A WIDE BANDGAP SEMICONDUCTOR DEVICE HAVING A HETEROJUNCTION

Abstract: A semiconductor device comprises two adjacent semiconductor layers (2, 3) of different materials forming a heterojunction (1) therebetween. A first (2) of said layers has a larger gap between the conduction band and the valence band than the other, second layer (3) and it is doped with impurities providing charge for forming a high mobility surface channel (7) in the second layer at the interface between said layers. The second layer (3) is made of SiC and the first layer (2) is made of one of a) A1N and b) an alloy of A1N and other Group 3B-nitrides.

Description: A WIDE BANDGAP SEMICONDUCTOR DEVICE HAVING A HETEROJUNCTION TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a semiconductor device comprising two adjacent semiconductor layers of different material forming a heterojunction therebetween, the first of saiά layers having a larger band gap between the con¬ duction band and valence band than the other, second layer, and being doped with impurities providing charge for forming a high mobility surface channel in the second layer at the interface between said layers. This type of semiconductor device is called a HE T (High Electron Mo¬ bility Transistor) due to the high mobility of charge car- riers in the surface channel, thanks to the fact that the free charge carrier in said channel are physically sepa¬ rated from the ionised impurities in said first layer re¬ ducing scattering of the charge carriers thereby. These semiconductor devices are gate controlled. The advantage thereof with respect to gate controlled Field Effect Transistors having an insulating layer, nor¬ mally Siθ2, between the gate and the semiconductor layer is that the amorphous nature of such an insulating layer as Siθ2 gives rise to additional scattering of carriers in the inversion channel at said interface, particularly for the case of high carrier densities where strong carrier localisation occurs at the semiconductor-insulating layer interface, so that the mobility of carriers will be con- siderably below the bulk carrier mobility, whereas a high quality heterojunction is known to be nearly free from in- terface scattering and carrier confinement can also bring about a rise in carrier mobilities, since the impurities are spatially separated from the mobile carriers, which is called modulation doping. Thanks to the high mobility HEMTs may operate under high frequencies. However, known devices of this type, which may have a heterojunction of for example GaAs/AlGaAs, may not be obtained with such high carrier densities that they may be used in high power devices, and the material will also be unable to take the heat created when high currents are transported. It is therefor a desire to be able to use SiC as said sec¬ ond layer in such semiconductor devices, since it would then be possible to benefit from the superior properties thereof in comparison with other semiconductors, espe¬ cially Si, namely the capability of SiC to function well under extreme conditions. SiC has a high thermal stability and it will have a stable function at much higher tempera¬ tures than for instance Si, namely well up to 1000 K. Fur- thermore, it has a high thermal conductivity, so -hat SiC devices may be arranged at a high density, and they may accordingly also carry high currents. However, no High Electron Mobility Transistor having SiC as said second layer has been suggested until now. Thus, the use of SiC in Field Effect Transistors has been re¬ stricted to gate-controlled semiconductor devices having an insulated gate having the disadvantages mentioned above of poor channel mobility in the inversion channel in SiC. SUMMARY OF THE INVENTION The object of the present invention is to provide a semi¬ conductor device of the type defined in the introduction, which has a high quality heterojunction and may operate under high frequencies and carry high currents while main¬ taining a good function stability. This object is in accordance with the invention obtained by making the second layer of SiC and the first layer of one of a) AIN and b) an alloy of AlN and other Group SB- nitrides. AIN has a very good lattice match with SiC with a misfit of only 0,7%, so that very high quality hetero- junctions between SiC and AIN nearly free from interface scattering may be grown. It also has nearly the same coef¬ ficient of thermal expansion as SiC and it is stable at very high temperatures. Furthermore, AlN has a band gap of about 6,2 eV, which is considerably larger than all polytypes of SiC, which have band gaps between 2,3 and 3,3 eV. This results in a considerably larger energy band off¬ set (energy difference between the bands) of a system of SiC/AlN than in heterojunctions in known semiconductor de¬ viceε. For the sake of clearness, we will hereinafter speak about electrons as charge carriers in said surface channel, since it is much more interesting to use elec¬ trons than holes as charge carriers in this type of de¬ vices because of the much lower mobility of holes, and said impurities are therefore donors, but it is emphasised that the invention is not in any way