TRANSISTOR HAVING A VERTICAL CHANNEL

Abstract: An IGBT of SiC comprises superimposed a drain (1), a highly doped p-type substrate layer (2), a highly doped n-type buffer layer (3), a low doped n-type drift layer (4), a highly doped p-type base layer (5), a highly doped n-type source region layer (6) and source (7). The transistor also comprises a vertical trench (8) extending through the source region layer and the base layer and to the drift layer. It also comprises an additional low doped p-type layer (13) arranged laterally to the base layer, connecting it to an insulating layer (11) and extending vertically at least over the extension of the base layer. A gate electrode (12) is applied on the insulating layer for, upon applying a voltage to the gate electrode, forming a conducting inversion channel at the interface between said additional layer (13) and the insulating layer for electron transport from the source to the drain.

Description: TRANSISTOR HAVING A VERTICAL CHANNEL TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, said transistor comprising superimposed in the order mentioned a drain, a highly doped substrate 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, a low doped n-type drift layer, a p-type base layer, a highly doped n-type source region layer and a source, said transistor further comprising a vertical trench extending through the source region layer and the base layer and at least to said drift layer and having a lateral wall next to these layers, said transistor further compris¬ ing a gate electrode extending vertically along said wall at least over the vertical extension of said base layer and an insulating layer arranged between the gate electrode and at least said base layer, for, upon applying a voltage to the gate electrode, form a conducting inversion channel at the interface of the SiC to said insulating layer for electron transport from the source to the drain, as well as a method for producing such a transistor. Such transistors of SiC may especially be used as switching devices in power applications, where they are very advanta¬ geous, owing to the possibility to turn them on and off very fast. Such transistors made of SiC as semiconductor material are especially well suited for high power applications, since, if it would be possible to fully benefit from the inherent properties of SiC, namely the capability of SiC to function well under extreme conditions, such devices would be superior to devices made of Si for high power applications. Besides the possible high operation temperatures of devices fabricated from SiC and the high thermal conductivity of SiC, SiC also has a more than five times higher breakdown field than Si, so that it is well suited as a material in high power devices operating under conditions where high voltages may occur in the blocking state of a device, and an IGBT made of this material will have particularly low conduction losses in the on-state compared to a MISFET, but the invention comprises also such MISFET 's. A major example of drawbacks for IGBTs already known arises from the fact that an IGBT has three pn-junctions in series and thereby a thyristor-like structure, which results in problems with so called latch-up in the on-state thereof, and this problem as well as other drawbacks related thereto and also to MISFET' s of this type will now be explained while referring to Figure 1 of the appended drawings illustrating a prior art IGBT having superimposed a drain 1', a highly doped p-type substrate layer 2' for forming a good ohmic contact to said drain, a highly doped n-type buffer layer 3', a low doped n-type drift layer 4' , a p- type base layer 5', a highly doped n-type source region layer 6' and a source 7' . The device has also a trench 8' carried out through the source region layer and the base layer and into the drift layer, and a wall 9' of the trench is covered with an insulating layer 11 ' , and said insulat¬ ing layer and the bottom 10' of the trench are covered with a gate electrode 12' on top thereof. By applying a positive voltage above a threshold voltage value to the gate elec¬ trode 12' a conducting inversion channel, i.e. an n-type channel, may be created in the base layer 5' at the inter¬ face between the base layer 5' and the insulating layer 11' for conduction of electrons from said source region layer 6' to the drift layer 4' and turning the device on. The de- vice may very fast be turned off by cutting off the voltage supply to said gate electrode. This is the normal operation of the IGBT, but this function is only there within a so called safe operating area (SOA), and outside this area the following mechanism will appear. Since an IGBT has a highly doped p-type substrate layer 2 ' , the electron current flowing through said inversion channel and towards the drain will cause a substantial hole injection from the substrate layer 2' into the drift layer 4' . The holes move across the drift layer taking a variety of paths and reach the base layer 5', and they will move towards the source 7' for recombining with electrons from the source. Some of these holes will take paths having a partially lateral extension, and these holes will feel a lateral spreading resistance of the base layer, which in its turn will result in a lateral voltage drop in the base layer along the junction between the base layer 5' and the source region layer 6' . This will tend to forward bias said junction, and if the voltage drop is large enough substantial injection of electrons from the source region layer into the base layer will occur and a parasitic thyristor