A BIPOLAR SEMICONDUCTOR DEVICE HAVING SEMICONDUCTOR LAYERS OF SiC AND A METHOD FOR PRODUCING A SEMICONDUCTOR DEVICE OF SiC

Abstract: A bipolar semiconductor device having semiconductor layers of SiC and at least one pn-junction with charge carrier transport there across between two subsequent such layers in a conducting forward state of the device has a member (2) having such a resistance that it substantially lowers the current as of which the voltage drop over the device increases with the temperature and which is arranged in series with said pn-junction.

Description: A bipolar semiconductor device having semiconductor layers of SiC and a method for producing a semiconductor TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a bipolar semiconductor device having semiconductor layers of SiC and at least one pn- junction with charge carrier transport thereacross between two subsequent such layers in a conducting forward state of the device, as well as a method for producing a semiconductor device of SiC according to the preamble of the appended independent method claim. All bipolar devices are comprised, that is devices in which there will be a flow of minority charge carriers in the forward conducting state, such as an IGBT behaving like a MOS- FET in series with a diode in that state. A disadvantage of known such bipolar semiconductor devices having semiconductor layers of SiC is that they have a negative temperature coefficient of the on-state voltage drop up to very high current densities due to the fact that SiC-de- vices require a much thinner drift region due to the high dielectric strength of SiC, so that the contribution of the resistive voltage drop of the n-type layer at said junction to the total on-state voltage will be relatively smaller in a bipolar device of SiC than in such a device of for instance Si. Thus, the diode relation I(V)=I 0 (e^ v / k -l) will dominate the influence upon the total on-state voltage in a bipolar device of SiC until very high current densities, and the cross-over point where the temperature coefficient shifts from negative to positive is typically of the order of 1000 A/cm 2 in the case of SiC as compared to 50-100 A/cm 2 in the Si case. This is illustrated in Figs. 1 and 2, which show the current density J versus the total on-state voltage of corresponding bipolar devices of Si and SiC. The continuous line represents a temperature being lower than that of the dashed line, and Fig. 1 indicates that the cross-over point will be at a much lower current density for Si than for SiC, for which, as indicated in Fig. 2, the lines in question for different temperatures will still run substantially in parallel with each other in the same current density region as said cross-over point for Si is located. The reason for the great importance of securing a positive temperature coefficient of the on-state voltage drop at normal working current densities is that it is often necessary to connect several such bipolar devices, such as diodes, in parallel within one semiconductor chip or connect several small device chips in parallel with each other for realising bipolar devices, such as IGBTs, being able to together conduct a current of a certain desired intensity at a given voltage. The following will happen if the temperature coefficient is negative: it is always so that different parts within the same device have different qualities and different devices connected in parallel with each other will also have different qualities - this may never be avoided -, and this will mean that the current density will not be totally uniform through a device and from one device to another, so that spots will be created where the current density is higher. This will lead to a higher temperature in this spots and by that to a still higher current density if the temperature coefficient is negative. The conditions will be the opposite for spots having a lower current density, so that the current will be concentrated to some hot spots and the device will be very unstable and may be destroyed. If on the other, the temperature coefficient is positive, said spots having a higher current density resulting in a higher temperature will get a higher resistance, so that the current density and the temperature will go down resulting in a uniform current density and a stable operation of an assembly of parallel connected such devices without any risk of destruction by extremely high temperatures. SUMMARY OF THE INVENTION The object of the present invention is to provide a bipolar semiconductor device having semiconductor layers of SiC which will have a stable operation at normal working current densities and may be connected in parallel with other such devices to an assembly for which a stable operation is secured at such working current densities. This object is according to the invention obtained by providing such a device with a member having such a resistance that it substantially lowers the current as of which the voltage drop over the device increases with the temperature and arranging this member in series with said pn-junction. Such an extra resistance will increase the contribution of the resistive voltage drop to the total on-stage voltage drop over the device at normal working current densities, so that the temperature coefficient will shift from negative to positive at much lower current densities. It will in this way be possible to obtain a cross-over point where the temperature coefficient shifts from negative to positive for SiC in the same region as for Si. Thus, said member having a resistance will compensate for the band-gap change with the temperature in bipolar devices and take the voltage drop away from the pn-junction, so