Switching element

Abstract

A switching element is provided with a semiconductor substrate (12) comprising silicon carbide, and a gate electrode (22) facing the semiconductor substrate with a gate insulating film interposed therebetween. The semiconductor substrate has an n-type source layer (32), a p-type body layer (36), an n-type first drift layer (41), an n-type second drift layer (42), and an n-type drain layer (48), and the n-type first drift layer (41) is separated from the source layer by the body layer and has an n-type impurity concentration of 8 * 1015 cm-3 or more. An n-type second drift layer (42) is in contact with the first drift layer from the lower side and has an n-type impurity concentration 12-26 times the n-type impurity concentration of the first drift layer, and an n-type drain layer (48) is disposed on the lower side of the second drift layer and has a higher n-type impurity concentration than the second drift layer. As a result, it is possible to suppress a decrease in withstand voltage caused by cosmic rays in the switching element.

Description

Technical Field

[0001] The technology disclosed in this specification relates to switching elements. Background Technology

[0002] As described in Patent Document 1, when cosmic rays are incident on the interior of a switching element, electron-hole pairs are generated inside the semiconductor substrate, causing a decrease in the breakdown voltage of the switching element. In the technology of Patent Document 1, the breakdown voltage decrease caused by cosmic rays is suppressed by adjusting the n-type impurity concentration distribution within the drift layer.

[0003] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2003-338624 Summary of the Invention

[0004] This specification proposes techniques for more appropriately suppressing voltage drop caused by cosmic rays in switching elements.

[0005] According to one embodiment disclosed in this specification, a switching element has a semiconductor substrate made of silicon carbide and a gate electrode facing the semiconductor substrate through a gate insulating film. The semiconductor substrate has: an n-type source layer in contact with the gate insulating film; a p-type body layer in contact with both the gate insulating film and the source layer; and an n-type first drift layer in contact with both the gate insulating film and the body layer, separated from the source layer by the body layer, having an area of ​​8 × 10⁻⁶. 15 cm -3 The above-mentioned n-type impurity concentration; an n-type second drift layer, which is connected to the first drift layer from below, having an n-type impurity concentration of 12 to 26 times that of the first drift layer; and an n-type drain layer, which is disposed below the second drift layer, having a higher n-type impurity concentration than the second drift layer.

[0006] Figure 1 This represents the relationship between the electric field intensity E generated inside the semiconductor substrate when cosmic rays are incident on the switching element and the concentration ratio B / A. The concentration ratio B / A is obtained by dividing the n-type impurity concentration B of the second drift layer by the n-type impurity concentration A of the first drift layer. Furthermore, in... Figure 1 In this context, the electric field strength E is represented by a standardized value obtained by setting the concentration ratio B / A to 1 when it is 38. For example... Figure 1 As shown, the electric field strength E varies with the concentration ratio B / A. The electric field strength E is minimum when the concentration ratio B / A is approximately 20. In the above-described switching element, since the concentration ratio B / A is 12 to 26, the electric field strength E generated inside the semiconductor substrate when cosmic rays are incident is low. Therefore, in this switching element, the voltage drop caused by cosmic rays can be suppressed. Attached Figure Description

[0007] Figure 1 It is a graph showing the relationship between the concentration ratio B / A and the electric field strength E.

[0008] Figure 2 This is a cross-sectional view of the switching element in Embodiment 1.

[0009] Figure 3 This is a cross-sectional view of the switching element of a modified example of Embodiment 1.

[0010] Figure 4 This is a cross-sectional view of the switching element in Embodiment 2.

[0011] Figure 5 This is a cross-sectional view of the switching element in Embodiment 3. Detailed Implementation

[0012] [Example 1] Figure 2 The switching element 10 of Embodiment 1 shown is a trench-gate MOSFET (metal-oxide-semiconductor field-effect transistor). The switching element 10 is designed for use at altitudes of the stratosphere or above (e.g., outer space) and has a structure capable of suppressing the effects of cosmic rays. The switching element 10 has a semiconductor substrate 12, a gate electrode 22, a gate insulating film 20, a source electrode 26, and a drain electrode 28.

[0013] The semiconductor substrate 12 is made of silicon carbide (i.e., SiC). Hereinafter, a direction parallel to the upper surface 12a of the semiconductor substrate 12 will be referred to as the x-direction, a direction parallel to the upper surface 12a and orthogonal to the x-direction will be referred to as the y-direction, and the thickness direction of the semiconductor substrate 12 will be referred to as the z-direction. A plurality of trenches 14 are provided on the upper surface 12a of the semiconductor substrate 12. Each trench 14 extends linearly along the y-direction on the upper surface 12a. The plurality of trenches 14 are arranged at intervals in the x-direction.

[0014] A gate insulating film 20 covers the inner surface of each trench 14. A gate electrode 22 is disposed within each trench 14. The gate electrode 22 is insulated from the semiconductor substrate 12 by the gate insulating film 20. The upper surface of the gate electrode 22 is covered by an interlayer insulating film 24.

