WO2001054204A1

WO2001054204A1 - A semiconductor device

Info

Publication number: WO2001054204A1
Application number: PCT/SE2001/000072
Authority: WO
Inventors: Johan Hammersberg; Olof Hjortstam; Mark Irwin; Jan Isberg
Original assignee: ABB AB
Current assignee: ABB AB
Priority date: 2000-01-17
Filing date: 2001-01-17
Publication date: 2001-07-26
Anticipated expiration: 2002-07-17
Also published as: AU2001228975A1; SE0000115D0

Abstract

In a semiconductor device comprising a first semiconductor layer (4) doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature a contact layer (11) is of a metal having a work function (ζ) being for a n-type doping substantially as high as or higher than the electron affinity (ψ) of the semiconductor material and for a p-type doping substantially as high as or lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material. The device comprises an irradiation source adapted to emit radiation of an energy being high enough for activating said dopants and thereby controlling the barrier against charge transport between the contact layer and the semiconductor layer.

Description

A SEMICONDUCTOR DEVICE

TECHNICAL FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a semiconductor device comprising a first semiconductor layer doped according to either a) n-type or b) p-type and a metal layer forming a contact thereto.

Accordingly, the present invention relates to all types of semiconductor devices having a metal layer forming a contact to a doped semiconductor layer. Such semiconductor devices may normally assume either a blocking state, in which the leakage current therethrough should be as low as possible, or a con- ducting state, in which the power losses generated through the current through the device should be as low as possible. The features of the contacts of such a semiconductor device are essential for obtaining this object.

The present invention is in particular, but not exclusively, directed to semiconductor devices in which it is very important that such a contact has two active functions, namely a blocking function in a blocking state of the device and a low ohmic function in the conducting on-state of the device. In normal semi- conductor technologies these two functions are solved separately with two types of contacts, namely a blocking Schottky- contact and a low resistance ohmic contact.

However, ohmic contacts require a very high concentration of thermally activated dopants in said semiconductor layer close to the contact layer, which is not achievable for all types of semi- conductor materials, such as for instance diamond. This fact will make it difficult to produce semiconductor devices made of such materials having excellent properties in said two states also from the contacting point of view with an acceptable low contri- bution from the contacts to leakage current and conducting losses.

Accordingly, it would be of particular interest to be able to improve contacts for these types of materials, so that the other ex- cellent properties of especially diamond could be used for producing semiconductor devices. Some of the advantages of using such a material for a semiconductor device will therefore be briefly discussed here, but it is emphasized that the present invention is directed to all types of semiconductor materials.

Diamond has some properties making it extremely interesting as a material in a device for high power applications, one of which is the very high breakdown field strength, which means that the number of devices to be connected in series for holding a volt- age of a certain magnitude may be reduced considerably with respect to devices of other known materials involving important cost reduction even if such a device itself would be much more expensive than the prior art devices, which for the rest is not an evident fact. Other interesting properties of diamond is a very high thermal conductivity and high charge mobility.

Besides the desire to provide a contact having low forward losses in the on-state of the device for semiconductor materials not possible to dope to very high concentrations for realizing ohmic contacts it would also be interesting, in particular for such materials, to combine a good blocking function and a low ohmic function in the same contact. SUMMARY OF THE INVENTION

The object of the present invention is to provide a semiconductor device of the type defined in the introduction having in some respects discussed above an improved function of the contacts thereof with respect to other such devices and making it possible to obtain a contact with a blocking function in a blocking state of the device and a low ohmic function in a conducting state of the device for in particular wide band gap semiconductor materials, such as diamond.

This object is according to the present invention obtained by providing such a semiconductor device, in which the semiconductor layer is doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature, the contact layer is of a metal having a work function being for a) substantially as high as or higher than the electron affinity of said semiconductor layer and for b) substantially as high as or lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material, and the device comprises an irradiation source adapted to emit radiation of an energy being high enough for activating said dopants and thereby controlling the barrier against charge transport between the contact layer and the semiconductor layer through irradiation of the region of the semiconductor layer closest to the contact layer.