that a stable operation is secured at normal working current densities. The definition that said member is arranged in series with said pn-junction does also include the arrangement of said member outside the very device, for instance in a line connecting the device to a potential. According to a preferred embodiment of the invention said member is an extra-layer of SiC built in by epitaxy in the device and having a considerable resistance with respect to the on-state resistance of the device. By forming such a so called ballast resistance in the very device, it will be possible to in a simple way increase the contribution of the resistive voltage drop of the device to the total on-state voltage sufficiently for making the temperature coefficient of the device positive as of much lower currents than without such an extra-layer. This extra-layer needs to have a certain resistance and according to another preferred embodiment of the invention it is therefore made thick as seen in the direction of the current flow in the forward state of the device with respect to other layers of the device, and this thickness is according to another embodiment of the invention over 200 μm and preferably over 300 μm. By making the extra-layer thick it will be possible to keep the doping concentration in said layer at a moderate level, which is desired, since it may be hard to produce such an extra-layer having an extremely low doping concentration when a substrate layer from which the growth of the other layers is started is used as said extra- layer. It will be advantageous to use the substrate as said extra-layer and according to a preferred embodiment the de- vice has a doped n-type substrate layer, and said extra- layer is a sub-layer of said substrate layer. This definition also includes that the extra-layer is identical to said substrate layer. According to another preferred embodiment of the invention constituting a further development of the embodiment last mentioned a highly doped n-type layer is arranged on top of said extra-layer for efficient charge injection into a low doped n-type drift layer of the device arranged on top of said highly doped layer and between the layer last mentioned and said pn-junction of the device. In fact the pn-junction will extend into said drift layer and that is intended to be covered by this definition. It is preferred to arrange such a highly doped n-type layer on top of said extra-layer, since the extra-layer has to have a comparatively low doping concentration in order to have the resistance required for substantially lowering said cross-over point of the temperature coefficient, and this will mean that the injection of charges into the drift layer would be too inefficient would the drift layer be arranged directly on top of said extra - layer. A method for producing a semiconductor device of SiC having a low doped drift layer and a highly doped layer placed next thereto for charge injection into said drift layer is ac- cording to the invention characterised in that said highly doped layer is epitaxyally grown on top of a doped substrate layer and the drift layer is epitaxyally grown on top of said highly doped layer. This method makes it possible to form the highly doped n-type layer on top of said extra- layer when such an extra-layer is present, in a very favourable way, since this means that the interface between the drift layer and the layer next thereto will in this way be of a much higher quality than if the drift layer were grown directly upon the substrate layer. However, it is emphasised that this method is not in any way restricted to the case in which an extra-layer having a considerable resistance with respect to the on-state resistance of the device is there, but it will be advantageous for all types of semiconductor devices having a doped substrate layer and a drift layer. This method finds a remedy to the inconvenience that the total resistance per unit area in already known such devices of SiC is not given solely by the sum of the resistance represented by the drift layer, the substrate layer and the contacts of the device, but also an interface resistance increasing the total voltage drop for a certain current. A lower crystal quality in the vicinity of the interface will decrease the injection efficiency of the highly doped layer of n-type as well as of p-type, if a p- type layer is regrown, and further increase the drift layer resistance due to a higher electron-hole recombination process in this region, i.e. a shorter carrier life time. By producing said highly doped layer by epitaxyally growing it on top of a substrate layer the interface between this layer and the drift layer will be of a much higher quality than if the substrate layer were used as said highly doped layer and the drift layer were grown on top thereof. It is important to point out that the function of the highly doped layer on top of the substrate layer is to act as the active injec - ing-emitter, i.e. the substrate will only be used as device carrier. According to a very preferred embodiment of the invention said highly doped layer and said drift layer are grown on top of said substrate layer in the same growth run. This means a possibility to obtain a very high quality of the interface between these two layers resulting in the advantages mentioned above. According to another preferred embodiment of the device ac- cording to the invention said member having a resistance having a resistivity of about 0,04 Ωcm. Such a resistivity has turned out to be suitable for obtaining a positive temperature coefficient of the on-state voltage drop at normal working current densities of bipolar semiconductor devices of SiC. According to another