High-breakdown-voltage semiconductor element

Abstract: In a high-breakdown-voltage diode, a high-concentration p-type layer (2) is selectively formed in an n-type silicon layer (1), and a high-concentration n-type layer (3) is formed in the same separate from the layer (2) by a predetermined distance. An insulation film (8) having a dielectric constant larger than silicon is formed on that portion of the n-type silicon layer (1) which extends between the layers (2, 3), for relaxing concentration of an electric field caused in the surface of the substrate.

Description: This invention relates to a high-breakdown-voltage semiconductor element. A technique of providing a guard ring around an end pn-junction formed in an element region to be supplied with a high voltage is known as a method of increasing the breakdown voltage of a semiconductor element. The guard ring reduces the degree of electric field concentration at a tip portion of a depletion layer extending from the pn-junction, thereby increasing the breakdown voltage of the element. The effect of reducing the degree of the electric field concentration is obtained by optimally designing the depth of diffusion of the guard ring layer, the distance between the guard ring layer and a terminal region, and the distance between guard ring layers (if a plurality of guard ring layers are employed). Fig. 7 shows an end portion of a high-breakdown-voltage diode having such a guard ring structure. A high-concentration p-type layer 2, which is to be an anode region, is formed in the upper surface of a high-resistance n-type silicon layer 1 (substrate). Further, a high-concentration n-type layer 3 is formed in the upper surface of the layer 1 separated from the layer 3 by a predetermined distance, for stopping the extension of a depletion layer. The layer 3 is provided with an electrode 5 for supplying a voltage identical to that of a cathode. High-concentration, p-type layers 21 and 22 serving as guard rings are formed in the surface between the layers 2 and 3. An electrode 7 is formed on the lower surface of the layer 1 with a high-concentration n-type layer 6 interposed therebetween. In such a structure, it is difficult to determine the distance between a guard ring layer and a terminal region, and the distance between the guard ring layers in the case of employing a plurality of guard ring layers. As described above, the conventional high-breakdown voltage semiconductor element of a guard ring structure is hard to design. This invention has been made in consideration of the above circumstances, and aims to provide a high-breakdown-voltage semiconductor element easy to design and having superior high-breakdown-voltage characteristics. The present invention provides, a high-breakdown-voltage semiconductor device comprising: a semiconductor active layer having main and back surfaces one on either side; an insulation support layer arranged on the back surface of the semiconductor active layer; a semiconductor support layer arranged on the insulation support layer such that the insulation support layer is interposed between the semiconductor active and semiconductor support layers; a first conductivity-type region and a second conductivity-type region which are formed in the semiconductor active layer along the main surface; and first and second electrodes provided to the semiconductor layer such that they are located one on either side of a junction between the first and second conductivity-type regions, wherein, a first insulation film is interposed between the semiconductor active and insulation support layers, and arranged along the back surface of the semiconductor active layer and between the first and second electrodes, the first insulation film being made of a material having a dielectric constant larger than a material constituting the semiconductor active layer. As will be hereinafter explained in detail, the invention utilizes the characteristic that the state of an electric field caused in the insulation film depends upon conditions given thereto without being influenced by the ambient state, when the dielectric constant of the insulation film is sufficiently large. That is, where an insulation film having a dielectric constant larger than the substrate is provided on the outside of a pn-junction, such as an end one, in an element having a structure as in the invention, a uniform electric field is formed in the insulation film, so that electric field concentration in a region near the insulation film can be avoided when a high reverse bias voltage is applied to the element. Thus, the invention can provide a high-breakdown-voltage semiconductor element employing an insulation film capable of reducing the degree of electric field concentration caused in that portion of the substrate which is located in the vicinity of the insulation film. Use of insulation films made of a material having a dielectric constant larger than that of the substrate is known from EP-A-0 224 968 and JP-A-2 001 980. However, the devices so fabricated are diodes and not transistors. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view showing a high-breakdown-voltage diode useful in explaining a reduction in the degree of concentration of an electric field by virtue of an insulating film having a large dielectric constant; Fig. 2 is a view, useful in explaining a reduction in the degree of concentration of an electric field by virtue of an insulating film having a large dielectric constant; Fig. 3 is a cross sectional view, showing a high-breakdown-voltage diode according to a first embodiment of the invention; Fig. 4 is a cross sectional view, showing a high-breakdown-voltage diode according to a second embodiment of the invention; Fig. 5 is a cross sectional view, showing a high-breakdown-voltage diode according to a third embodiment of the invention; Fig. 6 is a cross sectional view, showing a high-breakdown-voltage diode according to a fourth embodiment of the invention; Fig. 7 is a cross sectional view, showing a conventional high-breakdown-voltage diode. The embodiments of the invention will be explained with reference to the accompanying drawings. Fig. 1 shows a high-breakdown-voltage diode useful in explaining a reduction in the degree of concentration of an electric field by virtue of an insulating film having a large dielectric constant. A high-concentration p-type layer 2 serving as an anode terminal is selectively formed in the surface of a high-resistance n-type silicon layer (substrate) 1, and a high-concentration n-type layer 3 for stopping the extension of a depletion layer is formed in the surface, separated from the layer 2 by a predetermined distance. An anode electrode 4 and an electrode 5 supply a voltage indentical to that of a cathode are formed on the layers 2 and 3, respectively. An insulation film 8 made of TiO 2 and having a dielectric constant larger than that of Si is formed on the upper surface of the n-type silicon layer 1 between the electrodes 4 and 5. A cathode electrode 7 is formed on the lower surface of the layer 1 with a high-concentration n-type layer 6 interposed therebetween. When a reverse bias voltage is applied to the above high-breakdown-voltage diode between the anode and cathode electrodes 4 and 7, and between the electrodes 4 and 5, a depletion layer grows from the high-concentration p-type layer 2 into the n-type silicon layer 1. If the dielectric constant of the insulation film 8 is sufficiently larger than that of silicon, an electric field caused in the film 8 when the reverse bias voltage is applied is determined by the electrodes 4 and 5 irrespective of the ambient conditions. That is, the electric field is uniform in the horizontal direction, i.e., has a uniform potential gradient. As a result, the degree of concentration of the electric field is reduced in that portion of the silicon layer 1 which is located under the insulation film 8. The effect of the insulation film 8 will be explained referring to Fig. 2. Fig. 2 shows a state in which a dielectric member A having a dielectric constant ε 1 is in contact with a dielectric member B having a dielectric constant ε 2 . As regards a fine region Ω including the boundary surface between the dielectric members A and B, the following equation (1) can be obtained by using the Gauss' theorem: n ; outward unit vector perpendicular to surface of region Ω where Q represents the total charge accumulated in the region Ω. If the thickness d of the region Ω is sufficiently smaller than the length 1 of the region, the equation (1) can be replaced with the following equation (2): where ε 1 represents the dielectric constant of the portion of the region Ω 1 on the side of the dielectric member A, ε 2 the dielectric constant of the portion of the region Ω 2 on the side of the dielectric member B, and σ the density of charge accumulated in a line extending between a and b . The following equation (3) is obtained by dividing each side of the equation (2) by ε 2 : If ε 2 is sufficiently larger than ε 1 and than σ ℓ , then the first term of the left side and the term of the right side will be very smaller than the second term of the left side. Thus, the following equation (4) can be obtained approximately: From the equation (4), the following equation (5) is obtained: ∂ψ ∂n = 0 where ψ represents the potential. This equation (5) indicates a condition for a reflection-type boundary. Specifically, where ε 2 is sufficiently larger than ε 1 and than σℓ, the internal electric field caused in the region Ω 2 of the dielectric member B is determined by only a fixed boundary condition applied to the region, without being affected by the dielectric member A. The electric-field concentration-reducing effect of the invention is similar to that of a so-called field plate using a high-resistance film such as an SIPOS. However, the invention is basically different from the field plate in that the former utilizes a uniform electric field formed in the insulation film, while the latter utilizes potential distribution obtained by causing a very small amount of current to flow. To obtain the above effect of the invention, the dielectric constant of the insulation film must be sufficiently larger than that of silicon as the element material, i.e., be large enough to substantially satisfies the equation (4). Practically, it is preferable to use an insulation material having a dielectric constant five times larger than silicon. TiO 2 , BaTiO 3 , or the like is used as the insulation material. Further, it is more preferable to use an insulation film having a dielectric constant ten times larger than silicon. Figs. 3 to 6 are cross sectional views, showing high-breakdown-voltage diodes according to first to fourth embodiments of the invention. In these figures, like elements are designated by like signs, and