This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-244378, filed Aug. 25, 2005, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
Power semiconductor devices such as Schottky barrier diodes have been requested to have their on resistances reduced and their breakdown voltages increased. In response to such a request, efforts have been made to achieve both reduced on resistance and increased breakdown voltage by providing a drift layer and adjusting the concentration and thickness of the drift layer.
In recent years, to further reduce the on resistance and to further increase the breakdown voltage, a structure has been proposed in which a floating semiconductor layer (buried semiconductor layer) is provided in the drift layer (see JP-A 9-191109 (KOKAI)).
However, the above conventional structure requires electric charges (for example, holes) to be supplied to the buried semiconductor layer upon turn-on in order to neutralize electric charges (for example, electrons) accumulated in the buried semiconductor layer. Thus, upon turn-on, a forward voltage must be raised substantially to the built-in voltage of a pn junction to supply electric charges (for example, holes) to the buried semiconductor layer. This disadvantageously increases a switching loss.
Thus, a problem with the conventional power semiconductor device is that the switching characteristic is degraded upon turn-on.
According to a first aspect of the present invention, there is provide a semiconductor device comprising: a semiconductor substrate of a first conductivity type; a semiconductor region of the first conductivity type formed on a top surface of the semiconductor substrate; a lower electrode formed on a bottom surface of the semiconductor substrate; an upper electrode formed on a top surface of the semiconductor region; a buried semiconductor layer of a second conductivity type formed in the semiconductor region; a first semiconductor layer of the second conductivity type, formed on the top surface of the semiconductor region and connected to the upper electrode; and a second semiconductor layer of the second conductivity type, formed on a side surface of the semiconductor region and connected to the buried semiconductor layer and the first semiconductor layer, the second semiconductor layer having a lower second conductivity type impurity concentration than the buried semiconductor layer.
According to a second aspect of the present invention, there is provide a semiconductor device comprising: a semiconductor substrate of a first conductivity type; a semiconductor region of the first conductivity type formed on a top surface of the semiconductor substrate and having a hole; a lower electrode formed on a bottom surface of the semiconductor substrate; an upper electrode formed on a top surface of the semiconductor region; a buried semiconductor layer of a second conductivity type formed in the semiconductor region; and a second conductivity type semiconductor layer formed on a side surface of the hole of the semiconductor region, the second conductivity type semiconductor layer being connected to the buried semiconductor layer and the upper electrode, and the second conductivity type semiconductor layer having a lower second conductivity type impurity concentration than the buried semiconductor layer.
According to a third aspect of the present invention, there is provide a semiconductor device comprising: a semiconductor substrate of a first conductivity type; a semiconductor region of the first conductivity type formed on a top surface of the semiconductor substrate; a lower electrode formed on a bottom surface of the semiconductor substrate; an upper electrode formed on a top surface of the semiconductor region; a buried semiconductor layer of a second conductivity type formed in the semiconductor region; a first semiconductor layer of the second conductivity type, formed on the top surface of the semiconductor region and connected to the upper electrode; and a stacked structure formed on a side surface of the semiconductor region and comprising a first conductivity type semiconductor layer and a second semiconductor layer of the second conductivity type, the second semiconductor layer being connected to the buried semiconductor layer and the first semiconductor layer.
According to a fourth aspect of the present invention, there is provide a semiconductor device comprising: a semiconductor substrate of a first conductivity type; a semiconductor region of the first conductivity type formed on a top surface of the semiconductor substrate and having a hole; a lower electrode formed on a bottom surface of the semiconductor substrate; an upper electrode formed on a top surface of the semiconductor region; a buried semiconductor layer of a second conductivity type formed in the semiconductor region; and a stacked structure formed on a side surface of the hole of the semiconductor region, the stacked structure comprising a first conductivity type semiconductor layer and a second conductivity type semiconductor layer connected to the buried semiconductor layer and the upper electrode.
Embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
With reference to FIGS. 1 to 3, description will be given of a power semiconductor device (Schottky barrier diode) in accordance with a first embodiment.
For example, silicon carbide (SiC) is used for an n-type semiconductor substrate (n+-type semiconductor substrate) 11. An n-type semiconductor region (n−-type semiconductor region) 12 is formed on a top surface (principal surface) of the n-type semiconductor substrate 11. The n-type semiconductor region 12 functions as a drift layer of the Schottky barrier diode. A cathode electrode (lower electrode) 13 is formed on a bottom surface (back surface) of the n-type semiconductor substrate 11. Further, an anode electrode (upper electrode) 14 is formed on a top surface of the n-type semiconductor region 12.
A p-type buried semiconductor layer (p−-type buried semiconductor layer) 15 is formed in the n-type semiconductor region 12. As shown in
A p-type semiconductor layer 21 connected to the anode 14 is formed on a top surface of the n-type semiconductor region 12. As shown in
A p-type semiconductor layer (p−-type semiconductor layer) 22 is formed on side surfaces of the n-type semiconductor region 12; the p-type semiconductor layer 22 is connected to the p-type buried semiconductor layer 15 and p-type semiconductor layer 21. As seen in
As is understood from the above description, the anode 14 and the p-type buried semiconductor layer 15 are connected together via the p-type semiconductor layer 21 and p-type semiconductor layer 22. The p-type semiconductor layer 22 is not provided in the conventional configuration. In the present embodiment, the p-type semiconductor layer 22 is provided on the side surfaces of the n-type semiconductor region 12. Consequently, the switching characteristic obtained upon turn-on can be improved as described below.
While a reverse bias is being applied to the Schottky barrier diode, electrons are accumulated in the p-type buried semiconductor layer 15. Thus, to turn on the Schottky barrier diode (to change the Schottky barrier diode from the reverse bias state to a forward bias state), it is necessary to supply holes to the buried semiconductor layer 15 in order to neutralize the electric charges (electrons) accumulated in the buried semiconductor layer 15. However, in the conventional device, only the p-type semiconductor layer 21 is provided on the top surface of the n-type semiconductor region 12. Consequently, holes are supplied to the buried semiconductor layer 15 via the pn junction between the p-type semiconductor layer 21 and the n-type semiconductor region 12. Thus, the forward voltage rises substantially to the built-in voltage (for SiC, about 3 V) of the pn junction. This increases a switching loss upon turn-on.
In the present embodiment, the p-type semiconductor layer 22 is provided on the side surfaces of the n-type semiconductor region 12. Consequently, the anode 14 can supply holes to the p-type buried semiconductor layer 15 via the p-type semiconductor layer 21 and p-type semiconductor layer 22. This makes it possible to suppress a rise in voltage upon turn-on to reduce the switching loss.
Thus, in the present embodiment, the p-type semiconductor layer 22 is provided to enable the switching characteristic obtained upon turn-on to be improved. This makes it possible to reduce the switching loss upon turn-on. However, if the p-type semiconductor layer 22 is not entirely depleted when a reverse bias voltage (for example, at least several hundred volts) is applied to the Schottky barrier diode, the resistance between the anode 14 and the n-type semiconductor substrate 11 is not increased. This reduces the breakdown voltage provided upon the application of the reverse bias.
To prevent the above problem, it is important to set the thickness and impurity concentration of the p-type semiconductor layer 22 so as to completely deplete the p-type semiconductor layer 22 in the reverse bias state. In the present embodiment, the p-type semiconductor layer 22 has a lower impurity concentration (per unit volume) than the p-type buried semiconductor layer 15. The low impurity concentration of the p-type semiconductor layer 22 enables an increase in the width of the depletion layer of the p-type semiconductor layer 22. As a result, in the reverse bias state, the p-type semiconductor layer 22 can be depleted easily and completely. This makes it possible to prevent the breakdown voltage from lowering upon the application of the reverse bias.