It is here first of all to be mentioned that this way of doping, which is particularly interesting in the case of diamond as a semiconductor layer in a semiconductor device, since there are no shallow dopants for diamond, i.e. with dopants being thermally activated at the operation temperature of the device in question, so that some devices of interest for other materials may not be envisaged for diamond, has been disclosed in the unpublished Swedish patent application No. 9903149-4 of the applicant for the doping of the active layers of the semiconductor device. This way of deep energy level doping of diamond combined with activation by irradiation makes it possible to benefit from the superior properties of diamond in semiconductor devices of this type. Accordingly, the active layers in question may hold a very high voltage when not irradiated thanks to the high breakdown field strength of diamond, but conduct a high current with a low on-state voltage and thereby low losses when irradiated. However, the problem with a contact having two active functions - a blocking function in the dark off-state and low ohmic function in the on-state activated by irradiation - are not solved there. However, this is done by combining these features with an adaption of the work function of the metal used for the contact layer to the physical properties of electron affinity and/or band gap and electron affinity of the semiconductor material in question, i.e. by selecting a metal having an appropriate work function.

When a metal having a work function being for a) substantially as high as the electron affinity of the semiconductor material and for b) substantially as high as said sum is put in contact with the semiconductor layer the Fermi-level of the metal will be on substantially the same energy level as the conduction band of the n-type semiconductor layer (for a) and the valence band of a p-doped semiconductor layer (for b). When this system relaxes into thermal equilibrium, a potential barrier will be formed next to the metal-semiconductor interface due to electron transfer from the metal to the semiconductor on the n- type surface and conversely on the p-type surface. The height of this potential barrier will be given by the energy difference between the conduction band and the Fermi-level deep into the semiconductor material. Would the work function of the metal be higher than the electron affinity for a) and lower than said sum for b) the potential barrier will be higher. Since the potential barrier and the metal-semiconductor interface is given by the free charge density and the Fermi-level in the semiconductor it will be possible to adjust the potential barrier by using irradiation activation of said dopants located on deep energy levels. Accordingly, said barrier will result in a blocking function of the contact when the semiconductor material next thereto is not irradiated and a low ohmic function when the region in question of the semiconductor material is irradiated, preferably by light. For instance for diamond a barrier height around 1 eV will lead to a contact having blocking properties and a barrier height below 0.3 eV would result in low losses of the contact in the forward conducting case, and the concentration of said deep energy level dopants and the intensity of the irradiation source should be selected so as to achieve this.

According to a preferred embodiment of the invention the metal of the contact layer has a work function being for a) substantially higher than the electron affinity of the material of the semiconductor layer and for b) substantially lower than said sum for forming a Schottky-barrier at the interface between the contact layer and the semiconductor layer. By selecting such a metal for the contact layer there will be a sharp barrier of a certain magnitude directly at the metal surface next to the semiconductor layer and not a barrier growing from zero at this surface into the bulk of the semiconductor material as would be the case when said work function would be substantially the same as the electron affinity in case a) and substantially the same as said sum in the case b). This results in improved properties in the blocking state of the device in the form of a better blocking function, i.e. the leakage current will be further reduced by such a Schottky- barrier. The height of the Schottky-barrier will be independent of the irradiation by the irradiation source, but the width of the Schottky-barrier can be varied by irradiation. The irradiation of the deep levels in question will have the same effect as a thermally activated doping and the Schottky-barrier width will de- crease as the illumination increases. If the Schottky-barrier is thin enough the charges might tunnel through the barrier in the same way as in a conventional ohmic Schottky tunnelling contact, and this behaviour has to be obtained in the on-state of the device. It is true that the blocking function will be improved when the height of the Schottky-barrier is increased, but this will then also result in a wider barrier at a given irradiation intensity. Accordingly it would mostly be appropriate to use a metal resulting in a Schottky-barrier being not much higher than a minimum barrier necessary for obtaining an acceptably good blocking function, and this would typically mean that the Schottky- barrier at the interface between the contact layer and the semiconductor layer should be at least 0.6 eV.

According to another preferred embodiment of the invention the device comprises means promoting tunnelling of charge carriers through said Schottky-barrier when said region of the semiconductor layer is irradiated. This will make it possible to be able to tunnel through a wider barrier than otherwise improving the tunnel probability for a barrier of a certain width, i.e. a certain intensity of the irradiation source, or reducing the required inten- sity of the irradiation source while maintaining the tunnel probability.

