A FIELD CONTROLLED SEMICONDUCTOR DEVICE OF SiC AND A METHOD FOR PRODUCTION THEREOF

Abstract: A field controlled semiconductor device of SiC comprises superimposed in the order mentioned at least a drain (12), a highly doped substrate layer (1) and a low doped n-type drift layer (2). It has also a highly doped n-type source region layer (6) and a source (11) connected thereto. A doped channel region layer (4) connects the source region layer to the drift layer, and a current is intended to flow therethrough when the device is in an on-state. The device has also a gate electrode (9). The channel region layer has a substantially lateral extension and is formed by a low doped n-type layer (4). The gate electrode (9) is arranged to influence the channel region layer from above for giving a conducting channel (17) created therein from the source region layer to the drift layer a substantially lateral extension.

Description: A field controlled semiconductor device of SiC and a method for production thereof TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a field controlled semicon¬ ductor device of SiC comprising superimposed in the order mentioned at least a drain, a highly doped substrate layer and a low doped n-type drift layer, said device further com- prising a highly doped n-type source region layer and a source connected thereto, a doped channel region layer con¬ necting said source region layer to said drift layer and through which a current is intended to flow when the device is in an on-state, and a gate electrode arranged to, upon applying a voltage thereto, influence the charge carrier distribution of said channel region layer and by that the conductivity thereof, as well as a method for producing such a semiconductor device. All type of field controlled semiconductor devices are com¬ prised, such as for example field controlled transistors and field controlled thyristors. Such semiconductor devices of SiC may especially be used as switching devices in power applications owing to the possi¬ bility to turn them on and off very rapidly. Such devices made of SiC are particularly well suited for high power ap¬ plications, since such applications make it possible to benefit from the superior properties of SiC in comparison with especially Si, namely t e capability of SiC to function well under extreme conditions. SiC has a high thermal sta- bility due to a large bandgap energy, such that devices fab¬ ricated from said material are able to operate at high tem¬ peratures, namely up to 1000K. Furthermore, it has a high thermal conductivity, so that SiC devices may be arranged at a high density. SiC also has a more than five times higher breakdown field than Si, so that it is well suited as a ma¬ terial in high power devices operating under conditions where height voltages may occur in the blocking state of a device. Such field controlled semiconductor devices may be divided into two main groups, namely so called nor ally-off devices and normally-on devices, which means that said channel re¬ gion layer will have no conducting channel allowing an elec- tron transport from the source region layer to the drift layer when the gate electrode is laid on a zero potential and that such a conducting channel is there when the gate electrode is laid on zero potential, respectively. Normally- off devices are more interesting and have more applications, since no voltage has to be applied to the gate electrode for cutting the electron transport of the device off, but the present invention covers both types of devices. A field controlled semiconductor device of the type defined in the introduction is known through the US patent 5 323 040 and this device has vertical channel regions, which are lat¬ erally restricted by trenches receiving a gate electrode. The channel region layer has to be very thin, in the order of a couple of μm, for creating a normally-off device. The trenches in this known device has to be created by etching, and it is very delicate to etch for obtaining such thin lay¬ ers, so that it will be difficult and complicated to give a device constructed in that way a normally-off characteris¬ tic. SUMMARY OF THE INVENTION The object of the present invention is to provide a field controlled semiconductor device of SiC of the type defined in the introduction, through which the problem of the prior art device described above is solved to a large extent and which may be produced by processing techniques suitable for SiC. This object is in accordance with the invention obtained by providing such a field controlled semiconductor device in which said channel region layer has a substantially lateral extension and is formed by a low doped n-type layer, which further comprises a p-type base layer arranged under the low doped n-type channel region layer on the opposite side thereof with respect to the gate electrode for forming a n- type conducting channel in the channel region layer at a distance from an upper surface of said channel region layer, and in which the gate electrode is arranged to influence said channel region from above for giving a conducting channel created therein from the source region layer to the drift layer a substantially lateral extension. Thanks to the provision of said low doped n-type layer with a lateral extension as said channel region layer, it will be possible to without any problem vary the thickness of said channel region layer according to the wishes in the particu¬ lar case, so that a normally-off device may be created just as well as a normally on-device. No fine lithography will be required as for the production of such a channel region layer as is required for the prior art device discussed above. Said low doped n-type layer may thanks to the lateral extension thereof easily be created by epitaxial growth to an arbitrary thickness. The p-type base layer located under the n-type channel region layer will be used to deplete the channel region layer from one side and the gate electrode