Switching element

Abstract

In switching elements, the reduction in breakdown voltage caused by cosmic rays is suppressed. [Solution] A switching element comprising a semiconductor substrate (12) made of silicon carbide and a gate electrode (22) facing the semiconductor substrate via a gate insulating film. The semiconductor substrate comprises an n-type source layer (32), a p-type body layer (36), and a body layer separated from the source layer by the body layer and 8 × 10 15 cm -3 The apparatus comprises an n-type first drift layer (41) having the above-mentioned n-type impurity concentration, an n-type second drift layer (42) in contact with the first drift layer from below and having an n-type impurity concentration 12 to 26 times that of the first drift layer, and an n-type drain layer (48) located below the second drift layer and having a higher n-type impurity concentration than the second drift layer.

Description

[Technical Field]

[0001] The technology disclosed herein relates to switching elements.

[0002] As described in Patent Document 1, when cosmic rays enter the interior of a switching element, electron-hole pairs are generated inside the semiconductor substrate, reducing the breakdown voltage of the switching element. The technology in Patent Document 1 suppresses the breakdown voltage reduction caused by cosmic rays by adjusting the n-type impurity concentration distribution in the drift layer. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2003-338624 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] This specification proposes a technique for more effectively suppressing the breakdown voltage reduction caused by cosmic rays in switching elements. [Means for solving the problem]

[0005] The switching element disclosed herein comprises a semiconductor substrate (12) made of silicon carbide and a gate electrode (22) facing the semiconductor substrate via a gate insulating film. The semiconductor substrate comprises an n-type source layer (32) in contact with the gate insulating film, a p-type body layer (36) in contact with the gate insulating film and the source layer, and a gate electrode in contact with the gate insulating film and the body layer, separated from the source layer by the body layer, and 8 × 10 15 cm -3The apparatus comprises an n-type first drift layer (41) having the above-mentioned n-type impurity concentration, an n-type second drift layer (42) in contact with the first drift layer from below and having an n-type impurity concentration 12 to 26 times that of the first drift layer, and an n-type drain layer (48) located below the second drift layer and having a higher n-type impurity concentration than the second drift layer.

[0006] Figure 1 shows the relationship between the electric field strength 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 the value 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. Also, in Figure 1, the electric field strength E is shown as a value normalized to 1 when the concentration ratio B / A is 38. As shown in Figure 1, the electric field strength E changes according to the concentration ratio B / A. The electric field strength E is minimum when the concentration ratio B / A is approximately 20. In the above switching element, the concentration ratio B / A is 12 to 26, so the electric field strength E generated inside the semiconductor substrate when cosmic rays are incident is low. For this reason, the breakdown voltage reduction due to cosmic rays can be suppressed in this switching element. [Brief explanation of the drawing]

[0007] [Figure 1] A graph showing the relationship between the concentration ratio B / A and the electric field strength E. [Figure 2] Cross-sectional view of the switching element in Example 1. [Figure 3] Cross-sectional view of a switching element in a modified example of Example 1. [Figure 4] Cross-sectional view of the switching element in Example 2. [Figure 5] Cross-sectional view of the switching element in Example 3. [Modes for carrying out the invention] [Examples]

[0008] The switching element 10 in Example 1 shown in Figure 2 is a trench-gate type MOSFET (metal-oxide-semiconductor field effect transistor). The switching element 10 is designed for use in the stratosphere or higher altitudes (e.g., outer space) and has a structure that can suppress 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.

[0009] The semiconductor substrate 12 is made of silicon carbide (i.e., SiC). Hereinafter, the direction parallel to the upper surface 12a of the semiconductor substrate 12 will be referred to as the x-direction, the direction parallel to the upper surface 12a and perpendicular 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. Multiple trenches 14 are provided on the upper surface 12a of the semiconductor substrate 12. Each trench 14 extends linearly in the y-direction on the upper surface 12a. The multiple trenches 14 are spaced apart in the x-direction.

[0010] The gate insulating film 20 covers the inner surface of each trench 14. The gate electrode 22 is located 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 the interlayer insulating film 24.

[0011] 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 the interlayer insulating film 24. The drain electrode 28 covers the lower surface 12b of the semiconductor substrate 12.

[0012] The semiconductor substrate 12 has a plurality of source layers 32, a plurality of contact layers 34, a body layer 36, a first drift layer 41, a second drift layer 42, and a drain layer 48.

[0013] Each source layer 32 is an n-type layer having a high n-type impurity concentration. Each source layer 32 makes an ohmic contact with the source electrode 26 on the upper surface 12a. Each source layer 32 contacts the gate insulating film 20 at the upper end of the side surface of the corresponding trench 14.