composed of the two parasitic npn- and pnp-transistors created in this way will latch on and a "latch-up" of the IGBT has then oc¬ curred. Once the IGBT is in latchup, the gate no longer has any control of the drain current. The IGBT may then only be turned off in the same way as a conventional thyristor. If the latchup is not terminated quickly, the IGBT will be de¬ stroyed by excessive power dissipation. There is a critical value of drain current which will cause a large enough lateral voltage drop to activate said thyristor. Thus, this will mean a severe restriction of the safe operating area of such an IGBT, so that the advantageous features of SiC, especially the ability to withstand high temperatures and the high thermal conductivity thereof, may not be utilized as desired for high power applications. One possible solution to this problem is to keep the p-base resistance low, i.e. by doping the base layer highly, so that the safe operating area of the device may be extended and higher drain currents may be allowed. However, this so¬ lution presents severe problems to the formation of an inversion channel at the interface between the insulating layer and the base layer, since the threshold voltage value to be exceeded will be very high and accordingly limit the charge carrier mobility in said channel and thereby in¬ creasing the on-state restistance of the device. A MISFET does not suffer from the latch-up mechanism described above, however, there is also a trade off in doping level of the p base layer. The restriction of doping density imposed by threshold voltage and channel mobility implies that the base layer is necessarily thicker to support high voltage. SUMMARY OF THE INVENTION The object of the present invention is to provide a tran- sistor of the type defined in the introduction, in which the drawback mentioned above is reduced to a large extent. This object is in accordance with the invention obtained by providing such a transistor with an additional low doped p- type layer arranged laterally to the base layer, connecting it to the insulating layer and extending vertically at least over the extension of the base layer, and by making said base layer highly doped. The low doping concentration of said additional layer, in which a vertical conducting inversion channel will be formed upon applying a voltage to the gate electrode, allows a low necessary gate voltage, i.e. threshold voltage value, to produce the inversion channel. Thus, this low doping implies a higher channel mobility and hence a larger possible forward current. The base layer may therefore be highly doped without any negative influence upon the on- state losses of the device, so that the base layer may be given a low resistivity extending the safe operating area of the device to higher drain currents, i.e. the operation area in which the thyristor action of the device is pre¬ vented. These advantages are equally applicable to both an IGBT and a MISFET. "Highly doped" and "low doped" are in connection with said base layer and said lateral layer, respectively, to be interpreted broadly and cover all cases in which the doping concentration of said lateral layer is substantially lower than the doping concentration of said base layer. Accord¬ ingly, these claim definitions only state that this rela- tion is present. According to another preferred embodiment of the invention said trench extends into said drift layer. This facilitates the proper application of the gate electrode and still ensuring a passivating isolation of the trench bottom. A further object of the invention is to provide a method for producing a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, for which the drawback mentioned of prior art transistors is considerably reduced. This object is in accordance with the invention obtained by providing such a method comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped p-type 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, a low doped n-type drift layer and a highly doped p-type base layer, 2 ) etching at least through said base layer for forming a trench having a wall extending vertically, 3 ) epitaxially growing an additional low doped p-type layer on said wall at least over the vertical extension of the base layer, 4) one of a) implanting n-type dopants in the additional layer for forming a highly doped n-type source region layer therein and b) epitaxially growing such a source reigon layer on top of said additional layer, 5 ) applying an insulating layer with a gate electrode thereon on said additional layer at least over the vertical extension of the base layer and a source on said source re¬ gion layer. This object is also obtained by providing such a method comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate 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, a low doped n-type drift layer, a highly doped p-type base layer and a highly doped n-type source region layer, 2 ) etching at least through said source region layer and said base layer for forming a trench having a wall extend¬ ing vertically, 3) epitaxially growing an additional low doped p-type layer on said wall at least over the vertical extension of the base layer, 4) applying an insulating layer with a gate electrode thereon on said additional layer at least over the vertical extension of the base layer and a source on said source re¬ gion layer. The use of lateral regrowth in this way to produce said ad- ditional low doped layer, in which the conducting channel is created is the key to obtaining a transistor having the preferred properties discussed above. This