction. Also, there are cases where the Z-direction is described as upward and the direction opposite to the Z-direction is described as downward. 
       FIG.  1    is a schematic cross-sectional view showing a semiconductor device  1  according to an embodiment. The semiconductor device  1  is, for example, a fast recovery diode (FRD). The semiconductor device  1  has, for example, an SOI (Silicon on Insulator) structure. 
     The semiconductor device  1  includes a semiconductor substrate  10 , a first insulating film  20 , a semiconductor part  30 , and a second insulating film  40 . The semiconductor substrate  10  is, for example, a silicon substrate. The first insulating film  20  is provided on the semiconductor substrate  10 . The first insulating film  20  is, for example, a silicon oxide film. The semiconductor part  30  is provided on the first insulating film  20 . The semiconductor part  30  is, for example, silicon. The semiconductor part  30  is, for example, a portion of a semiconductor layer bonded to the semiconductor substrate  10  via the first insulating film  20 . The semiconductor part  30  has a Z-direction thickness of, for example, 20 micrometers. The semiconductor part  30  includes a bottom surface  30 B contacting the first insulating film  20 , and an upper surface  30 T at the side opposite to the bottom surface  30 B. 
     The semiconductor layer on the first insulating film  20  includes a separation trench  40   g  that reaches the first insulating film  20  from the upper surface  30 T. The separation trench  40   g  is filled with the second insulating film  40 . The second insulating film  40  is, for example, a silicon oxide film. The separation trench  40   g  surrounds the semiconductor part  30 . The second insulating film  40  surrounds the semiconductor part  30  and electrically insulates the semiconductor part  30  from, for example, other portions of the semiconductor layer on the first insulating film  20 . 
     The semiconductor part  30  includes a first semiconductor layer  31  of a first conductivity type, a second semiconductor layer  33   a  of a second conductivity type, a third semiconductor layer  33   b  of the second conductivity type, a fourth semiconductor layer  35  of the first conductivity type, a first contact region  37   a  of the second conductivity type, a second contact region  37   b  of the second conductivity type, and a third contact region  39  of the first conductivity type. In the following description, the first conductivity type is an n-type, and the second conductivity type is a p-type. 
     The first semiconductor layer  31  is, for example, an n-type silicon layer. The first-conductivity-type impurity concentration of the first semiconductor layer  31  is, for example, not more than 1×10 16  cm −3 . The first semiconductor layer  31  extends in a plane along the first insulating film  20 . The first semiconductor layer  31  extends in an X-direction and a Y-direction, for example, and contacts the second insulating film  40 . 
     The second semiconductor layer  33   a , the third semiconductor layer  33   b , and the fourth semiconductor layer  35  are provided on the first semiconductor layer  31  and are arranged in a first direction, e.g., the X-direction. The fourth semiconductor layer  35  is provided between the second semiconductor layer  33   a  and the third semiconductor layer  33   b . The fourth semiconductor layer  35  is apart from the second and third semiconductor layers  33   a  and  33   b  with the first semiconductor layer  31  interposed. The first semiconductor layer  31  includes a portion that extends between the second semiconductor layer  33   a  and the fourth semiconductor layer  35  and between the third semiconductor layer  33   b  and the fourth semiconductor layer  35 . 
     The second semiconductor layer  33   a  and the third semiconductor layer  33   b  are, for example, p-type anode layers. The second semiconductor layer  33   a  and the third semiconductor layer  33   b  contact the second insulating film  40  in the X-direction. 
     The first semiconductor layer  31  extends between the first insulating film  20  and the second semiconductor layer  33   a  and between the first insulating film  20  and the third semiconductor layer  33   b . The first semiconductor layer  31  contacts the second insulating film  40  between the first insulating film  20  and the second semiconductor layer  33   a  and between the first insulating film  20  and the third semiconductor layer  33   b.    
     The first contact region  37   a  is provided on the second semiconductor layer  33   a . The first contact region  37   a  has a second-conductivity-type impurity with a higher concentration than a concentration of a second-conductivity-type impurity in the second semiconductor layer  33   a . The second-conductivity-type impurity concentration of the second semiconductor layer  33   a  is, for example, 5×10 16  to 5×10 17  cm −3 . The second-conductivity-type impurity concentration of the first contact region  37   a  is, for example, not less than 1×10 18  cm −3 . 