Therefore, in the present embodiment, it is possible to improve the switching characteristic obtained upon turn-on and to prevent the breakdown voltage from lowering upon the application of the reverse bias. A power semiconductor device can thus be obtained which has excellent characteristic and a high reliability.
Desirable values for the impurity concentration and thickness of the p-type semiconductor layer 22 are, for example, as described below.
To obtain a favorable turn-on characteristic, it is important that a current path be present between the anode 14 and the p-type buried semiconductor layer 15. In a zero bias state (in which the voltage between the cathode 13 and the anode 14 is zero), the completely depleted p-type semiconductor layer 22 prevents the formation of a current path between the anode 14 and the p-type buried semiconductor layer 15. This precludes a favorable turn-on characteristic from being obtained.
The expression shown below represents a condition for preventing the p-type semiconductor layer 22 from being completely depleted in a zero bias state.
Na×L>(2×Nd×ε×Eg/e)1/2
In this expression:
For example, when Nd=1×1016 cm−3, Na×L>5.7×1011 cm−2. That is, 5.7×1011 cm−2 is the lower limit of Na×L. However, too large a value for Na×L precludes the p-type semiconductor layer 22 from being completely depleted in the reverse bias state (for example, at least several hundred volts). This lowers the breakdown voltage. For example, a value about 10 times as large as the lower limit of Na×L(5.7×1012 cm−2) is the upper limit of Na×L. Accordingly, the following condition is desirable.
5.7×1012 cm−2>Na×L>5.7×1011 cm−2
(Embodiment 2)
With reference to
In the present embodiment, p-type semiconductor layers (p−-type semiconductor layers) 23 are formed on the side surfaces of respective holes penetrating the n-type semiconductor region 12. The p-type semiconductor layers 23 connect the anode 14 to the p-type buried semiconductor layer 15. The concentration (per unit volume) of p-type impurities in the p-type semiconductor layers 23 is lower than that in the p-type buried semiconductor layer 15. A method for forming p-type semiconductor layers 23 is similar to that for forming a p-type semiconductor layer 22 as described in the first embodiment. That is, with the substrate rotated, oblique ion implantation is used to implant ions of p-type impurities in the side surface regions of the holes formed in the n-type semiconductor region 12. The hole surrounded by the p-type semiconductor layer 23 is filled with an insulator (for example, silicon oxide) 24.
As described above, in the present embodiment, the anode 14 and the p-type buried semiconductor layer 15 are connected together by the p-type semiconductor layers 23. Consequently, upon turn-on, the anode 14 can supply holes to the p-type buried semiconductor later 15 via the p-type semiconductor layers 23. As in the case of the first embodiment, this makes it possible to suppress a rise in the voltage upon turn-on to reduce the switching loss.
In the present embodiment, to obtain a sufficient breakdown voltage upon the application of the reverse bias, it is important to set the thickness and impurity concentration of the p-type semiconductor layers 23 so as to completely deplete the p-type semiconductor layer 23 in the reverse bias state, as in the case of the first embodiment. In the present embodiment, the concentration (per unit volume) of impurities in the p-type semiconductor layers 23 is lower than that in the p-type buried semiconductor layer 15. The low impurity concentration of the p-type semiconductor layer 23 serves to increase the width of the depletion layer of the p-type semiconductor layer 23. As a result, in the reverse bias state, the p-type semiconductor layers 23 can be depleted easily and completely. This makes it possible to prevent the breakdown voltage from lowering upon the application of the reverse bias.
Therefore, in the present embodiment, it is possible to improve the switching characteristic obtained upon turn-on and to prevent the breakdown voltage from lowering upon the application of the reverse bias, as in the case of the first embodiment. A power semiconductor device can thus be obtained which has excellent characteristic and a high reliability.
Further, in the present embodiment, the p-type semiconductor layers 23 are formed on the side surfaces of the holes formed in the n-type semiconductor region 12. This enables an increase in the number of paths between the anode 14 and the p-type buried semiconductor layer 15. It is thus possible to efficiently supply holes to the p-type buried semiconductor layer 15 upon turn-on.