lateral regrowth also means that the SiC surface forming the interface to the insulating layer is created by epitaxial growth and will thereby have a considerably lower density of traps with respect to prior art transistors having an insulated gate, in which said interface is provided by a surface created through etching, so that the mobility of charge carriers in the channel will be increased and thereby the on-state losses be reduced. Further advantages and preferred features of the invention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a specific description of preferred embodiments of the invention cited as examples. In the drawings: Fig 1 is a schematic cross-section view of an IGBT accord¬ ing to the prior art, Fig 2 is a cross-section view corresponding to that of Fig 1 of an IGBT according to a first preferred embodiment of the invention, and Fig 3 is a cross-section view corresponding to Fig 2 of an IGBT according to a second preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig 2 illustrates an insulated gate bipolar transistor (IGBT) according to a first preferred embodiment of the in¬ vention, and the parts corresponding to the parts of the prior art transistor described above are provided with the same reference numerals with the index ' removed, so that the transistor has superimposed a drain 1, a highly doped p-type substrate layer 2 for forming a good ohmic contact to the drain, a highly doped n-type buffer layer 3 arranged for preventing a reach-through of the depletion layer to the substrate layer (this layer is optional), a low dope n- type drift layer 4, a p-type base layer 5, which here is highly doped, a highly doped n-type source region layer 6 and a source 7. A trench 8 is carried out through etching through the source region layer and the base layer and into the drift layer. An additional low dope p-type layer 13 is epitaxially grown, preferably by using the CVD-technique, en a wall 9 of said trench and here also on top of a part of the base layer. The source reigon layer is located on a lateral part of this additional layer, possibly formed by carrying out an implantation step in the additional layer 13. An insulating layer 11, for instance of Siθ2, is applied in the trench on said additional layer 13. Finally, a gate electrode 12, preferably of poly-crystalline sili¬ con, is applied on the insulating layer. The function of the insulated gate bipolar transistor according to Fig 2 will be as follows: upon application of a voltage over a certain threshold voltage level on the gate electrode 12 a vertical conducting inversion channel will be created at the interface between the additional low doped layer 13 and the insulating layer 11 for transport of electrons between the source region layer 6 and the drift layer 4 and turn the transistor on if a forward bias is ap¬ plied over the source and drain thereof. Thus, an electron flow from the source 7 to the drain 1 will result. This electron flow will cause hole injection from the substrate layer 2 into the drift layer 4. These holes will try to reach the source for recombining with the electrons thereof. These holes will reach the base layer 5, which thanks to the high doping thereof will have a low resistiv¬ ity, so that the movement of these holes in partial lateral paths in the base layer will cause only low voltage drops in the lateral direction along the source region layer. Furthermore, only a low positive voltage has to be applied to the gate for creating the inversion channel as a conse¬ quence of the low doping of the additional layer 13, in which the inversion channel is created. This means a high mobility in the inversion channel and low on-state losses of the device, which situation is further improved by the fact that the layer 13 is formed by a lateral regrowth, which means that the surface thereof will be of a higher quality, i.e. have a lower trap density, than an etched surface. Accordingly, thanks to the low doping of the additional layer 13 the base layer may be highly doped without causing any problem with a high threshold voltage value. The doping concentration of the lateral layer 13 may be as low as lθ cm ~ 3 or even lower and the doping concentration of the base layer may be as high as lO^-lO^l C m ~ 3, but the invention is not in any way restricted to these figures. Fig 3 shows an IGBT according to a second preferred embodi¬ ment of the invention, which differs from that shown in Fig 2 by the fact that the trench 8 was carried out after the epitaxial growth of the base layer 5 and the source region layer 6 and the layer 13 was then grown on the wall 9 formed by the base layer and the source region layer 6. The rest of the steps for producing this IGBT are the same as for the IGBT according to Fig 2. The function of this IGBT will also be substantially the same as of the one shown in Fig 2, but Fig 3 is only shown for indicating that the embodiment shown in Fig 2 may be varied in different ways within the scope of the invention. 