restricted to elec- trons as charge carriers in said surface channel. This larger band offset means that more electrons may be put in said interface channel from said ionised donors before the conduction band in said first layer has been bent to the Fermi-level. Accordingly, it will be possible to obtain far higher carrier densities and by that currents than in HEMTs already known. Thanks to the characteristics of the materials forming the two layers in a semiconductor device according to the invention such a device may be operated at high frequencies and carry high currents, thus function well as high power devices. The first layer may also be made of an alloy of AlN and other Group 3B-nitrides, through which a high quality heterojunction may also be obtained. The presence of such Group 3B-nitrides may make it easier to obtain a well defined doping of said first layer, but these other Group 3B-nitrides have smaller gaps between the valence band and the conduction band, so that they counteract the advantages of the large band-offset of AlN with respect to to SiC, so that it will mostly be suitable to have a high concentration of AlN and lower concentration of one or more of the other Group 3B-nit.ride in any case close to said heterojunction. According to a preferred embodiment of the invention a first region of said first layer closest to said junction is made of AlN. This means that the lattice match at the heterojunction and by that the quality of the heterojunc¬ tion will be at an optimum, so that the mobility in the two dimensional interface channel will be very high. According to another preferred embodiment of the invention a second region of said first layer adjacent to said first region of AlN and separated from said junction there¬ through is made of an alloy of AlN and other Group 3B-ni- trides. This means that the high quality heterojunction is ensured by the first region of AlN at said heterojunc- tion at the same time as it will be easier to dope said first layer by the presence of said second region of an alloy of AlN and other Group 3B-nitrides. According to a further embodiment to the invention the region of said first layer closest to said junction is not doped with i - purities, so that a spacer layer is obtained and the ion¬ ised donors are well separated from the surface channel and by that do not affect the mobility of the electrons therein, so that thiε mobility will be excellent. This em¬ bodiment in combination with the embodiment last mentioned will be very advantageous. According to a still further preferred embodiment of the invention the concentration of said Group 3B-nitrides in said alloy is gradually increasing in said at least one region in the direction away from said junction. By pro- viding εuch a graded layer there will be no abrupt change in the composition of the layer material, which would lead to strains in the material influencing the mobility in said surface channel, so that such strains are reduced and the mobility in the surface channel will be higher. In the case of said first region of AlN and the second region of an alloy of AlN and other Group 3B-nitrides, this will mean no abrupt change from AlN to said alloy. Thuε, when growing said first layer the concentration cf for instance gallium may be gradually increased in the direction away from said heterojunction. According to a further preferred embodiment of the inven¬ tion said alloy is an alloy having a content of GaN, which is very advantageous, since gallium nitride and aluminium nitride have a complete miεcibility, so that a high qual¬ ity layer may be obtained. According to another embodiment of the invention said sec¬ ond layer is made of 3C-SiC. The use of this particular polytype of SiC is advantageous in thiε type of devices, in which the mobility is of most importance, since this polytype is characterised by a particularly high mobility. Further advantages and preferred features of the invention will appear from the following description and the other dependent claims . BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of preferred embodiments of the in- vention cited as examples. In the drawings: Fig. 1 is a view illustrating the difference in band gap of two layers at a heterojunction in a semiconduc¬ tor device having AlN or an alloy of AIN and ether Group 3B-nitrides as one layer and SiC as the other layer for illustrating how a surface channel having charge carriers is obtained by auto-ioniεa- tion of impurities in the layer with the largest band gap, Fig. 2 is a view showing the extension of the valence and conduction bands close to said heterojunction il¬ lustrating the high mobility surface channel at the interface between the two layers and how the ionised donors will influence the mobility therein, Fig. 3 illustrates the concentration of AlN in said first layer in relation to the distance from the hetero- junction in the semiconductor device having the conduction band extension shown in Fig. 2, and Fig. 4 a High Electron Mobility Transistor (HEMT) , which may be provided with the layers made of the raate- rial according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 