In the present embodiment, the p-type semiconductor layer 22 is provided as in the case of the first embodiment. However, the p-type semiconductor layer 22 need not necessarily be provided.
(Embodiment 3)
With reference to
In the present embodiment, a stacked structure is provided on the side surfaces of the n-type semiconductor region 12; the stacked structure is formed of a p-type semiconductor layer (p−-type semiconductor layer) 25 and an n-type semiconductor layer (n−-type semiconductor layer) 26. Similarly to the p-type semiconductor layer 22 in accordance with the first embodiment, the p-type semiconductor layer 25 is connected to the p-type buried semiconductor layer 15 and p-type semiconductor layer 21. The concentration (per unit volume) of p-type impurities in the p-type semiconductor layers 25 is lower than that in the p-type buried semiconductor layer 15. Further, the concentration (per unit volume) of n-type impurities in the n-type semiconductor layers 26 is higher than that in the n-type semiconductor region 12.
A method for forming a p-type semiconductor layer 25 is similar to that for forming a p-type semiconductor layer 22 as described in the first embodiment. That is, with the substrate rotated, oblique ion implantation is used to implant ions of p-type impurities in the side surface region of the n-type semiconductor region 12. A method for forming an n-type semiconductor layer 26 is also similar to that for forming a p-type semiconductor layer 25. With the substrate rotated, oblique ion implantation is used to implant ions of n-type impurities in the side surface region of the n-type semiconductor region 12. Further, ion implantation energy is adjusted to increase the implantation depth of n-type impurities above that of p-type impurities. Consequently, the n-type semiconductor layer 26 is formed inside the p-type semiconductor layer 25.
As described above, in the present embodiment, the anode 14 and the p-type buried semiconductor layer 15 are connected together by the p-type semiconductor layer 25. Consequently, upon turn-on, the anode 14 can supply holes to the p-type buried semiconductor later 15 via the p-type semiconductor layer 25. As in the case of the first embodiment, this makes it possible to suppress a rise in the voltage upon turn-on to reduce the switching loss.
Further, in the present embodiment, the n-type semiconductor layer 26 is provided which has a higher n-type impurity concentration than the n-type semiconductor region 12. This makes it possible to increase the concentration of p-type impurities in the p-type semiconductor layer 25 and to increase the width of the depletion layer of the p-type semiconductor layer 25. That is, in the pn junction, the positive and negative electric charges in the depletion layer are balanced. Accordingly, when the concentration of p-type impurities in the p-type semiconductor layer 25 is increased without forming an n-type semiconductor layer 26, the width of the depletion layer of the p-type semiconductor layer 25 necessarily decreases. In the present embodiment, the n-type semiconductor layer 26 with a high n-type impurity concentration is provided. This enables an increase in the width of the depletion layer in the p-type semiconductor layer 25 even with an increase in the p-type impurity concentration of the p-type semiconductor layer 25. Therefore, in the present embodiment, the p-type semiconductor layer 25 can be depleted easily and completely in the reverse bias state as in the case of the first embodiment. This makes it possible to prevent the breakdown voltage from lowering upon the application of the reverse bias.
The p-type impurity concentration of the p-type semiconductor layer 25 is preferably equivalent to the n-type impurity concentration of the n-type semiconductor layer 26. Specifically, each of the p- and n-type impurity concentrations is desirably at most 1×1014 cm−2 and higher than 5.7×1012 cm−2.
As described above, in the present embodiment, it is also possible to improve the switching characteristic obtained upon turn-on and to prevent the breakdown voltage from lowering upon the application of the reverse bias. A power semiconductor device can thus be obtained which has excellent characteristic and a high reliability.
Further, the present embodiment can increase the p-type impurity concentration of the p-type semiconductor layer 25 to reduce the resistance of the p-type semiconductor layer 25, as described above. It is thus possible to reduce the resistance of the current path between the anode 14 and the p-type buried semiconductor layer 15. This also serves to improve the switching characteristic obtained upon turn-on.