duplication of explanation is avoided. Fig. 3 shows a first embodiment of the invention, which employs insulation films 13 (13 1 , 13 2 , 13 3 ) having a large dielectric constant. The diode according to this embodiment has a dielectric isolation structure obtained, for example, by adhering a second silicon substrate 11 to the first silicon layer 1 with a silicon oxide film 10 interposed therebetween. A plurality of insulation films 13 of a large dielectric constant are formed in boundary portions between the silicon layers 1 and 10. The reason why a plurality of insulation films 13 are formed in the diode is that they are not connected to the anode and cathode electrodes 4 and 7. In the above structure, when the n-type silicon layer 1 becomes a complete depletion layer at the time of applying a reverse bias voltage thereto, the entire voltage is applied to that portion of the depletion layer which extends between the high-concentration p-type layer 2 and high-concentration n-type layer 6. At this time, the potentials of the insulation films 13, which are in a floating state, are determined depending upon the positions thereof, and the degree of electric field in each insulation film 13 is sufficiently lower than in the ambient circumstance. Accordingly, in a bottom portion of the silicon layer 1, the potential varies in stepwise such that it is flattened in the insulation films 13 in the horizontal direction. Thus, concentration of an electric field caused in the insulation films 13 by the influence of the side of the substrate 11 is relaxed, thereby providing high-breakdown-voltage characteristics. Fig. 4 shows a second embodiment of the invention which employs an insulation film 13 formed on the over all lower surface of the n-type silicon layer 1. In this embodiment, the both opposite ends of the insulation film 13 are connected to an anode electrode 4 and a cathode electrode 7 which extend to the bottom of the element. For the same reason described in the case of Fig. 1 where the insulation film 8 is provided on the upper surface of the element, concentration of an electric field in a lower portion of the layer 1 is relaxed. Fig. 5 shows a third embodiment which is a substantial combination of the structures shown in Figs. 1 and 4. In the third embodiment, insulation films 8 and 13 of a large dielectric constant are formed on the lower and upper surfaces of an end portion of the n-type silicon layer 1, respectively, thereby providing a diode having a higher breakdown voltage. Fig. 6 shows a fourth embodiment which is a combination of the structures of Figs 1 and 3. This embodiment also provides superior high-breakdown-voltage characteristics.

Claims: A high-breakdown-voltage semiconductor device comprising: a semiconductor active layer (1) having main and back surfaces one on either side thereof; an insulation support layer (10) arranged on the back surface of the semiconductor active layer (1); a semiconductor support layer (11) arranged on the insulation support layer (10) such that the insulation support layer is interposed between the semiconductor active and semiconductor support layers (1,11); a first conductivity-type region (2) and a second conductivity-type region (6) which are formed in the semiconductor active layer (1) along the main surface; and first and second electrodes (4, 7) provided to the semiconductor layer (1) such that they are located one on either side of a pn-junction between the first and second conductivity-type regions (2, 6); characterized in that a first insulation film (13) is interposed between the semiconductor active and insulation support layers (1, 10), and arranged along the back surface of the semiconductor active layer (1) and between the first and second electrodes (4, 7), the first insulation film (13) being made of a material having a dielectric constant larger than a material constituting the semiconductor active layer (1). A device according to claim 1, characterized in that the first and second electrodes (4, 7) are in contact with the first and second conductivity-type regions (2, 6), respectively. A device according to claim 1 or 2, characterized in that the first insulation film (13) is in contact with the first and second electrodes (4, 7). A device according to claim 1 or 2, characterized in that the first insulation film (13) consists of a plurality of portions (13 1 , 13 2 , 13 3 ) separate from one another. A device according to any one of claims 1 to 4, characterized in that the semiconductor active layer (1) is of a first conductivity type having a high resistance. A device according to any one of claims 1 to 5, characterized in that the second insulation film (8) is arranged along the main surface of the semiconductor active layer and between the first and second electrodes (4, 7), the second insulation film (8) being made of a material having a dielectric constant larger than a material constituting the semiconductor active layer (1). A device according to claim 6, characterized in that the second insulation film (8) is in contact with the first and second electrodes (4, 7).

Inventors: MATSUSHITA KEN ICHI, NAKAGAWA AKIO, OMURA ICHIRO, MATSUSHITA, KEN'ICHI, NAKAGAWA, AKIO, OMURA, ICHIRO, Matsushita, Ken'ichi, c/o Intellectual Prop. Div., Nakagawa, Akio, c/o Intellectual Prop. Div., Omura, Ichiro, c/o Intellectual Prop. Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H01L29
IPC codes: H01L29