from the other side, and a normally-off device may be easily created if desired by appropriate choice of the width of the channel region layer, the gate electrode material and the doping concentration of the channel region layer and said base layer. This base layer does also act as a voltage blocking layer. Such a structure, which leads to the formation of a conducting channel at a distance from an upper surface of the channel region layer is very preferred, since the channel mobility equals bulk mobility due to the channel conduction being independent of the surface conditions of said channel region layer. This is an important advantage with respect to a conventional device, in which the conducting channel is created next to the surface and thus in regions having a considerably higher trap density than the bulk of the layer. According to another preferred embodiment of the invention said p-type base layer is highly doped. This will facilitate the realisation of a normally-off device and \r 11 also make it easier to turn a normally-on device off by depleting the entire cross-section area of said channel region layer be¬ tween the base layer and the upper surface of the channel region layer. Furthermore, the device may thanks to this characteristic be able to take higher voltages. According to another preferred embodiment of the invention said gate electrode is insulated with respect to the channel region layer by an interposed insulating layer. This will make it possible to fabricate a field controlled semiconduc- tor device of SiC having an insulated gate, but in which the conducting channel is not created at the interface between the insulating layer and the semiconductor layer, where the trap density is comparatively high, but at a distance there¬ from in the bulk. According to another preferred embodiment of the invention the thickness and doping concentration of the low doped channel region layer are co-ordinated with the doping con- centration of the base layer and the choice of the gate so as to form a p-type depletion region in the channel region layer totally blocking the passage of electrons from said source region layer to the drift layer when a zero potential is laid on the gate electrode for making the device a nor- mally-off device. A device having this preferred character¬ istic may in this way easily be realised, essentially through the lateral extension of the channel region layer and the possibility to form it by epitaxial growth. According to another preferred embodiment of the invention said device is a MISFET having a highly doped n-type sub¬ strate layer, said low doped n-type channel region layer is a part of the drift layer extending laterally in between the insulating layer and said base layer, which is buried in the drift layer, for forming, upon applying a voltage to said gate electrode, a conducting channel from the source region layer to the drift layer at the interface between the base layer and said channel region layer at a distance from said insulating layer. In this way the acceptor like traps formed through the injection of the electrons from the SiC into the insulating layer of conventional MISFETs having a conducting inversion channel at the interface between the SiC layer and the insulating layer will no longer be created, since the conducting channel is in this device formed in the bulk of the material, so that the channel will have a bulk carrier mobility rather than the degraded mobility of the interface channel, so that a lower on-state resistance is obtained. That the insulating layer is removed from the channel makes degradation of the insulating layer due to injected charge largely neglectable. According to other preferred embodiments of the invention said device is a transistor and a thyristor, respectively, so that field controlled semiconductor devices of SiC of these two types having advantageous features, especially low on-state losses and a normally-off characteristic when de¬ sired, may be provided, which will be of interest in differ¬ ent types of high power applications. According to another preferred embodiment of the invention said device comprises an additional highly doped n-type layer arranged to connect said low doped n-type channel re¬ gion layer to the drift layer. Such a highly doped addi¬ tional layer will contribute to reduce the on-state losses of such a device. According to another preferred embodiment of the invention the source region layer and said additional n-type layer are arranged at a mutual lateral distance leaving a part of said channel region layer exposed to a voltage of said gate elec- trode therebetween. It will in this way be easy to regulate the cross-section area of the conducting channel between the source region layer and said additional n-type layer and if desired cut it totally off by varying the voltage applied to the gate electrode. According to another preferred embodiment of the invention said device comprises an additional highly doped p-type layer arranged laterally with respect to the low doped n- type channel region layer and being in contact with the source. Such a highly doped p-type layer is particularly ad¬ vantageous when the device is a thyristor, since the elec¬ tron current flowing from the source region layer into the drift layer will create an injection of holes into the drift layer from the substrate layer, and these holes may be effi- ciently collected by said highly doped p-type layer thereby reducing the latchup problem of such a device. A further object of the invention is to provide a method for producing a field controlled semiconductor device of SiC be¬ ing one of a) a transistor and b) a thyristor, for which the drawback of the prior art device of this type discussed above is considerably reduced. Such a method comprises according to the invention the steps of: 1 ) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped sub¬ strate layer being for a) of n-type and for b) of p- type, for b) on top