A SEMICONDUCTOR ELEMENT

Abstract: A semiconductor element comprises an insulating outer layer (1) that includes electric contact connections of a first conductive type. These connections are connected to contact areas (5, 6) located beneath the insulating surface layer (1), of which connections at least one is of a first conductive type. At least one of the contact areas (5, 6) and a further area (9, 10) comprised of two layers of mutually different conductive types disposed between the contact areas (5, 6) are surrounded by a layer (8) of a second conductive type of material. This second layer (8) is, in turn, covered with an insulating layer (11) on at least that side which lies distal from the surface layer.

Description: A SEMICONDUCTOR ELEMENT The present invention relates to a semiconductor element and then particularly to an improved semiconductor element espe- 5 cially for use in a transistor. U.S. Patent Specification 5,146,298, for example, teaches a semiconductor element, particularly a high-voltage OS- transistor, that includes a substrate of a first conductive w type on which there is mounted a layer of material of a second conductive type that includes cells which are comprised of material of a first conductive type in which the electrical contact areas, source area and gate area, are placed and also an electric contact area of a second conductive type, is drain area. According to this prior publication, the first type of conductive material is normally p-type silicon doped with about 5xl0 14 atoms per cm 3 , whereas the second conductive material is n-type silicon doped with about 3xl0 12 atoms per cm 2 . A further area formed by two layers of the first and the 0 second conductive types of material respectively is disposed between drain area and the other two contact areas. The substrate has a typical thickness of from 100 to 500 μm. An effective breakdown voltage of about 300 V is obtained with these dimensions and doping levels. The semiconductor element 5 also includes an insulating outer layer through which the electric contacts extend. The object of the present invention is to provide an improved semiconductor element of this type, in which the dimensions o of said semiconductor element can be reduced substantially without detracting from its performance. The object of the invention is achieved with a semiconductor element in which the layer of said second conductive material is covered with an insulating layer on its side that lies distal from the surface layer. By providing an insulating layer on the opposite side of the second conductive type of material in relation to the insulating surface layer, the layer of second conductive type material can be made thinner, in the order of magnitude of 1 μm or less, as distinct to present-day thicknesses of 4-5 μm. This enables the charge carrier concentration, doping, to be increased from about 10 15 to about 10A The breakdown voltage is determined by the area that has the lowest critical field. In the case of the prior publication, the p " area doped with 5xl0 14 atoms per cm 2 has a critical field of about 20V/μm, which is the determinant field. When the p " area is replaced with an insulating layer in accordance with the invention, the breakdown voltage will, m- stead, be determined by the doping and the critical field in the layer comprising said second type of conductive material surrounded by the insulating layer. According to the aforesaid prior patent publication, doping is about 5X10 1 " and the critical field is 30V/μm. The aforesaid significant improvement is achieved when this layer is made thinner, now about 0.3 μm, while retaining a total charge of 3xl O l -/ c . Doping is thus increased to 10 17 and the critical field to 80V/μm. The transistor breakdown voltage is herewith increased, determined by the critical field, from « 20V/μm to « 80V/μm, i.e. an increase by a factor of 4. The distance between source area and drain area L D , the operative distance, for a given voltage can be reduced to 1/4 at the same time as the resistance of the active transistor, R on , decreases to 1/4. The quality factor, R on * A (the surface) , will thereby be improved by a factor of 16. In another embodiment of the invention, there is disposed between the insulating layer and the layer of said second conductive type of material a thin layer of a first conductive type of material, therewith enabling doping in the layer of said second conductive type of material to be further increased by a factor of 2. According to further embodiments of the invention, an IGBT- transistor (Insulated Gate Polar Transistor) and also a bipolar transistor can be constructed in a corresponding fashion. The invention will now be described with reference to two non-limiting embodiments and with reference to the accompanying drawings, in which Fig. 1 is a schematic side sectioned view of a MOS-transistor in accordance with the invention; Fig. 2 is a corresponding view of another embodiment of the transistor shown in Fig. 1; Fig. 3 is a schematic side sectioned view of an IGBT-transistor in accordance with the invention; and Fig. 4 is a corresponding view of another embodiment of the transistor shown in Fig. 3. Fig. 1 is a schematic sectioned side view of an inventive semiconductor element, a transistor. The transistor includes an insulating outer layer 1 which includes electric contact connectors. These connectors may be connectors for the source 2, gate 3 and drain 4 of the transistor. The electric contact connectors are connected to the contact areas 5, 6 located beneath the insulating surface layer 1. The contact area 5 includes connections to the source connectors and a part 1 enclosed therein, this part normally being referred to as the source area, whereas the contact area 6, referred to as the drain area, includes connections to the drain connectors. The contact area 5 is comprised of a material of a first conductive type, normally a p-type material, while the part 7 enclosed therein is comprised of a second conductive type of material, normally an n-type material. The contact area 6 is also comprised of a second conductive type of material, nor- mally an n-type material. The contact areas 5, 6 are surrounded by a layer 8 comprised of the second conductive type of material, normally an n-type material. The material is usually in the form of a thin sili- con plate, Si. This layer or plate has a thickness in the order of 1 μm or less, and has a doping ratio of 10 16 to 10 11 atoms per cm 2 . A further area comprised of two layers 9, 10 of said first and said second conductive types of material respectively are disposed between the contact areas 5 and 6. The layer 8 is surrounded by an insulating layer 11 on its side distal from the outer layer 1. The insulating layer 11 may suitably comprise silicon dioxide, SiO-. The insulating layer 11 and therewith the remaining parts of the inventive semiconductor element may be supported by a carrier, a substrate 12, e.g. a silicon substrate Si. Shown in the drawings is a symmetry line 13 on whose other side a second half of a transistor is located as a mirror image of the transistor shown on the left of the symmetry line as seen in the drawings. It will be understood that the contact area 6 including the transistor drain connector 4 is common to the two transistors of the semiconductor element. As before mentioned, the use of an insulator 11 surrounding the doped layer 8 comprised of a material of the second conductive type enables the doped layer to be made thinner and therewith enables the distance between source area 5 and drain area 6, the operative distance L D , to be reduced to 1/4. This enables the dimensions of the semiconductor element as a whole to be reduced in the order of 1:16. In addition to space benefits, these reductions in dimensions also benefit manufacturing costs, which are essentially directly proportional to size. The circuit illustrated in Fig. 1 can also function as a bipolar transistor, with the area 7 functioning as an emitter, the area 5 functioning as a base, and the layer 8, the area 10 and the area 6 functioning as an extended collector. Fig. 2 illustrates another embodiment of an inventive semiconductor element, a transistor, in a view corresponding to the Fig. 1 illustration. This embodiment differs from the Fig. 1 embodiment solely by virtue of a further thin layer of material 14 disposed between the doped layer 8 of the second conductive type of material and the isolator, the insulating layer 11. This further thin layer 14 is comprised of the first type of conductive material, normally a p-type material, and, similar to the doped layer 8 of said second type of conductive material, is normally in the form of a thin silicon plate, Si. The inclusion of the further thin layer 14 enables doping of the doped layer 8 of said second type of conductive material to be increased by a factor of 2. This enables the dimensions of the semiconductor element, and therewith the costs thereof, to be further reduced. As with the first embodiment, the insulating layer 11 and therewith the remaining parts of the inventive semiconductive element may be supported by a carrier, a substrate 12, for instance a silicon (Si) substrate. Fig. 3 illustrates another type of semiconductor element, in this case an IGBT-transistor, which has generally the same construction as the aforedescribed MOS-transistor illustrated in Fig. 1, although with the difference that the contact area 6 has been replaced by a contact area 15 comprised of a first type of conductive material, normally a p-type. Fig. 4 illustrates a variant of an IGBT-transistor according to the invention that has the same construction as the MOS- transistor described above and illustrated in Fig. 1, al- though with the difference that the contact area 6 has been supplemented with a further area 16 that while being similar to the area 6 is comprised of a first type of conductive material, normally a p-type material. This further area 16 is joined by metal to the contact area 6, in the same way as the areas 5 and 7 are interconnected. Similar to the embodiment shown in Fig. 2, the semiconductor elements according to Figs. 3 and 4 may be provided with a further thin layer of material disposed between the doped layer 8 of said second conductive type of material and the isolator, the insulating layer 11. Although the contact area 5 of the illustrated embodiments is fully surrounded by the layer 8, it will be understood that the contact area 5 may instead extend beyond the layer 8. However, the layer 8 must surround respective contact areas 6; 15; 6, 16, and the further contact area 9, 10. Neither is it necessary for the insulating layer 11 to surround the layer 8, since it is sufficient for the insulating layer 11 to cover that side of the layer 8 which is distal from the surface layer 1. It will be understood that the first and the second types of conductive materials in the inventive semiconductor element may exchange places in relation to what has been described above .