     The second contact region  37   b  is provided on the third semiconductor layer  33   b . The second contact region  37   b  has a second-conductivity-type impurity with a higher concentration than a concentration of a second-conductivity-type impurity in the third semiconductor layer  33   b . The second-conductivity-type impurity concentration of the third semiconductor layer  33   b  is, for example, 5×10 16  to 5×10 17  cm −3 . The second-conductivity-type impurity concentration of the second contact region  37   b  is, for example, not less than 1×10 18  cm −3 . 
     The third contact region  39  is provided on the fourth semiconductor layer  35 . The fourth semiconductor layer  35  is, for example, an n-type cathode layer. The third contact region  39  has a first-conductivity-type impurity with a higher concentration than a concentration of a first-conductivity-type impurity in the fourth semiconductor layer  35 . The first-conductivity-type impurity concentration of the fourth semiconductor layer  35  is, for example, 1×10 17  to 1×10 18  cm −3 . The first-conductivity-type impurity concentration of the third contact region  39  is, for example, not less than 1×10 18  cm −3 . As shown in  FIG.  1   , the semiconductor device  1  further includes a third insulating film  50 , a fourth insulating film  60 , a first electrode  70   a , a second electrode  70   b , a third electrode  80 , and a fourth electrode  90 . 
     The third insulating film  50  covers the upper surface  30 T of the semiconductor part  30 . The third insulating film  50  is, for example, an inter-layer insulating film. The third insulating film  50  is, for example, a silicon oxide film. 
     The fourth insulating film  60  is partially provided between the semiconductor part  30  and the third insulating film  50 . The fourth insulating film  60  is, for example, a STI (Shallow Trench Isolation). The fourth insulating film  60  is, for example, a silicon oxide film formed by thermal oxidation of the semiconductor part  30 . The fourth insulating film  60  surrounds the fourth semiconductor layer  35  on the first semiconductor layer  31 . 
     The first electrode  70   a , the second electrode  70   b , and the third electrode  80  are provided on the third insulating film  50 . The first electrode  70   a  and the second electrode  70   b  are, for example, anode electrodes. The third electrode  80  is, for example, a cathode electrode. The first electrode  70   a , the second electrode  70   b , and the third electrode  80  are, for example, metal films that include tungsten (W). 
     The first electrode  70   a  is electrically connected to the first contact region  37   a  through a contact hole  51  provided in the third insulating film  50 . The second electrode  70   b  is electrically connected to the second contact region  37   b  through a contact hole  53  provided in the third insulating film  50 . The third electrode  80  is electrically connected to the third contact region  39  through a contact hole  55  provided in the third insulating film  50 . 
     The fourth electrode  90  is provided between the third insulating film  50  and the fourth insulating film  60 . The fourth electrode  90  is, for example, a field plate. The fourth electrode  90  extends along, for example, the outer edge of the fourth semiconductor layer  35  and surrounds the fourth semiconductor layer  35  in a plan view parallel to the upper surface  30 T of the semiconductor part  30  (see  FIG.  2   ). The fourth electrode  90  is electrically connected to, for example, the third electrode  80  and relaxes the electric field concentration at the boundary between the first semiconductor layer  31  and the fourth semiconductor layer  35 . The fourth electrode  90  is, for example, conductive polysilicon. 
       FIG.  2    is a schematic plan view showing the semiconductor device  1  according to the embodiment.  FIG.  2    is a plan view showing the layout of the upper surface  30 T of the semiconductor part  30 . 
     The second insulating film  40  surrounds the semiconductor part  30  and fills the separation trench  40   g . The second semiconductor layer  33   a  and the third semiconductor layer  33   b  extend in a second direction, e.g., the Y-direction, and contact the second insulating film  40 . The fourth semiconductor layer  35  extends in the Y-direction and is apart from the second insulating film  40 . In  FIG.  2   , the fourth electrode  90  is shown by a dot line. As seen in  FIG.  2   , the fourth electrode  90 , for example, surrounds the fourth semiconductor layer  35  in a plan view. 
     The second semiconductor layer  33   a  contacts the second insulating film  40  between the first semiconductor layer  31  and the second insulating film  40 . The third semiconductor layer  33   b  also contacts the second insulating film  40  between the first semiconductor layer  31  and the second insulating film  40 . 