[0015] The source electrode 26 covers the upper surface 12a of the semiconductor substrate 12. The source electrode 26 is insulated from the gate electrode 22 by an interlayer insulating film 24. The drain electrode 28 covers the lower surface 12b of the semiconductor substrate 12.

[0016] The semiconductor substrate 12 has multiple source layers 32, multiple contact layers 34, a body layer 36, a first drift layer 41, a second drift layer 42, and a drain layer 48.

[0017] Each source layer 32 is an n-type layer with a high n-type impurity concentration. Each source layer 32 is in ohmic contact with the source electrode 26 on its upper surface 12a. Each source layer 32 is connected to the gate insulating film 20 at the upper end of the side of the corresponding trench 14.

[0018] Each contact layer 34 is a p-type layer with a high p-type impurity concentration. Each contact layer 34 has an ohmic contact with the source electrode 26 on its upper surface 12a.

[0019] Body layer 36 is a p-type layer with a lower p-type impurity concentration than contact layer 34. Body layer 36 is disposed below source layer 32 and contact layer 34. Body layer 36 is connected to source layer 32 and contact layer 34 from below. Body layer 36 is connected to the gate insulating film 20 on the side of trench 14 below source layer 32.

[0020] The first drift layer 41 is an n-type layer with a lower n-type impurity concentration than the source layer 32. The n-type impurity concentration A of the first drift layer 41 is 8 × 10⁻⁶. 15 cm -3 The above. For example, the n-type impurity concentration A of the first drift layer 41 can also be 3 × 10⁻⁶. 16 cm -3 That's all. For example, the n-type impurity concentration A of the first drift layer 41 is 1 × 10⁻⁶. 18 cm -3 The first drift layer 41 is disposed on the lower side of the body layer 36. The first drift layer 41 is connected to the body layer 36 from the lower side. The first drift layer 41 is separated from the source layer 32 through the body layer 36. The first drift layer 41 is connected to the gate insulating film 20 on the side of the trench 14 on the lower side of the body layer 36. The first drift layer 41 is distributed from the position where it is connected to the body layer 36 to a position further lower than the lower end of each trench 14.

[0021] The second drift layer 42 is an n-type layer, which is connected to the first drift layer 41 from the bottom. The n-type impurity concentration B of the second drift layer 42 is more than 12 times and less than 26 times the n-type impurity concentration A of the first drift layer 41. That is, the n-type impurity concentration B of the second drift layer 42 is set such that the concentration ratio B / A is 12 to 26.

[0022] Drain layer 48 is an n-type layer with a higher n-type impurity concentration than the second drift layer 42. Drain layer 48 is connected to the second drift layer 42 from the bottom. The n-type impurity concentration of drain layer 48 is 1 × 10⁻⁶. 19 cm -3 The drain layer 48 is in ohmic contact with the drain electrode 28 on the lower surface 12b of the semiconductor substrate 12.

[0023] When using the switching element 10, a higher potential than that of the source electrode 26 is applied to the drain electrode 28. If a potential above the gate threshold is applied to the gate electrode 22, a channel is formed in the body layer 36 adjacent to the gate insulating film 20. The source layer 32 and the first drift layer 41 are then connected through the channel. Electrons flow from the source layer 32 through the channel, the first drift layer 41, and the second drift layer 42 to the drain layer 48. That is, the switching element 10 is turned on. If the potential of the gate electrode 22 is reduced to a value below the gate threshold, the channel disappears, and the switching element 10 is turned off. If the switching element 10 is turned off, a depletion layer extends from the body layer 36 into the drift layers 41 and 42. The voltage applied between the drain electrode 28 and the source electrode 26 is maintained by the depletion layer.

[0024] In the off state of the switching element 10, if cosmic rays are incident on the drift layers 41 and 42, electron-hole pairs are generated within the drift layers 41 and 42. This generates an electric field within the drift layers 41 and 42. If an electric field is generated due to the incident cosmic rays, insulation breakdown occurs in the switching element 10 even when the drain-source voltage is below the rated value. Thus, the withstand voltage of the switching element 10 decreases due to the incident cosmic rays on the drift layers 41 and 42.

[0025] Figure 1 This indicates that the concentration A of the n-type impurities in the first drift layer 41 is 8 × 10⁻⁶. 15 cm -3 Under the above conditions, the relationship between the electric field intensity E and the concentration ratio B / A generated when cosmic rays are incident on drift layers 41 and 42 was obtained through simulation. For example... Figure 1 As shown, the electric field strength E is minimum when the concentration ratio B / A is approximately 20. Furthermore, within the range of a concentration ratio B / A of 12 or higher and 26 or lower, the rate of change of the electric field strength E relative to the concentration ratio B / A is small, and the electric field strength E remains stable at a low value of 0.3 or lower. Therefore, if the concentration ratio B / A is adjusted to 12 or higher and 26 or lower, the electric field strength E generated when cosmic rays are incident on the drift layers 41 and 42 can be suppressed. In the switching element 10 of Embodiment 1, since the concentration ratio B / A is 12 or higher and 26 or lower, the electric field strength E generated when cosmic rays are incident is low, and the voltage drop caused by cosmic rays can be suppressed. Additionally, the concentration ratio B / A can also be in the range of 14 or higher and 24 or lower, and the electric field strength E remains stable at a low value of 0.25 or lower.