[0014] Each contact layer 34 is a p-type layer having a high p-type impurity concentration. Each contact layer 34 makes an ohmic contact with the source electrode 26 on the upper surface 12a.

[0015] The body layer 36 is a p-type layer having a p-type impurity concentration lower than that of the contact layer 34. The body layer 36 is disposed below the source layer 32 and the contact layer 34. The body layer 36 contacts the source layer 32 and the contact layer 34 from below. The body layer 36 contacts the gate insulating film 20 at the side surface of the trench 14 below the source layer 32.

[0016] The first drift layer 41 is an n-type layer having an n-type impurity concentration lower than that of the source layer 32. The n-type impurity concentration A of the first drift layer 41 is 8×10 15 cm -3 or more. For example, the n-type impurity concentration A of the first drift layer 41 may be 3×10 16 cm -3 or more. The first drift layer 41 is disposed below the body layer 36. The first drift layer 41 contacts the body layer 36 from below. The first drift layer 41 is separated from the source layer 32 by the body layer 36. The first drift layer 41 contacts the gate insulating film 20 at the side surface of the trench 14 below the body layer 36. The first drift layer 41 extends from the position where it contacts the body layer 36 to below the lower end of each trench 14.

[0017] The second drift layer 42 is an n-type layer and is in contact with the first drift layer 41 from below. The n-type impurity concentration B of the second drift layer 42 is 12 times or more and 26 times or less than 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.

[0018] The drain layer 48 is an n-type layer having an n-type impurity concentration higher than that of the second drift layer 42. The drain layer 48 is in contact with the second drift layer 42 from below. The n-type impurity concentration of the drain layer 48 is 1×10 19 cm -3 or more. The drain layer 48 makes ohmic contact with the drain electrode 28 on the lower surface 12b of the semiconductor substrate 12.

[0019] When the switching element 10 is in use, a potential higher than that of the source electrode 26 is applied to the drain electrode 28. When a potential equal to or higher than the gate threshold is applied to the gate electrode 22, a channel is formed in the body layer 36 within the range adjacent to the gate insulating film 20. Then, the source layer 32 and the first drift layer 41 are connected by the channel. Therefore, electrons flow from the source layer 32 to the drain layer 48 through the channel, the first drift layer 41, and the second drift layer 42. That is, the switching element 10 is turned on. When the potential of the gate electrode 22 is lowered to a potential less than the gate threshold, the channel disappears and the switching element 10 is turned off. When 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 held by the depletion layer.

[0020] When cosmic rays enter the drift layers 41 and 42 in the switching element 10 in the off state, electron-hole pairs are generated in the drift layers 41 and 42. As a result, an electric field is generated in the drift layers 41 and 42. When an electric field is generated due to the incidence of cosmic rays in this way, dielectric breakdown occurs in the switching element 10 even when the drain-source voltage is below the rated value. Thus, when cosmic rays enter the drift layers 41 and 42, the breakdown voltage of the switching element 10 decreases.

[0021] FIG. 1 shows the result of obtaining by simulation the relationship between the electric field strength E generated when cosmic rays enter the drift layers 41 and 42 and the concentration ratio B / A when the n-type impurity concentration A of the first drift layer 41 is 8×10 15 cm -3 or more. As shown in FIG. 1, the electric field strength E is minimized when the concentration ratio B / A is about 20. Also, in the range where the concentration ratio B / A is 12 or more and 26 or less, the rate of change of the electric field strength E with respect to the concentration ratio B / A is small, and the electric field strength E stabilizes at a low value of 0.3 or less. Therefore, if the concentration ratio B / A is adjusted to be 12 or more and 26 or less, the electric field strength E generated when cosmic rays enter the drift layers 41 and 42 can be suppressed. In the switching element 10 of Example 1, since the concentration ratio B / A is 12 or more and 26 or less, the electric field strength E generated when cosmic rays enter is low, and the decrease in breakdown voltage due to cosmic rays is suppressed. Also, the range where the concentration ratio B / A is 14 or more and 24 or less may be used, and the electric field strength E stabilizes at a low value of 0.25 or less.

[0022] Note that 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 may be 400 to 600 nm. The thickness of the body layer 36 may be 300 to 1,000 nm. The thickness of the first drift layer 41 may be 3 to 10 μm. The thickness of the second drift layer 42 may be 0.3 to 2.0 μm. The thickness of the drain layer 48 may be 50 to 300 μm. The breakdown voltage class of the switching element may be 1,200 to 3,300 V.