thereof one of c) a highly doped n- type buffer layer and d) no such layer, and a low doped n-type drift layer, 2) implanting p-type dopants into a surface layer of said drift layer for forming a p-type base layer, 3) epitaxially growing a low doped n-type channel region layer on top of said base layer, 4) etching away said channel region layer for forming a wall extending to the drift layer, 5) one of a) implanting n-type dopants into a surface layer of said channel region layer and b) epitaxially growing a n-type layer onto said channel region layer, for forming a highly doped n-type source region layer, 6) one of a) implanting n-type dopants into said channel region layer at a lateral distance from said source re¬ gion layer and in connection with said wall and into said wall and b) epitaxially growing a n-type layer onto said channel region layer at a lateral distance from said source region layer and in connection with said wall and on said wall for forming an additional n-type layer extending from said channel region layer to the drift layer, 7) applying a gate electrode on said channel region layer between the source region layer and said addi- tional n-type layer and a source on said source region layer, the steps 4) and 5) being carried out in one of the follow¬ ing orders: a) 4) and then 5) and b) 5) and then 4) . By using this method field controlled transistors and thyristors of SiC with a lateral conducting channel and the same preferred characteristics as those previously mentioned may be produced. By using regrowth on an implanted layer said low doped n-type channel region layer may easily be given an arbitrary thickness and also be made very thin when so desired, at the same time as the base layer created by an implantation step may be provided at an arbitrary depth. The diffusitivity of different dopants in SiC is dramatically reduced with respect to Si, but in this way it will be pos¬ sible to obtain a layer buried in a layer with another type of doping by using the implantation technique and without damaging the upper one of the layers. Another method through which said object of the present in¬ vention may be obtained comprises the following steps for producing a field controlled semiconductor device of SiC be¬ ing one of a) a transistor and b) a thyristor: 1 ) epitaxially growing on top of each other the follow- ing semiconductor layers of SiC: a highly doped sub¬ strate layer being for a) of n-type and for b) of p- type, for b) on top thereof one of c) a highly doped n- type buffer layer and d) no such layer, and a low doped n-type drift layer, 2) etching a groove into said drift layer, 3) epitaxially growing a p-type base layer in said groove, 4) epitaxially growing a low doped n-type channel region layer on top of said base layer and a surface of said drift layer located laterally to said base layer, 5) etching away said channel region layer at a lateral distance from one lateral border of said base layer while forming a wall of the channel region layer, 6) one of a) implanting n-type dopants into said channel region layer at a lateral distance from said border and in connection with said wall and into said wall and b) epitaxially growing a n-type layer on said channel re¬ gion layer at a lateral distance from said border and in connection with said wall and onto said wall for forming a highly doped n-type source region layer, 7) applying a gate electrode on said channel region layer in the lateral direction between the source region layer and said border and a source on said source region layer. This method constitutes another preferred way to form a de¬ vice having the preferred characteristics mentioned above, and here the base layer is obtained by an epitaxial re¬ growth in the etched groove with the channel region layer epitaxially grown on top thereof. Another method through which said object of the present in¬ vention may be obtained comprises the following steps for producing a field controlled semiconductor device of SiC in the form of a MISFET: 1) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped n-type substrate layer and a low doped n-type drift layer, 2) forming a buried p-type base layer in the drift layer by one of a) implanting p-type dopants with such an en¬ ergy that a buried implanted base layer is formed at a distance from the upper surface of the drift layer and b) implanting p-type dopants into a surface layer of said drift layer and after that epitaxially growing a further portion of said drift layer thereupon for form¬ ing a buried p-type base layer, said base layer being formed so that a vertical portion of said drift layer passes it laterally on a first side, 3) implanting n-type dopants into a surface layer of said drift layer over said base layer at a distance from said first side for forming a highly doped source region layer, 4) applying an insulating layer with an electrode thereon on said portion of said drift layer located above said p-type base layer and a source on said source region layer. By combining the implantation of the p-type dopants into the drift layer with a subsequent epitaxial growth of a further portion of the drift layer thereupon a buried base layer may without any problems be formed in a MISFET of SiC, so that a conducting channel created therein will be at a distance from the interface to said insulating layer and the carrier mobility will be considerably higher than in known MISFETs of SiC. Further advantages and preferred features of the invention will appear from the description and the other claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of preferred embodiments of the inven¬ tion cited as examples. In the drawings: Fig. 1-4 are schematic cross-section views illustrating different steps of a method for producing a field controlled transistor according to Fig. 4, Fig. 5 is an enlarged cross-section view of primarily the channel region layer of the transistor in Fig. 4, which illustrates how the cross-section area