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 to a man skilled in the art without departing from the basic idea of the invention. It is emphasized that the thicknesses of the different layers in the Figures cannot be interpreted as limiting the scope of protection, but any thickness relations are intended to be covered by the claims. Furthermore, the definition of "superimposed" in the claims is to be interpreted broadly and means that there is a contact in the vertical direction between such subsequent layers. The dopants used may be of any material suitable as dopants for this application. The independent patent claims concerning the methods for producing an insulated gate bipolar transistor do of course not include every step necessary for producing such a transistor, but several conventional steps of semiconductor producing technologies will also be used. "Trench" as defined in the claims only means that a part of the semiconductor layers of SiC is etched away, but this "trench" does not necessarily have to have a bottom with a considerable extension. Furthermore, "vertical" means that the vertical component of the direction in question is substantially larger than the horizontal component. The number of layers mentioned in the claims is a minimum number, and it is within the scope of the invention to arrange further layers in the devices or dividing any layer into several layers by selective doping of different regions thereof. "Substrate layer" is in this disclosure to be interpreted 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 substrate 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 restricted 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 whi hin the scope of the claims. For in- stance, 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 transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, said transistor comprising superimposed in the order mentioned a drain ( 1 ) , a highly doped substrate layer (2) 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 (3) and d) no such layer, a low doped n-type drift layer (4), a p-type base layer (5), a highly doped n-type source region layer (6) and a source (7), said transistor further comprising a vertical trench ( 8 ) extending through the source region layer and the base layer and at least to said drift layer and having a wall (9) next to these layers, said transistor further comprising a gate electrode (12) extending vertically along said lateral wall at least over the vertical extension of said base layer and an insulating layer (11) arranged between the gate electrode and at least said base layer, for, upon applying a voltage to the gate electrode, form a conducting inversion channel at the interface of the SiC to said insulating layer for electron transport from the source to the drain, characterized in that it comprises an additional low doped p-type layer (13) arranged laterally to the base layer (5) connecting it to the insulating layer and extending vertically at least over the extension of the base layer, and that said base layer is highly doped. 2. A transistor according to claim 1, characterized in that said trench (8) extends into said drift layer (4). 3. A transistor according to claim 1 or 2, characterized in that said insulating layer (11) and the gate electrode (12) extend over substantially the entire wall (9) of the trench. 4. A transistor according to any of claims 1-3, charac¬ terized in that said insulating layer (11) covers substan¬ tially the entire trench (8). 5. A transistor according to any of claims 1-4, charac¬ terized in that said additional p-type layer (13) extends vertically over substantially the entire extension of the trench ( 8 ) . 6. A method for producing a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer (2) 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 (3) and d) no such layer, a low doped n-type drift layer (4) and highly doped p-type base layer (5), 2 ) etching at least through said base layer for forming a trench (8) having a wall (9) extending vertically, 3) epitaxially growing an additional low doped p-type layer ( 13 ) on said wall at least over the vertical extension of the base layer, 4) one of a) implanting n-type dopants in the additional layer for forming a highly doped n-type source region layer (6) therein and b) epitaxially growing such a source region layer on top of said additional layer, §) applying an insulating layer (11) with a gate electrode (12) thereon on said additional layer (13) at least over the vertical extension of the base layer and a source (7) on said source region layer. 7. A method for producing a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer (2) 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 (3) and d) no such layer, a low doped n-type drift layer (4), a highly doped p-type base layer (5) and a highly doped n-type source region layer (6), 2) etching at least through said source region layer and said base layer for forming a trench (8) having a wall (9) extending vertically, 3 ) epitaxially growing an additional low doped p-type layer (13) on said wall at least over the vertical extension of the base layer, 4) applying an insulating layer (11) with a gate electrode (12) thereon on said additional layer at least over the vertical extension of the base layer and a source ( 7 ) on said source region layer. 8. A method according to claim 6 or 7, characterized in that said epitaxial growth of the different layers is carried out by using the Chemical Vapour Deposition (CVD) technique. 9. A method according to any of claims 6-8, characterized in that said additional layer (13) is grown on the whole wall (9) of the trench (8). 10. A method according to any of claims 6-9, characterized in that said etching is carried out so that the vertical trench extends into said drift layer.

Inventors: HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREI, HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREI
Applicants: ABB RESEARCH LTD, HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREI, ABB RESEARCH LIMITED, HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREI
Application year: 1997
Main classification: H01L
IPC codes: H01L21, H01L29