[0026] Furthermore, the thickness of each semiconductor layer is arbitrary, but the thickness of each layer can be set as follows: The thickness of the source layer 32 and the contact layer 34 can also be 400nm~600nm. The thickness of the body layer 36 can also be 300nm~1000nm. The thickness of the first drift layer 41 can also be 3μm~10μm. The thickness of the second drift layer 42 can also be 0.3μm~2.0μm. The thickness of the drain layer 48 can also be 50μm~300μm. The voltage rating of the switching element can also be 1200V~3300V.

[0027] Furthermore, in Embodiment 1, a switching element having a trench-type gate structure was described, but as... Figure 3 As shown, in a switching element with a planar gate structure, the concentration ratio B / A can be adjusted to 12 or more and 26 or less. Furthermore, in a switching element having a planar gate structure... Figure 2 , Figure 3 In any switching element with different top structures, the B / A concentration ratio disclosed in this specification can also be applied to suppress the voltage drop caused by cosmic rays. Similarly, in other embodiments described later, the top structure of the switching element can be of any construction.

[0028] [Example 2] Figure 4 The switching element of Embodiment 2 shown has a third drift layer 43 between the second drift layer 42 and the drain layer 48. Except for the presence of the third drift layer 43, the switching element of Embodiment 2 has the same structure as the switching element 10 of Embodiment 1. The third drift layer 43 has an n-type impurity concentration 2 to 10 times that of the second drift layer 42. The third drift layer 43 is connected to the second drift layer 42 from below. The drain layer 48 has a higher n-type impurity concentration than the third drift layer 43. The drain layer 48 is connected to the third drift layer 43 from below. With the addition of the third drift layer 43, the electric field generated within the drift layer when cosmic rays are incident becomes smaller. Therefore, the switching element of Embodiment 2 is less prone to voltage drop caused by cosmic rays.

[0029] [Example 3] like Figure 5 As shown, the switching element of Embodiment 3 has a fourth drift layer 44 between the third drift layer 43 and the drain layer 48. The fourth drift layer 44 has an n-type impurity concentration that is 2 to 10 times that of the third drift layer 43. The fourth drift layer 44 is connected to the third drift layer 43 from below. The drain layer 48 has a higher n-type impurity concentration than the fourth drift layer 44. The drain layer 48 is connected to the fourth drift layer 44 from below. According to this configuration, the breakdown voltage drop caused by cosmic rays can be further suppressed. In addition, the switching element may have more drift layers.

[0030] Furthermore, in the above embodiments, the concentration of drift layers 41-44 can be adjusted either during epitaxial growth or by ion implantation after epitaxial growth. Additionally, in the above embodiments, the concentration of n-type impurities at the boundaries between drift layers can vary in a step-like manner or in a sloping manner.

[0031] In any of the above configurations, the electric field strength generated inside the semiconductor substrate when cosmic rays are incident can be further reduced.

[0032] The embodiments have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes technologies obtained by various modifications and alterations to the specific examples described above. The technical elements illustrated in this specification or drawings exert their technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of application. Furthermore, the technology illustrated in this specification or drawings achieves multiple objectives simultaneously, and achieving one of these objectives is itself technically useful.

Claims

1. A switching element, characterized in that, have: Semiconductor substrate, which is made of silicon carbide; and The gate electrode is positioned opposite the semiconductor substrate, separated by a gate insulating film. The semiconductor substrate has: An n-type source layer, which is in contact with the gate insulating film; A p-type body layer, which is in contact with the gate insulating film and the source layer; The first drift layer of the n-type structure is in contact with the gate insulating film and the body layer, and is separated from the source layer through the body layer, having an area of ​​8 × 10⁻⁶. 15 cm -3 The above n-type impurity concentrations; A second n-type drift layer, which is in contact with the first drift layer from below, has an n-type impurity concentration that is 12 to 26 times that of the first drift layer; and An n-type drain layer is disposed below the second drift layer and has a higher n-type impurity concentration than the second drift layer.

2. The switching element as described in claim 1, characterized in that, The n-type impurity concentration in the first drift layer is greater than 3 × 10⁻⁶. 16 cm -3 .

3. The switching element as described in claim 1 or 2, characterized in that, It also has an n-type third drift layer, which is in contact with the second drift layer from below, and has an n-type impurity concentration that is 2 to 10 times that of the second drift layer. The drain layer is disposed below the third drift layer and has a higher n-type impurity concentration than the third drift layer.

4. The switching element as described in claim 1 or 2, characterized in that, The switching element is used at a height above the stratosphere.

5. The switching element as described in claim 1 or 2, characterized in that, The switching element is used in outer space.

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