of the conducting channel may be varied, Fig. 6-9 are views corresponding to Fig. 1-4 illustrating different steps of a second method according to the invention for producing a field controlled thyristor illustrated in Fig. 9, Fig. 10 is a schematic cross-section view of a field controlled thyristor according to a third pre¬ ferred embodiment of the invention, Fig. 11 is an enlarged cross-section view of the channel region layer of the thyristor according to Fig. 10 illustrating how the cross-section area of the conducting channel may be varied, Fig. 12 is a schematic cross-section view of a field controlled thyristor according to a fourth pre¬ ferred embodiment of the invention, Fig. 13 is a schematic cross-section view of a field controlled transistor according to a fifth pre¬ ferred embodiment of the invention and in which a way of varying the cross-section of the con¬ ducting channel has been illustrated, and Fig. 14 is a schematic cross-section view of a MISFET according to a sixth preferred embodiment of the invention, in which it is illustrated how the conducting channel may be formed and the cross- section area thereof may be varied. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A method for producing a field controlled transistor having a channel region layer with a substantial lateral extension will now be described with reference to Fig. 1-4. Conven¬ tional semiconductor device producing steps having nothing to do with the invention, such as masking, demasking and so on, have for the sake of clearness not been shown in the figures. First of all a highly doped n-type substrate layer 1 and on top thereof a low doped n-type drift layer 2 are epitaxially grown (see Fig. 4). After that p-type dopants, preferably aluminium or boron, are implanted into the sur¬ face layer of said drift layer for forming a highly doped p- type base layer 3. That step is followed by a step of epi¬ taxial growth (see Fig. 2) of a low doped n-type channel re¬ gion layer 4 on top of said base layer. This channel region layer 4 has preferably a very low doping concentration, so that it may easily be depleted for forming a blocking deple- tion layer therein, which will be described further below. This epitaxial growth is preferably carried out by using the Chemical Vapour Deposition technique involving a high tem¬ perature, preferably above 1500°C, and an annealing of the implanted layer 3 may be carried out in connection thereto by raising the temperature to 1700°C or more for making the dopants implanted electrically active. After that a MESA- etch is carried out in said channel region layer 4 for form¬ ing a wall 5 extending to the drift layer 2. Then are n- type dopants implanted into the surface layer of said chan- nel region layer 4 for forming a highly doped n-type source region layer 6 and n-type dopants are implanted into the channel region layer 4 at a lateral distance from the source region layer and in connection with said wall and into said wall and into a surface layer of the drift layer for forming an additional n-type highly doped layer 7 extending from the surface of the channel region layer 4 to the drift layer 2. This step is necessarily followed by an annealing step for making said n-type dopants implanted electrically active. The implantation for forming the source region layer 6 may as an alternative be carried out separately before the etch- ing step. An insulating layer 8, for instance of Siθ2, with a gate electrode 9 thereon is applied on the channel region layer with an extension between the source region layer and the additional n-type layer. Said gate electrode is preferably of poly-crystalline silicon. After that a passivating addi¬ tional insulating layer 10 is applied over the gate elec¬ trode and the upper surface of the device leaving an area of said source region layer 6 exposed and a source 11 is then applied on the layer 6. Finally, a drain contact 12 is ap¬ plied to the substrate layer 1. The function of this field controlled transistor will now be described with reference made to Fig. 5. This transistor may be of normally-off as well as normally-on type. We do now assume that it is of the normally-off type, which may be achieved by appropriately choosing the thickness and doping concentration of the channel region layer 4, the gate elec¬ trode material and the doping concentration of said base layer 3. A lower doping concentration of the channel region layer 4 and a higher doping concentration of the base layer 3 as well as an appropriate choice of the gate electrode ma¬ terial will make it possible to increase the thickness of the channel region layer and still obtain a normally-off de- vice. The gate electrode 9 will deplete the channel region layer located thereunder according to a dashed line having the lower extension 14 when a zero potential is laid on the gate electrode 9 and the base layer 3 from below according to the dashed line 13, so that a depleted layer 16 is formed between the source region layer 6 and the additional n-type layer 7 blocking an electron transport from the source re- gion layer to the drift layer. Application of a positive voltage to the gate electrode 9 will move the lower exten¬ sion 14 of the layer depleted by the gate electrode up¬ wardly, and the extension 15 corresponds to a certain level of the voltage applied on the gate electrode . In this way a lateral conducting n-type channel 17 is created between the source region layer 6 and the layer 7, and this conduct¬ ing channel is located at a considerable distance from the upper surface of the channel region layer 4 resulting in a channel mobility equaling bulk mobility due to the channel conduction being independent of the surface conditions, so that the on-state losses of such a device will be very low with respect to for instance a conventional MISFET. Thanks to the possibility to create a channel region layer 4 by epitaxial growth, which may be realised by conventional techniques thanks to the lateral