preferred embodiment of the invention said extra-layer being a layer of another material than SiC placed on one end of the device as seen in the direction of the current flow through the device in the forward state of the device. It will in this way be possible to apply an extra-layer of a material having totally different physical properties than SiC as said member if desired, and it may in fact be desired to do so, and this is the subject of another preferred embodiment of the invention, in which said member is made of a material having a much stronger positive temperature coefficient than SiC, and the on-state resistance of said member is much lower than the resistance of the device at normal operation temperatures of the device. This means that the on-state voltage will not be increased at normal operation temperatures of the device as much as would be the case if said member having a resistance is built in by epitaxy of a layer of SiC, but when the temperature is raised the much stronger positive temperature coefficient of this material than SiC will result in a comparatively large contribution to the resistive voltage drop of the device and turning the temperature coefficient positive for much lower current densities than without this member. According to a further preferred embodiment of the invention said member is connected in series with the device at a distance therefrom. Thus, this indicates that it is within the scope of the invention to connect any type of resistor having the desired physical properties in series with the device to obtain the change of temperature coefficient aimed at, and this embodiment may advantageously be combined with the embodiment last mentioned. Further advantages and preferred features of the invention will appear from the following description and the other de- pendent claims. RRTEF 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: Figs. 1 and 2 are graphs showing the on-state voltage drop versus the current density for prior art bipolar semiconductor devices of Si and SiC, respectively, at different operation temperatures, Fig. 3 is a diode according to a preferred embodiment of the invention. Fig. 4 is a graph corresponding to the graphs of Figs. 1 and 2 for the diode in Fig. 3, Fig. 5 illustrates a diode according to a second preferred embodiment of the invention, Fig. 6 is a graph corresponding to Fig. 4 for the diode in Fig. 5, Fig. 7 illustrates a diode according to a third preferred embodiment of the invention, Fig. 8 illustrates a diode grown through a method ac- cording to a preferred embodiment of the invention, and Fig. 9 illustrates very schematically an IGBT according to a fourth preferred embodiment of the inven- tion, which has been simplified by leaving source and drain contacts as well as the gate electrode and the gate insulator and passivation layers out. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 3 illustrates a rectifier diode according to a first preferred embodiment of the invention having superimposed a highly doped n-type layer 1 for forming a low ohmic contact to a cathode electrode not shown, a low doped n-type substrate layer 2, a highly doped n-type layer 3, a very lightly doped n-type drift layer 4 and a highly doped p-type layer 5 for hole injection and for forming a good ohmic contact with an anode electrode not shown. The pn-junction is formed by the anode region 5 and the drift layer, and normally the layer 2 having a low doping concentration and being comparatively thick will not be there, but a single substrate layer having a comparatively high doping concentration will form a substitute for the layers 1-3. The addition of the layer 2, which accordingly constitutes an extra-layer of SiC built in by epitaxy, will increase the on-state resistance of the device and contribute a resistive part to the total voltage drop over the device. For obtaining this said layer 2 has to be comparatively thick, and in this case it has a thickness of 350 μm and a doping concentration of 3xlθl7cm~3, w hich will result in a resistance being suitable to move the cross over point at which the temperature coefficient shifts from negative to positive to an acceptable current density level. This will mean a resistance having a resistivity of about 0,04 Ωcm for a 2,5 kV device. It is of course also possible to obtain the same resistivity by making the doping concentration lower and the thickness of the layer 2 thinner or conversely. It may be mentioned that the layers 3 and 5 have preferably a doping concentration being so high as possible, and this will mean about lO^cm"^ The drift layer will typically have a doping concentration of about lθl5 C jτι-3 # ^- ± s preferred to have a layer 3 not belonging to the substrate as the layer making the interface to the drift layer 4, since this means that this layer, which will act as an injecting-emitter, may be grown by epi- taxy on top of the substrate 2 in the same growth run as the drift layer, so that a high quality interface will result therebetween. This means that the efficiency of the injection of charge carriers into the drift layer and the carrier life time will increase, so that the electrical properties of the device will become better by using this production method than in prior art rectifier diodes. This is also true in the case of such growing of a layer corresponding to the layer 3 for producing unipolar devices. It is shown in Fig. 4 by a continuos line 6 and a dashed line 7 how the total on-state voltage drop over the device will change with changing density of the current therethrough if the extra layer 2 would not be there and there would instead be a highly doped substrate layer as shown for the device in Fig. 5. The