illustrates a heterojunction 1 between a first layer 2 and a second layer 3 in a semiconductor device. It is very schematically illustrated that the energy gap be¬ tween the valence band 4 and the conduction band 5 iε larger in said first layer 2 than in the second layer 3, so that a band-offset is obtained at said heterojunction. When the firεt layer is doped with donors these have their free electrons 6 at a higher energy level than the energy level at the other side of the heterojunction, which will result in a fall of said electrons as indicated by the ar¬ row to the lower energy level in the second layer while ioniεing said donors and leaving positive holes in the firεt layer 2. Thiε technique of providing only the layer with the largest band gap with impuritieε iε called εelec- tive doping or modulation doping. In this way an electri¬ cal field iε created at the heterojunction bending the conduction bands in the way shown in Fig. 2. The higher the concentration of donors in the first layer compara¬ tively close to the heterojunction the more electrons will "fall" to the lower energy level in the second layer 3 and the more will the conduction band be bent. The maximum limit of electrons in the two dimensional surface channel 7 so formed by the ionisation of the donors depends on the band offset between the conduction bands in the two lay¬ ers, and it iε only poεεible to put in electrons in said surface channel until the Fermi-level in both layers haε reached equilibrium as shown in Fig. 2. In the case of the second layer of SiC and the first layer of AlN the barrier height 8 will be about 1,7 eV in com¬ parison to a heterojunction between GaAs and AlGaAε, which haε a barrier height of 300 meV. Thanks to this increased barrier height it will be possible to raise the maximum density of free electrons in the two dimensional surface channel from about 10 12 cm ~2 to above 10 13 cπT 2 . The sharp¬ ness of the conduction band shown in Fig. 2 is defined by the concentration of the impurity, i. e. the doping, of the first layer 2. This higher electron density results in the possibility to carry much higher currents, so that such a semiconductor device may handle more power, which in its turn will result in more heat, which SiC as well as AlN are designed to take while maintaining a stable func- tion of the device. The higher band-offset also results in a tighter confinement of the electrons in the potential. It iε shown in Fig. 3 how the concentration of AlN in the firεt layer iε changed in the direction away from εaid heterojunction 1. In a firεt region 9 of εaid firεt layer closest to said junction the material consiεtε of only AlN. In thiε way the highest quality posεible of the het¬ erojunction may be obtained thanks to the excellent lat¬ tice match and match of physical properties of AlN and SiC. At a distance away from the heterojunction 1 another Group 3B-nitride is mixed with AlN to an alloy, in which the concentration of said Group 3B-nitride is gradually increasing in thiε second region 10 away from the junc¬ tion. This other Group 3B-nitride may be GaN or InN. In the latter case the concentration thereof may not be larger than 20%, since after that there will be hardly no band gap between the first layer and the second layer. The intermixing of gallium or indium may have two purposes, namely the content thereof may be used to vary the band- offset and they will make it easier to obtain a doping of said first layer. Preferably, the region of said first re¬ gion cloεeεt to εaid junction iε not doped with impuri- tieε, so that the attraction of the ioniεed donorε in εaid first layer excerted upon the electrons in the surface channel will be reduced and by that the mobility increaε . It is shown in Fig. 2 how the region closest to the het¬ erojunction is not doped with any impuritieε . It iε not absolutely necessary that said first region made of AlN corresponds to the region having no doping, but the former may be thicker or thinner. The gradual increase of the concentration of the other Group 3B-nitride in the first layer in the direction away from the heterojunction, which is obtained when the crys¬ tal is grown, reduces strains in the material as compared to an abrupt change in alloy composition, which also has a positive influence on the mobility in said surface chan¬ nel . In this way a Field Effect Transistor (FET) which may be used as an amplifier, with a gate voltage regulating the current in the two dimensional channel may be obtained. This device will be nearly free from interface scattering at said interface, εo that the electrons may be moved very fast and the device may operate at high frequencies. Thanks to the poεsibility of carrying very high currents the device will be well suited for use in high power ap¬ plications. Fig 4 shows a HEMT comprising a source 11 and drain 12 with metal contacts 13 and 14 respectively. The layer se¬ quence is made up from a first doped wide bandgap layer 2 on top a second layer 3 of smaller bandgap. Said first layer iε divided into two sub-layers, namely a first un¬ doped sub-layer 15 and a second doped layer 16. Dopants placed in the