extension thereof, this may be made arbitrarily thick, which means that it may be easily made very thin for the production of a normally-off device. Such a device may of course also be made as a normally-on device, so that a voltage of an opposite sign to that ac¬ cording to above has to be applied to the gate electrode for eliminating the conducting channel 17. Fig. 6-9 illustrates the different steps for producing a field controlled thyristor according to the second preferred embodiment of the invention, and this method comprises the same steps as the method described above with reference to Fig. 1-4 except for the initial growth of a highly doped p- type substrate layer 1 and a highly doped n-type buffer layer 18 (this layer is optional) on top thereof, and the additional step illustrated in Fig. 8 of etching a trench 19 into the channel region layer 4 for forming a wall 20. The step of implanting n-type dopants for forming a source re¬ gion layer 6 is then carried out by implanting said n-type dopants into the channel region layer 4 into the wall 20 thereof and laterally in the direction of the additional highly doped n-type layer 7 while leaving a lateral distance between the two layers 6 and 7. Furthermore, p-type dopants are implanted in the bottom of the trench 19 for forming an additional highly doped p-type layer 21 in a surface region of the base layer 3. When the source is applied this is also applied in contact with said additional p-type layer 21. This additional layer 21 will collect the holes injected into said drift layer 2 from the substrate layer 1 as a con¬ sequence of the electron current flowing from the source to the drain in the on-state of the device. A field controlled transistor may be produced in the same way with the excep¬ tion that the p-type substrate layer is replaced by a n-type substrate layer and the buffer layer will not be there. It would then also be possible to omit the additional highly doped p-type layer. The function of this device with respect to the formation of a lateral conducting channel in the channel region layer near the interface to the base layer will be the same as discussed above for the field controlled semiconductor device shown in Fig. 4. Fig. 10 illustrates a field controlled thyristor according to a third preferred embodiment of the invention, which has been produced by carrying out the following steps: epitax¬ ially growing on top of each other the following semiconduc- tor layers of SiC: a highly doped p-type substrate layer 1, a highly doped n-type buffer layer 18 and a low doped n-type drift layer 2. A groove 22 is then etched into the drift layer. Two devices produced in a mirror image relation are shown in Fig. 10, so that two such grooves 22 are realised. A p-type base layer 3 is epitaxially grown in said groove, and after that is a low doped n-type channel region layer 4 epitaxially grown on top of the base layer and a surface of said drift layer located laterally to said base layer. After that said channel region layer is etched away at a lateral distance from a first lateral border 23 of the base layer 3. n-type dopants are after that implanted into said channel region layer at a lateral distance from said border and in connection with a wall formed by the etching and into said wall for forming a highly doped n-type source region layer 6. p-type dopants are after that implanted into a surface layer of the base layer 3 for forming an additional highly doped p-type layer 21. Then an insulating layer 8 with a gate electrode 9 thereon are applied on said channel region layer between the source region layer and a second lateral border 24 of the base layer opposite to said first one. Fi- nally, a passivating additional insulating layer 10 is ap¬ plied over the gate electrode and the upper surface of the device leaving an area of said source region layer 6 and said highly doped additional p-type layer 21 exposed, and a source 11 is then applied on these two layers. The layers 6 and 21 could alternatively be created by epitaxial growth instead of implantation. The function of this device, which by leaving out the lower layer 1 could be a transistor instead of a thyristor, will appear from the previous description of the other preferred embodiments of the invention, but it is illustrated in Fig. 11 how a conducting channel 17 for conducting electrons from the source region layer to the drift layer may be created and how the width thereof may be varied by varying the volt- age applied to the gate electrode. It is indicated how a de¬ pleted layer 16 is formed and how this will cut the path for electrons from the source region layer 6 to the drift layer off in a position 14 of the portion depleted by the gate corresponding to a certain voltage applied to the gate elec- trode 9, which in the normally-off case will be a zero po¬ tential, and a conducting lateral channel 17 is formed in another position 15 corresponding to another voltage applied to the gate electrode. The advantages of this structure ap¬ pear from the description above and are strongly depending on the fact that the low doped channel region layer is cre¬ ated by epitaxial growth. Fig. 12 illustrates a field controlled thyristor according to a fourth preferred embodiment of the invention, which may be produced by a method with the same initial steps as the method previously described, but after the epitaxial growth of the base layer 3 a second groove 25 is etched into the base layer at a lateral distance from a first lateral border 23 thereof and beyond an opposite second lateral border 24 thereof. After that a low doped n-type channel region layer 4 is grown into this second groove 25. The rest of the steps will be the same as in the method previously described, with the exception that the source region layer 6 will have a lateral extension due to the planar structure. Two different extensions of a blocking depleted layer 16 formed in the channel region layer corresponding to two different