(Embodiment 4)
With reference to
In the present embodiment, stacked structures are provided on side surfaces of holes penetrating the n-type semiconductor region 12; each of the stacked structures is formed of a p-type semiconductor layer (p−-type semiconductor layer) 27 and an n-type semiconductor layer (n−-type semiconductor layer) 28. The p-type semiconductor layer 27 is connected to the anode 14 and p-type buried semiconductor layer 15. The p-type impurity concentration (per unit volume) of the p-type semiconductor layers 27 is lower than that of the p-type buried semiconductor layer 15. Further, the n-type impurity concentration (per unit volume) of the n-type semiconductor layers 28 is higher than that of the n-type semiconductor region 12. The hole surrounded by the p-type semiconductor layer 27 is filled with an insulator (for example, silicon oxide) 24.
As described above, in the present embodiment, the anode 14 and the p-type buried semiconductor layer 15 are connected together by the p-type semiconductor layer 27. Consequently, upon turn-on, the anode 14 can supply holes to the p-type buried semiconductor later 15 via the p-type semiconductor layer 27. As in the case of the already described embodiments, this makes it possible to suppress a rise in the voltage upon turn-on to reduce the switching loss.
Further, in the present embodiment, the n-type semiconductor layer 28 is provided which has a higher n-type impurity concentration than the n-type semiconductor region 12. This enables an increase in the width of the depletion layer in the p-type semiconductor layer 27 even with an increase in the p-type impurity concentration of the p-type semiconductor layer 27 as described in the third embodiment. Therefore, in the present embodiment, the p-type semiconductor layer 25 can also be depleted easily and completely in the reverse bias state. This makes it possible to prevent the breakdown voltage from lowering upon the application of the reverse bias. Furthermore, as described in the third embodiment, it is possible to reduce the resistance of the p-type semiconductor layer 27 and thus the resistance of the current path between the anode 14 and the p-type buried semiconductor layer 15.
As described above, in the present embodiment, it is also possible to improve the switching characteristic obtained upon turn-on and to prevent the breakdown voltage from lowering upon the application of the reverse bias. A power semiconductor device can thus be obtained which has excellent characteristic and a high reliability.
In the present embodiment, the p-type semiconductor layer 25 and the n-type semiconductor layer 26 are provided as in the case of the third embodiment. However, the p-type semiconductor layer 25 and the n-type semiconductor layer 26 need not necessarily be provided.
(Embodiment 5)
With reference to
In the first embodiment, the single p-type buried semiconductor layer 15 is provided. However, in the present embodiment, a plurality of (in the present example, two) p-type buried semiconductor layers, that is, p-type buried semiconductor layers 15a and 15b, are provided. Even with the plurality of p-type buried semiconductor layers, the fifth embodiment has a basic configuration similar to that of the first embodiment and produces effects similar to those of the first embodiment.
The configuration provided with the plurality of p-type buried semiconductor layers is also applicable to Schottky barrier diodes such as those shown in the second to fourth embodiments.
In the above first to fifth embodiments, configurations such as those shown in the first to fifth embodiments can be adopted even if all the n-type components are changed to the p type, while all the p-type components are changed to the n type. In this case, it is possible to exert effects similar to those described in the first to fifth embodiments.
In the description of the first to fifth embodiments, the Schottky barrier diodes are examples of a power semiconductor device. However, configurations such as those shown in the first to fifth embodiments are applicable to power semiconductor devices such as a power MOSFET and a junction FET.
In the above first to fifth embodiments, the area of the n-type semiconductor substrate 11 is larger than the n-type semiconductor region 12. However, the area of the n-type semiconductor substrate 11 may be substantially the same as the n-type semiconductor region 12.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
Number | Date | Country | Kind |
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2005-244378 | Aug 2005 | JP | national |