doped layer 16 are auto-ioniεed and the charge tranεferred to the smaller gap layer 3 and located at the heterojunction with layer 15, thereby forming a channel 18 comprising a two dimensional charge sheet be¬ tween εource and drain. The device also comprises a gate 17, the potential of which may be varied so as to control the conductivity of the channel at the heterojunction 1, and a substrate layer 19. The invention is of course not in any way restricted to the preferred embodiments described above, but many possi- bilities to modificationε thereof will be apparent to a man with ordinary skill in the art. As already mentioned, the invention is also applicable to other devices than HEMTs, accordingly alεo when εaid first layer having a larger band gap is doped with acceptors and the two dimenεional channel at the interface between the two layers will contain holes as charge carriers. As mentioned above, it will be possible to use one cr a combination of Group 3B-nitrideε in εaid first layer ac¬ cording to the formula: Al x Ga y In (1 _ x . y) N where X+Y <. 1. The lattice match with SiC decreases in the direction AlN, GaN and InN, so that it will in most cases be preferred to have a part of the first layer of only AlN, at least close to the interface with the SiC se icon- ductor layer. However, it is well posεible to have a com¬ position changing with the distance from said interface in an arbitrary way according to the wisheε in every separate case. All definitionε concerning the materialε of the different device layerε do of course also include inevitable impuri¬ ties as well as intentional doping of said two layers. The definition layer is to be interpreted broadly and com- prises all types of volume extensions and shapeε. The definition "other Group 3B-nitrides" ir. the claim is also to be interpreted aε comprising the case of one sin¬ gle Group 3B-nitride, for instance GaN. The SiC layer 3 may be of any polytype of SiC, such as for example 6H, 4H, 3C and 15R.

Claims: Claims 1. A semiconductor device compriεing two adjacent εemicon- ductor layers (2, 3) of different materials forming a het- erojunction (1) therebetween, a firεt (2) of εaid layers having a larger gap between the conduction band and the valence band than the other, second layer (3) , and being doped with impurities providing charge for forming a high mobility surface channel (7) in the second layer at the interface between said layers, characterized in that the second layer (3) is made of SiC and the first layer (2) is made of one of a) AlN and b) an alloy of AlN and other Group 3B-nitrides. 2. A device according to claim 1, characterized in that a firεt region (9) of εaid firεt layer closest to said junc¬ tion is made of AlN. 3. A device according to claim 1 or 2, characterized in that at least one region (10) of said first layer (2) is made of an alloy of AIN and other group 3B-nitrides. 4. A device according to claim 2 or 2 and 3, characterized in that a second region (10) of said firεt layer adjacent to εaid firεt region of AIN and εeparated from εaid junc¬ tion (1) therethrough iε made of an alloy of AlN and other Group 3B-nitrideε. 5. A device according to claim 3, characterized in that the concentration of said Group 3B-nitrideε in said alloy is gradually increasing in said at least one region in the direction away from said junction (1) . 6. A device according to claim 4 and 5, characterized in that εaid region of gradually increaεing concentration of said Group 3B-nitride is separated from said junction by a region of AlN. 7. A device according to any of claims 1-6, characterized in that a region of said first layer closeεt to εaid junc¬ tion iε not doped with said impurities. 8. A device according to any of claims 1-7, characterized in that said alloy is an alloy having a content of GaN. 9. A device according to claim 8, characterized in that said alloy is an alloy of AlN and GaN. 10. A device according to any of claims 1-7, characterized in that said alloy is an alloy having a content of InN. 11. A device according to claim 10, characterized in that said alloy is and alloy of AlN and InN. 12. A semiconductor device according to claims 10 or 11 characterized in that the concentration of InN in said al¬ loy is less than 20%. 13. A device according to any of claims 1-12, character- ized in that said second layer (3) is made of 3C-SiC. 14. A device according to any of claims 1-13, character¬ ized in that said first layer (2) iε N-doped. 15. A device according to any of claimε 1-14, character¬ ized in that it iε a high power device. 16. A device according to any of claimε 1-15, character¬ ized in that it is adapted to operate under high frequen- cies . 17. A device according to any of claims 1-16, ized in that it is a High Electron Mobility Transistor (HEMT) .

Inventors: HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREY, HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREY
Applicants: ABB RESEARCH LTD, HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREY, ABB RESEARCH LTD., HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREY
Application year: 1996
Main classification: H01L29
IPC codes: H01L29