voltages applied to the gate electrode 9 are indicated by dashed lines, and the function of the device is clear from what has been said above. A field controlled transistor according to a fifth preferred embodiment of the invention is schematically illustrated in Fig. 13. In the method for producing this transistor the substrate layer 1, the drift layer 2 and the highly doped p- type base layer 3 are epitaxially grown on top of each other. After that a groove 26 is MESA-etched through the base layer to the drift layer. A low doped n-type channel region layer is after that epitaxilly grown on top of the base layer and in said groove 26. The channel region layer is etched for forming a wall 5 and a highly doped n-type source region layer 6 is created by implantation into said wall. The rest of the steps are as described above, and the different elements has been indicated by the same reference numbers as before. A depletion layer in two different posi¬ tions corresponding to two different voltages applied to the gate electrode 9 as well as a lateral conducting channel 17 created in one of the positions are indicated by dashed lines. Fig. 14 illustrates a MISFET according to a sixth preferred embodiment of the invention, and this is created by carrying out the following steps: A highly doped n-type substrate layer 1 and a low doped n-type drift layer 2 are epitaxially grown on top of each other. A buried p-type base layer 3, which preferably is highly doped, is then created by im- planting p-type dopants with such an energy that a buried implanted base layer is formed at a distance from the upper surface of the drift layer or better by implanting p-type dopants in a surface layer of said drift layer and after that epitaxially growing a further portion of said drift layer thereupon for forming a buried p-type base layer. This further portion may then preferably be made lower doped than the rest of the drift layer. The base layer is formed so that a vertical portion 27 of said drift layer 2 passes it laterally on a first side 28 thereof, n-type dopants are af- ter that implanted into a surface layer of said drift layer over said base layer at a distance from said first side for forming a highly doped source region layer 6, whereupon an insulating layer 8 with a gate electrode 9 thereon are ap¬ plied on said portion of said drift layer located above said p-type base layer, and a source 11 is applied on said source region layer. A mask oxide layer 29 is also shown in the figure. By appropriately doping the base layer 3 and said further portion of the drift layer, selecting the thickness of this portion 30 of the drift layer between the base layer and the insulating layer as well as choosing the gate this device may be normally-off, i.e. a p-type depletion layer will be formed in said portion 30 and covering the entire cross-section thereof. If a voltage over a threshold voltage is applied to the gate electrode a conducting lateral chan- nel will be created from the source region layer to the ver¬ tical portion 27 of the drift layer 2 along and at a dis- tance above the interface between said portion 30 and the base layer 3. As a bulk channel this channel will have a bulk carrier mobility rather than the degraded mobility of an inversion channel formed at the interface to the insulat- ing layer, where there will be a considerably higher trap density. Thus, the on-state resistance of this device will be considerably lower than of a conventional MISFET. MISFET is here defined as covering field effect transistors having an insulated gate, such as MOSFETs. The low doped n-type channel region layer in the devices ac¬ cording to the different embodiments of the invention de¬ scribed above will, except from in the embodiment described in Fig. 14, preferably have a doping concentration below or well below that of the drift layer, so that it will easily be depleted for controlling the conductivity thereof and making it possible to form a normally-off device. This dop¬ ing concentration may go down to the order of lO^^cm-^ or even lower. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibili¬ ties to modifications thereof will be apparent for a man skilled in the art without departing from the basic idea of the invention. It is emphasised that the thicknesses of different layers in the figures can not be interpreted as limiting the scope of the protection, but any thickness relations are intended to be covered by the claims. The dopants used may be of any material suitable as dopants for this application. The number of layers mentioned in the claims is a minimum number, and it is within the scope of the invention to ar¬ range further layers in the devices or dividing any layer into several layers by selective doping of different regions thereof. Especially the drift layer may be composed by sub¬ layers of different doping concentration, such as particu¬ larly low doping concentration close to the base layer. Although it has been shown in the figures that the gate electrode is insulated, the invention is not restricted thereto, but it may also be diffused, i.e. a so-called bipo¬ lar gate being highly doped, which however is not that ad¬ vantageous, since it will then take current. "Substrate layer" is in this disclosure to be inter¬ preted as the layer closest to the drain of the layers mentioned and it has not to be a substrate layer in the strict sense of this word within this field, i.e. the layer from which the growth is started. The real sub- strate layer may be any of the layers and is mostly the thickest one, which may be the drift layer. The method claims are to be interpreted as not re¬ stricted to a growth of the layers located on top of each other in the order they are mentioned, but any other order of growth of these layers is within the scope of the claims. For instance, the method may be started from the drift layer and the so called substrate layer and the drain may be grown at the very end of the method.