Claims: CLAIMS 1. A semiconductor element comprising an insulating surface layer (1) in which electric contact connections are 5 provided, said connections being connected to contact areas (5, 6) situated beneath the insulating surface layer (1), of which contact areas at least one is a first conductive type, wherein at least one of the contact areas (5, 6) and a further area (9, 10) formed by two layers of mutually different o conductive types disposed between the contact areas (5, 6) are surrounded by a layer (8) of a second conductive type of material, characterised in that the second layer (8) is covered with an insulating layer (11) at least on that side of said layer which lies distal from the surface layer. 5 2. A semiconductor element according to Claim 1, characterised in that the insulating layer (11) is comprised of silicon dioxide. 3. A semiconductor element according to Claim 1 or 2, characterised by a further, thin layer of material (14) between the layer (8) of said second conductive type of material and said insulating layer (11) . 4. A semiconductor element according to Claim 3, characterised in that the thin further layer (14) is comprised of silicon and is of the first conductive type. 5. A semiconductor element according to any one of the preceding Claims, characterised in that the insulating layer (11), and therewith the remaining parts of the semiconductor element, is supported by a carrier, a substrate (12) .

Inventors: EKLUND KLAS-HAAKAN, EKLUND, KLAS-HAAKAN
Applicants: EKLUND KLAS HAAKAN, EKLUND, KLAS-HAAKAN
Application year: 2000
Main classification: H01L29
IPC codes: H01L21, H01L27, H01L29