     The first contact region  37   a , the second contact region  37   b , and the third contact region  39  each are apart from the second insulating film  40 . The first contact region  37   a  extends in the Y-direction on the second semiconductor layer  33   a . The second contact region  37   b  extends in the Y-direction on the third semiconductor layer  33   b . The third contact region  39  extends in the Y-direction on the fourth semiconductor layer  35 . The first contact region  37   a  is provided inward of the outer edge of the second semiconductor layer  33   a  at the upper surface  30 T of the semiconductor part  30 . Also, the second contact region  37   b  is provided inward of the outer edge of the third semiconductor layer  33   b . The third contact region  39  is provided inward of the outer edge of the fourth semiconductor layer  35 . 
     In the semiconductor device  1 , in the on-state in which a forward voltage is applied between the first electrode  70   a  and the third electrode  80  and between the second electrode  70   b  and the third electrode  80  (see  FIG.  1   ), second-conductivity-type carriers referred to as holes herein are injected from the second and third semiconductor layers  33   a  and  33   b  into the first semiconductor layer  31 . Also, first-conductivity-type carriers referred to as electrons herein are injected from the fourth semiconductor layer  35  into the first semiconductor layer  31 . Thereby, a forward current flows toward the third electrode  80  from the first and second electrodes  70   a  and  70   b.    
     In the recovery process of transitioning to the off-state by switching the forward voltages between the first electrode  70   a  and the third electrode  80  and between the second electrode  70   b  and the third electrode  80  to the reverse voltage, the holes inside the first semiconductor layer  31  are discharged to the first electrode  70   a  via the second semiconductor layer  33   a  and the first contact region  37   a  and discharged to the second electrode  70   b  via the third semiconductor layer  33   b  and the second contact region  37   b . The electrons inside the first semiconductor layer  31  are discharged to the third electrode  80  via the fourth semiconductor layer  35  and the third contact region  39 . The first semiconductor layer  31  is depleted thereby, and the semiconductor device  1  transitions to the off-state. 
     In the recovery process, so-called recovery currents are caused by the discharge of the holes and electrons from the first semiconductor layer  31 . The recovery currents flow between the first electrode  70   a  and the third electrode  80  and between the second electrode  70   b  and the third electrode  80 . Therefore, switching loss is caused by the recovery current flowing in the recovery period from when the forward voltage is switched to the reverse voltage until the first semiconductor layer  31  is depleted. The switching loss increases as the recovery period increases. 
     In the semiconductor device  1 , the second semiconductor layer  33   a  and the third semiconductor layer  33   b  contact the second insulating film  40  so that the first semiconductor layer  31  is not interposed between the second semiconductor layer  33   a  and the second insulating film  40  and between the third semiconductor layer  33   b  and the second insulating film  40 . In other words, the side surfaces of the second and third semiconductor layers  33   a  and  33   b  other than the portions facing the fourth semiconductor layer  35  contact the second insulating film  40 . Thereby, the spatial distributions of the holes and electrons inside the first semiconductor layer  31  are limited to the regions between the second semiconductor layer  33   a  and the fourth semiconductor layer  35 , between the third semiconductor layer  33   b  and the fourth semiconductor layer  35 , between the second semiconductor layer  33   a  and the first insulating film  20 , and between the third semiconductor layer  33   b  and the first insulating film  20 . The discharge times of the holes and electrons inside the first semiconductor layer  31  can be reduced thereby, and the switching loss can be reduced. 
     In the semiconductor device  1 , a first distance “a” in the Y-direction to the second insulating film  40  from each of the first and second contact regions  37   a  and  37   b  is less than a second distance “b” in the Y-direction from the third contact region  39  to the second insulating film  40 . The current concentration at each end portion  37   e  in the Y-direction of the first and second contact regions  37   a  and  37   b  can be relaxed thereby. 
       FIG.  3    is a graph showing a characteristic of the semiconductor device  1  according to the embodiment. The horizontal axis is a ratio b/a of the second distance “b” to the first distance “a”. The vertical axis is the temperature of the first contact region  37   a . In  FIG.  3   , the temperatures of a central part  37   c  and the end portion  37   e  in the Y-direction of the first contact region  37   a  in the recovery period are shown versus b/a. 
     As shown in  FIG.  3   , when b/a is less than 1, the Y-direction lengths of the first and second contact regions  37   a  and  37   b  are less than the Y-direction length of the third contact region  39 ; therefore, the current densities in the first and second contact regions  37   a  and  37   b  increase, and the temperatures at the central part  37   c  and the end portion  37   e  rise. As b/a decreases, the current densities in the first and second contact regions  37   a  and  37   b  increase, and the temperature rise becomes pronounced. 