dashed line 7 shows said voltage versus the current density for the same temperature T^ as the line 6 of the device shown in Fig. 3, and it is illustrated that there will be an addition 8 to the on-state voltage drop of the device, which is a punishment for ar- ranging the extra-layer 2. However, it is also indicated by the dashed line 9 how the extension of the corresponding graph will be for the device in Fig. 3 for a temperature T2 being higher than the temperature T ] _. This graph may be compared to the dashed lines in Fig. 2, which there extend sub- stantially in parallel with the line 6. It is shown in Fig. 4 how the extension of this line 9 is changed with respect to the dashed lines in Fig. 2, so that the line 9 will cross the line 7 at a much lower current density than without the extra layer 2. A device according to a second preferred embodiment of the invention is shown in Fig. 5, and this device is a rectifier diode having a highly doped n-type substrate layer 10, a low doped n-type drift layer 11 and a highly doped p-type anode layer 12. Furthermore, this diode is provided with an extra- layer 13 in series with the pn-junction formed in the drift layer 11, and the layer 13 is made of another material than SiC, which preferably has a much stronger positive temperature coefficient than SiC, and the resistance of this extra layer 13 is selected to be preferably much lower than the on-state resistance of the device at normal operation temperatures of the device. This means that there will be preferably no increase of the total on-state voltage drop over the device at normal operation temperatures as shown through the dashed line 14 in Fig. 6 following the line 6 of an identical device except for the absence of the layer 13. However, as soon as the temperature is raised to a certain extent the stronger positive temperature coefficient of the layer 13 will result in an increase of the on-state voltage drop over the device being much higher with increasing current densities, which is illustrated by the dashed-dotted line 15 for a high temperature T2- Thus, the use of such an extra-layer as said member having a resistance will lead to a considerable reduction of the punishment in increased on- state voltage drop at normal operation temperatures. A rectifier diode according to a third preferred embodiment of the invention is shown in Fig. 7, and this diode is constituted in the same way as the diode shown in Fig. 5, but said extra-layer 13 is replaced by a resistor 16 located at a distance from the rest of the device. This resistor 16 may have the same characteristics as said extra layer 13 in Fig. 5 and the graphs for different temperatures may be the same as shown in Fig. 6 for the device according to Fig. 5. In this case the resistor 16 is considered to be included when speaking about the total on-state voltage drop over the device. Fig. 8 illustrates how a normal rectifier diode or a unipo- lar device may be produced by using a method according to a preferred embodiment of the invention thoroughly discussed above and which is based upon the step of regrowth of a highly doped n-type layer 17 on top of the substrate layer 10 in the same growth run as the drift layer 11 is grown. The interface between the drift layer 11 and the charge injection layer 17 will in this way be at a distance from the interface of lower quality to the substrate 10. Another bipolar semiconductor device of SiC according to a fourth preferred embodiment of the invention is shown in Fig. 9, and this is an IGBT (Insulated Gate Bipolar Transistor ) , which is formed by having superimposed a highly doped p-type layer 18, a low doped n-type extra-layer 19 corresponding to the layer 2 in the device in Fig. 3, a highly doped n-type layer 20, a very lightly doped n-type drift layer 21, a p-type base layer 22 and a highly doped n-type source region layer 23. Some other details as source, gate, gate insulating layer and so on having nothing to do with the invention are left out. The on-state characteristics of such an IGBT can be viewed to consist of two segments: a diode drop portion followed by a resistive portion. The diode voltage drop increases when the temperature is increased. This behaviour is typical for a diode, where the injection across the pn-junction becomes more favourable with in- creasing temperature. At the same time, the resistance of the second segment increases when the temperature increases. The influence of the extra-layer 19 having a resistance on the voltage over the current density curve will be the same as the corresponding influence of the layer 2 in the device according to Fig. 3. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modification thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention. It is emphasised that the thicknesses of the different layers in the figures cannot be interpreted as limiting the scope of protection, but any thickness relations are in- tended to be covered by the claims. The dopants used may be of any material suitable as dopants for this application. The independent patent claim concerning the method for producing a semiconductor device does of course not include every step necessary for producing such a device, but several conventional steps of semiconductor technologies will also be used. It is also pointed out that this method covers the production of unipolar devices as well as bipolar devices. "Substrate layer" is in this disclosure to be interpreted as a layer next to the drain with exception for a possible layer for forming a good ohmic contact to the drain of the layers mentioned and it has not to be a substrate layer in the strict sense of the 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.