Claims: Claims 1. A field controlled semiconductor device of SiC comprising superimposed in the order mentioned at least a drain (12), a highly doped substrate layer (1) and a low doped n-type drift layer (2), said device further comprising a highly doped n-type source region layer (6) and a source (11) con¬ nected thereto, a doped channel region layer (4) connecting said source region layer to said drift layer and through which a current is intended to flow when the device is in an on-state, and a gate electrode (9) arranged to, upon apply¬ ing a voltage thereto, influence the charge carrier distri¬ bution of said channel region layer and by that the conduc¬ tivity thereof, characterized in that said channel region layer has a substantially lateral extension and is formed by a low doped n-type layer, that it further comprises a p-type base layer (3) arranged under the low doped n-type channel region layer (4) on the opposite side thereof with respect to the gate electrode (9) for forming a n-type conducting channel (17) in the channel region layer at a distance from an upper surface of said channel region layer, and that the gate electrode is arranged to influence said channel region from above for giving a conducting channel (17) created therein from the source region layer to the drift layer a substantially lateral extension. 2. A device according to claim 1, characterized in that said p-type base layer (3) is highly doped. 3. A device according to claim 1 or 2, characterized in that said gate electrode ( ) is insulated with respect to the channel region layer by an interposed insulating layer (8) . 4. A device according to claim 1 or 2, characterized in that the thickness and doping concentration of the low doped channel region layer (4) are co-ordinated with the doping concentration of said base layer (3) and the choice of the gate so as to form a p-type depletion region (16) in the channel region layer totally blocking the passage of elec¬ trons from said source region layer to the drift layer when a zero potential is laid on the gate electrode for making the device a normally-off device. 5. A device according to claim 1 or 2 and 3, characterized in that it is a MISFET having a highly doped n-type sub- strate layer (1), that said low doped n-type channel region layer is a part of the drift layer (2) extending laterally in between the insulating layer and said base layer (3) , which is buried in the drift layer, for forming, upon apply¬ ing a voltage to said gate electrode, a conducting channel (17) from the source region layer (6) to the drift layer (2) at the interface between the base layer and said channel re¬ gion layer at a distance from said insulating layer (8) . 6. A device according to any of claims 1-4, characterized in that it has a highly doped n-type substrate layer (1) and is a transistor. 7. A device according to any of claims 1-4, characterized in that it is a thyristor and that it has a highly doped p- type substrate layer (1) and one of a) a highly doped n-type buffer layer and b) no such layer between said substrate layer and the drift layer. 8. A device according to any of claims 1-4, 6 and 7, charac- terized in that it comprises an additional highly doped n- type layer (7) arranged to connect said low doped n-type channel region layer (4) to the drift layer (2) . 9. A device according to claim 8, characterized in that the source region layer (6) and said additional n-type layer (7) are arranged at a mutual lateral distance leaving a part of said channel region layer exposed to a voltage of said gate electrode (9) therebetween. 10. A device according to any of claims 6-9, characterized in that it comprises an additional highly doped p-type layer (21) arranged laterally with respect to the low doped n-type channel region layer (4) and being in contact with the source (11). 11. A method for producing a field controlled semiconductor device of SiC being one of a) a transistor and b) a thyris¬ tor, comprising the steps of: 1) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped sub- strate layer (1) being for a) of n-type and for b) of p- "type, for b) on top thereof one of c) a highly doped n- type buffer layer (18) and d) no such layer, and a low doped n-type drift layer (2), 2) implanting p-type dopants into a surface layer of said drift layer for forming a p-type base layer (3), 3) epitaxially growing a low doped n-type channel region layer (4) on top of said base layer, 4) etching away said channel region layer for forming a wall (5) extending to the drift layer, 5) one of a) implanting n-type dopants into a surface layer (4) of said channel region layer and b) epitax¬ ially growing a n-type layer onto said channel region layer, for forming a highly doped n-type source region layer (6), 6) one of a) implanting n-type dopants into said channel region layer (4) at a lateral distance from said source region layer (6) and in connection with said wall and into said wall (5) and b) epitaxially growing a n-type layer onto said channel region layer at a lateral dis- tance from said source region layer and in connection with said wall and on said wall, for forming an addi- tional n-type layer (7) extending from said channel re¬ gion layer to the drift layer, 7) applying a gate electrode (9) on said channel region layer between the source region layer and said addi- tional n-type layer and a source (11) on said source re¬ gion layer, the steps 4) and 5) being carried out in one of the follow¬ ing orders: a) 4) and then 5) and b) 5) and then 4) . 