     On the other hand, when b/a is greater than 1, the Y-direction lengths of the first and second contact regions  37   a  and  37   b  are greater than the Y-direction length of the third contact region  39 ; the current densities in the first and second contact regions  37   a  and  37   b  are reduced; and the temperature rise at the central part  37   c  and the end portion  37   e  is suppressed. As b/a increases, the temperature of the end portion  37   e  drops below the temperature of the central part  37   c . In other words, the current distributions in the first and second contact regions  37   a  and  37   b  are dispersed, and the current density at the end portion  37   e  drops below the current density at the central part  37   c  that is proximate to the third contact region  39 . 
     Because the semiconductor device  1  has an SOI structure, the Joule heat dissipation via the semiconductor substrate  10  is suppressed. Therefore, in the semiconductor device  1 , a local temperature rise occurs easily due to concentration of the recovery current; and the immunity to current breakdown is reduced. For example, when b/a is less than 1, the current may concentrate at the end portion  37   e  in each of the first and second contact regions  37   a  and  37   b , resulting in the current breakdown. Accordingly, it is desirable to increase the breakdown immunity by dispersing the recovery current. 
     In the semiconductor device  1 , the immunity to current breakdown is improved by setting b/a to be greater than 1. According to  FIG.  3   , b/a is preferably not less than 1.2, and more preferably not less than 1.5. Thus, the breakdown immunity of the semiconductor device  1  can be increased by relaxing the current concentration due to the planar arrangement of the first contact region  37   a , the second contact region  37   b , and the third contact region  39 . 
       FIGS.  4 A and  4 B  are schematic plan views showing semiconductor devices  2  and  3  according to modifications of the embodiment.  FIGS.  4 A and  4 B  are plan views showing the layout in the upper surface  30 T of the semiconductor part  30 . 
     In the semiconductor device  2  shown in  FIG.  4 A , the second semiconductor layer  33   a  and the third semiconductor layer  33   b  respectively include extension parts  33   ae  and  33   be  that extend along the second insulating film  40 . The extension parts  33   ae  and  33   be  contact the second insulating film  40 . The Y-direction widths of the extension parts  33   ae  and  33   be  each are less than the second distance “b” from the third contact region  39  to the second insulating film  40 . 
     The extension part  33   ae  extends in the direction along the second insulating film  40  toward the third semiconductor layer  33   b . The extension part  33   be  extends in the direction along the second insulating film  40  toward the second semiconductor layer  33   a . The extension part  33   ae  faces, for example, the extension part  33   be  via the first semiconductor layer  31  in the X-direction. 
     The extension part  33   ae  extends toward the third semiconductor layer  33   b  from each of the two ends arranged in the Y-direction of the second semiconductor layer  33   a . The extension part  33   be  extends toward the second semiconductor layer  33   a  from each of the two ends arranged in the Y-direction of the third semiconductor layer  33   b.    
     By providing the extension parts  33   ae  and  33   be , the holes inside the first semiconductor layer  31  can be efficiently discharged to the first and second electrodes  70   a  and  70   b  (see  FIG.  1   ). The switching loss can be further reduced thereby. 
     In the semiconductor device  3  shown in  FIG.  4 B , the semiconductor part  30  further includes a fifth semiconductor layer  33   c  and a sixth semiconductor layer  33   d . The fifth semiconductor layer  33   c  and the sixth semiconductor layer  33   d  each contact the second insulating film  40  and link the second semiconductor layer  33   a  and the third semiconductor layer  33   b.    
     The fourth semiconductor layer  35  is positioned between the fifth semiconductor layer  33   c  and the sixth semiconductor layer  33   d.    
     The Y-direction widths of the fifth and sixth semiconductor layers  33   c  and  33   d  are less than the second distance “b” from the third contact region  39  to the second insulating film  40 . The fifth semiconductor layer  33   c  and the sixth semiconductor layer  33   d  are apart from the fourth semiconductor layer  35  with the first semiconductor layer  31  interposed. 
     Thus, the second semiconductor layer  33   a  and the third semiconductor layer  33   b  are formed to have a continuous body via the fifth and sixth semiconductor layers  33   c  and  33   d  that surrounds the fourth semiconductor layer  35 . The discharge of the holes of the first semiconductor layer  31  can be promoted thereby, and the switching loss can be reduced. Also, the current concentration at an end portion  37   ae  of the first contact region  37   a  and an end portion  37   be  of the second contact region  37   b  can be relaxed, and the immunity to current breakdown can be improved. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.