Claims: Claims 1. A bipolar semiconductor device having semiconductor layers of SiC and at least one pn-junction with charge carrier transport thereacross between two subsequent such layers in a conducting forward state of the device, characterized in that a member (2, 13, 16, 19) having such a resistance that it substantially lowers the current as of which the voltage drop over the device increases with the temperature is ar- ranged in series with said pn-junction. 2. A device according to claim 1, charaπterized in that said member is an extra-layer (2, 19) of SiC built in by epitaxy in the device and having a considerable resistance with re- spect to the on-state resistance of the device. 3. A device according to claim 2, characterized in that said extra-layer (2, 19) is thick as seen in the direction of the current flow in the forward state of the device with respect to other layers of the device. 4. A device according to claim 3, characterized in that said thickness is over 200 urn and preferably over 300 μm. 5. A device according to any of claims 2-4, characteri ed in that the doping concentration of said extra-layer (2, 19) is low. 6. A device according to claim 5, characterized in that the doping concentration of said extra-layer (2, 19) is below SxlO-^cm- , preferably below 3xl0 17 cm ~3 . 7. A device according to any of claims 2-6, -characterized in that the thickness of said extra-layer (2, 19) is about 350 μm, and the doping concentration of said extra-layer is between lxl0 17 cm ~3 and 5xl0 17 cm -3 . 8. A device according to any of claims 1-7, characterized in that said member (2, 13, 16, 19) having a resistance has a resistivity of 0,02-0,08 Ω cm. 9. A device according to claim 2, characterized in that it has a doped n-type substrate layer ( 2 ) , and that said extra- layer is a sub-layer (2) of said substrate layer. 10. A device according to claim 9, characterized in that a highly doped n-type layer (3, 20) is arranged on top of said extra-layer (2, 19) for efficient charge injection into a low doped n-type drift layer (4, 21) of the device arranged on top of said highly doped layer and between the layer last mentioned and said pn-junction of the device. 11. A device according to claim 1, characterized in that said member is an extra-layer (13) being a layer of another material than SiC placed on one end of the device as seen in the direction of the current flow through the device in the forward state of the device. 12. A device according to claim 1, characterized in that said member (16) is connected in series with the device at a distance therefrom. 13. A device according to claim 11 or 12, characterized in that said member (13, 16) is made of a material having a much stronger positive temperature coefficient than SiC, and that the resistance of said member is much lower than the on-state resistance of the device at normal operation temperatures of the device. 14. A method for producing a semiconductor device of SiC having a low doped drift layer (4, 11, 21) and a highly doped layer placed next thereto for charge injection into said drift layer, characterized in that said highly doped layer is epitaxially grown on top of a doped substrate layer (2, 10, 19) and the drift layer (4, 11, 21) is epitaxially grown on top of said highly doped layer (3, 17, 20). 15. A method according to claim 14, characterized in that said highly doped layer (3, 17, 20) and said drift layer (4, 11, 21) are grown on top of said substrate layer (2, 10, 19) in the same growth run. 16. A method according to claims 14 or 15, characterized in that said highly doped layer (3, 17, 20) has the same doping type as the substrate layer (2, 10, 19). 17. A method according to any of claims 14-16, characterized in that said highly doped layer (3, 17, 20) is of n-type. 18. A method according to any of claims 14-17, characterized in that it is used for producing a bipolar semiconductor de- vice. 19. A method according to any of claims 14-18, characterized in that a p-type anode layer (5, 12) is epitaxyally grown on top of said drift layer (4, 11) being of n-type for forming a diode.

Inventors: BAKOWSKI MIETEK, BLEICHNER HARRY, GUSTAVSSON ULF, BAKOWSKI, MIETEK, BLEICHNER, HARRY, GUSTAVSSON, ULF
Applicants: ABB RESEARCH LTD, BAKOWSKI MIETEK, BLEICHNER HARRY, GUSTAVSSON ULF, ABB RESEARCH LTD., BAKOWSKI, MIETEK, BLEICHNER, HARRY, GUSTAVSSON, ULF
Application year: 1997
Main classification: H01L29
IPC codes: H01L21, H01L29