[0023] In Example 1, a switching element having a trench-type gate structure was described, but as shown in Figure 3, a switching element having a planar-type gate structure may also have its concentration ratio B / A adjusted to be between 12 and 26. Furthermore, any switching element having a different superstructure than those shown in Figures 2 and 3 can have its breakdown voltage reduction due to cosmic rays suppressed by applying the concentration ratio B / A disclosed herein. Similarly, in other embodiments described later, any structure can be adopted as the superstructure of the switching element. [Examples]

[0024] The switching element of Example 2 shown in Figure 4 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 Example 2 has the same structure as the switching element 10 of Example 1. The third drift layer 43 has an n-type impurity concentration 2 to 10 times that of the n-type impurity concentration B of the second drift layer 42. The third drift layer 43 is in contact with 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 in contact with the third drift layer 43 from below. With the addition of the third drift layer 43 in this way, the electric field generated in the drift layer when cosmic rays are incident becomes smaller. For this reason, the switching element of Example 2 is less susceptible to a decrease in breakdown voltage due to cosmic rays. [Examples]

[0025] As shown in Figure 5, the switching element of Example 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 2 to 10 times that of the third drift layer 43. The fourth drift layer 44 is in contact with 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 in contact with the fourth drift layer 44 from below. With this configuration, the breakdown voltage reduction due to cosmic rays can be further suppressed. The switching element may also have multiple drift layers.

[0026] In each of the above embodiments, the drift layers 41-44 may have their concentrations adjusted during epitaxial growth, or their concentrations may be adjusted by ion implantation after epitaxial growth. Furthermore, in each of the above embodiments, the n-type impurity concentration may change in a stepwise manner or in a gradient manner at the boundaries between each drift layer.

[0027] The components of the technology disclosed herein are listed below. (Composition 1) A switching element, A semiconductor substrate (12) made of silicon carbide, A gate electrode (22) facing the semiconductor substrate via a gate insulating film, It has, The aforementioned semiconductor substrate An n-type source layer (32) in contact with the gate insulating film, The gate insulating film and the p-type body layer (36) in contact with the source layer, The gate insulating film and the body layer are in contact, and the body layer separates them from the source layer, and the 8 × 10 15 cm -3 The first drift layer (41) of n type having the above n-type impurity concentration, A second n-type drift layer (42) is in contact with the first drift layer from below and has an n-type impurity concentration 12 to 26 times that of the first drift layer, An n-type drain layer (48) is located below the second drift layer and has a higher n-type impurity concentration than the second drift layer. Having, Switching element. (Configuration 2) The n-type impurity concentration in the first drift layer is 3 × 10 16 cm -3 A switching element larger than the one described in Configuration 1. (Composition 3) The material further comprises an n-type third drift layer (43) that is in contact with the second drift layer from below and has an n-type impurity concentration 2 to 10 times that of the second drift layer, The drain layer is located below the third drift layer and has a higher n-type impurity concentration than the third drift layer. A switching element as described in configuration 1 or 2. (Composition 4) A switching element used at altitudes above the stratosphere, as described in any of configurations 1 to 3. (Composition 5) A switching element used in outer space, as described in any of configurations 1 to 4.

[0028] According to the above configuration 3, the electric field strength generated inside the semiconductor substrate when cosmic rays are incident can be further reduced.

[0029] Although embodiments have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings achieve multiple objectives simultaneously, and achieving even one of these objectives constitutes technical usefulness. [Explanation of Symbols]

[0030] 10: Switching element, 12: Semiconductor substrate, 20: Gate insulating film, 22: Gate electrode, 26: Source electrode, 28: Drain electrode, 32: Source layer, 34: Contact layer, 36: Body layer, 41: First drift layer, 42: Second drift layer, 48: Drain layer

Claims

1. A switching element, A semiconductor substrate made of silicon carbide, A gate electrode facing the semiconductor substrate via a gate insulating film, It has, The aforementioned semiconductor substrate An n-type source layer in contact with the gate insulating film, The gate insulating film and the p-type body layer in contact with the source layer, The gate insulating film and the body layer are in contact, and the body layer separates them from the source layer, and the dimensions are 8 × 10 １５ cm －３ An n-type first drift layer having the above n-type impurity concentration, A second n-type drift layer is in contact with the first drift layer from below and has an n-type impurity concentration 12 to 26 times that of the first drift layer. An n-type drain layer is located below the second drift layer and has a higher n-type impurity concentration than the second drift layer. Having, Switching element.

2. The n-type impurity concentration in the first drift layer is 3 × 10 １６ cm －３ A switching element according to claim 1, which is larger than the switching element described in claim 1.

3. The material further comprises a third n-type drift layer that is in contact with the second drift layer from below and has an n-type impurity concentration 2 to 10 times that of the second drift layer. The drain layer is located below the third drift layer and has a higher n-type impurity concentration than the third drift layer. A switching element according to claim 1 or 2.

4. A switching element according to claim 1 or 2, used at altitudes above the stratosphere.

5. A switching element according to claim 1 or 2, for use in outer space.

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