12. A method according to claim 11, characterized in that step 4) is carried out before step 5) and steps 5) and 6) are carried out at the same time as one single process step. 13. A method for producing a field controlled semiconductor device of SiC being one of a) a transistor and b) a thyris¬ tor, comprising the steps of 1) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped sub¬ strate layer (1) being tor a) of n-type and for b) of p- type, for b) on top thereof one of c) a highly doped n- type buffer layer (18) and d) no such layer, and a low doped n-type drift layer (2), 2) etching a groove (22) into said drift layer, 3) epitaxially growing a p-type base layer (3) in said groove, 4) epitaxially growing a low doped n-type channel region layer (4) on top of said base layer and a surface of said drift layer (2) located laterally to said base layer, 5) etching away said channel region layer (4) at a lat¬ eral distance from one lateral border of said base layer while forming a wall of the channel region layer, 6) one of a) implanting n-type dopants into said channel region layer at a lateral distance from said border and in connection with said wall (5) and into said wall and b) epitaxially growing a n-type layer on said channel region layer at a lateral distance from said border and in connection with said wall and onto said wall, for forming a highly doped n-type source region layer (6) , 7) applying a gate electrode on said channel region layer in the lateral direction between the source region layer and said border and a source (11) on said source region layer. 14. A method for producing a field controlled semiconductor device of SiC being one of a) a transistor and b) a thyris¬ tor, comprising the steps of: 1 ) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped sub¬ strate layer (1) being for a) of n-type and for b) of p- type, for b) on top thereof one of c) a highly doped n- type buffer layer (18) and d) no such layer, and a low doped n-type drift layer (2), 2) etching a groove (22) into said drift layer, 3) epitaxially growing a p-type base layer (3) in said groove, 4) etching a second groove (25) into said base layer at a lateral distance from a first lateral border (23 ) thereof and beyond an opposite second lateral border (24) thereof, 5) epitaxially growing a low doped n-type channel region layer (4) into the second groove, 6) one of a) implanting n-type dopants into a surface layer of said channel region layer between said two lat¬ eral borders of the base layer and b) epitaxially growi- ng a n-type layer on top of said channel region layer between said lateral borders of the base layer for form¬ ing a highly doped n-type source region layer (6), 7) applying a gate electrode (9) on said channel region layer (4) between said source region layer (6) and said second lateral border (24) of the base layer and a source (11) on said source region layer. 15. A method according to any of claims 11-14, characterized in that it comprises the step of one of a) implanting p-type dopants into a surface layer of said base layer (3) and b) epitaxially growing a p-type layer on said base layer, for forming an additional highly doped p-type layer (21 ) located laterally on the opposite side of the source region layer with respect to said channel region layer to be influenced by said gate electrode, and that the source (11) is applied also in contact with this additional p-type layer. 16. A method according to any of claims 11-15, characterized in that an insulating layer (8) is applied on the channel region layer (4) and the gate electrode (9) is applied on said insulating layer. 17. A method for producing a field controlled semiconductor device of SiC in the form of a MISFET, comprising the steps of: 1) epitaxially growing on top of each other the follow¬ ing semiconductor layers of SiC: a highly doped n-type substrate layer (1) and a low doped n-type drift layer (2), 2) forming a buried p-type base layer (3) in the drift layer by one of a) implanting p-type dopants with such an energy that a buried implanted base layer is formed at a distance from the upper surface of the drift layer and b) implanting p-type dopants into a surface layer of said drift layer and after that epitaxially growing a further portion (30) of said drift layer thereupon for forming a buried p-type base layer, said base layer be¬ ing formed so that a vertical portion (27) of said drift layer (2) passes it laterally on a first side (28) , 3) implanting n-type dopants into a surface layer of said drift layer over said base layer at a distance from said first side (28) for forming a highly doped source region layer (6), 4) applying an insulating layer (8) with a gate elec¬ trode (9) thereon, on said portion (30) of said drift layer located above said p-type base layer (3) and a source (11) on said source region layer. 18. A field controlled semiconductor device of SiC formed by carrying out the method according to any of claims 11-17.

Inventors: ANDERSSON MATS, BAKOWSKI MIETEK, GUSTAFSSON ULF, HARRIS CHRISTOPHER, ANDERSSON, MATS, BAKOWSKI, MIETEK, GUSTAFSSON, ULF, HARRIS, CHRISTOPHER
Applicants: ABB RESEARCH LTD, ANDERSSON MATS, BAKOWSKI MIETEK, GUSTAFSSON ULF, HARRIS CHRISTOPHER, ABB RESEARCH LIMITED, ANDERSSON, MATS, BAKOWSKI, MIETEK, GUSTAFSSON, ULF, HARRIS, CHRISTOPHER
Application year: 1997
Main classification: H01L
IPC codes: H01L21, H01L29