According to a preferred embodiment of the invention said tunnelling promoting means is formed by defects introduced close to the contact layer. The introduction of such defects in the Schottky-barrier will increase the tunnelling probability, since the charge carriers may "jump" from one defect level to another defect level within the barrier and thereby come through the barrier, so that they will be able to tunnel through a wider barrier resulting in the options just mentioned. According to another preferred embodiment of the invention this may be obtained by arranging a third thin layer between said semiconductor layer and the contact layer, which is of a material introducing said defects for promoting tunnelling of charge carriers between the contact layer and the semiconductor layer, and such a layer may in the case of diamond as said semiconductor material be TiC, which is also transparent for the irradiation to be used then. It would also be possible to form the defects by radiation damage of regions of the semiconductor layer next to the contact layer.

According to another preferred embodiment of the invention said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV, and diamond and SiC are of particular interest for such a device thanks to the physical proper- ties thereof. In the case of diamond and a doping of said semiconductor layer according to n-type, it is preferred to use a metal having a work function exceeding 2.9 eV for forming the contact layer, which will result in a Schottky-barrier exceeding 0.6 eV. Candidates as metals for such a contact layer are for instance Nd and Sc. In the case of a doping of the semiconductor layer with deep level dopants of p-type in diamond a metal having a work function lower than 7.2 eV is to be selected for the contact layer for obtaining a Schottky-barrier height above 0.6 eV. Pt and Pd are candidates as metal for such a contact layer.

According to another preferred embodiment of the invention the device is adapted to switch between a state of conducting current and a state of blocking transport of charge carriers between two terminals of the device upon applying a voltage thereacross, and said irradiation source is adapted to irradiate said region of the semiconductor layer closest to the contact layer when the device is in the conducting state and interrupt such irradiation when it is in the blocking state. It is then possible that the de- vice comprises means for switching between the conducting state and the blocking state by irradiating material layers between said two terminals, which means that not only the properties of the contact, blocking or conducting, is controlled by irradiation, but also the state of the entire device, and it will then be particularly preferred to use the same irradiation source for controlling both the contacts and the other active layers of the device for switching between said two states.

The invention also comprises the uses according to the ap- pended use-claims, and the advantages thereof resides in the possibility to use materials as diamond while obtaining good contact functions and the inherent properties of such materials, in particular diamond and SiC.

Further advantages as well as advantageous 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 invention cited as examples.

In the drawings:

Figs 1 and 2 are very simplified cross-section views of the terminals and the layers located therebetween of two types of switching devices to which the present invention may be ap- plied,

Fig 3 is a schematic cross-section view of another type of a semiconductor device, to which the present invention may be applied,

Fig 4 is an energy band diagram of a contact metal layer adjacent to a semiconductor layer doped with deep level dopants in the form of donors in a blocking state of a device according to a first preferred embodiment of the invention, Fig 5 is a view corresponding to Fig 4 in a conducting state of said device,

Fig 6 is a view corresponding to Fig 4 for a device according to a second preferred embodiment of the invention,

Fig 7 is a view corresponding to Fig 5 for said device according to the second preferred embodiment of the invention,

Fig 8 is a view corresponding to Figs 5 and 7 for a device according to a third preferred embodiment of the invention in the conducting state,

Fig 9 illustrates an energy band diagram of a metal adjacent to a semiconductor layer doped with dopants in the form of acceptors on deep energy levels of a device according to a fourth preferred embodiment of the invention in the blocking state thereof, and

Fig 10 is a view corresponding to Fig 9 when said device is in a conducting state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Devices of types to which the present invention is particularly applicable will now be described with reference to Figs 1 -3. Fig 1 illustrates a semiconductor device having two terminals 1 , 2 for connecting the device to an electric current path. The device has also one first semiconductor layer 3 of diamond doped by dopants assuming such deep energy levels that the majority thereof will not be thermally activated at room temperature, and this means that the activation energy of said dopants should be higher than 0.3 eV. The device comprises another first semiconductor layer 4 of diamond doped according to the same conditions as the first layer 3. A second layer 5 of intrinsic diamond is arranged between the two first layers 3 and 4. The layer 5 has typically a thickness of 3 to 200 μm, whereas the two doped layers 3, 4 have a thickness of 1 to 20 μm. It is within the scope of the present inven- tion to dope the two layers 3, 4 with dopants of the same conductivity type, n or p, but it is preferred to dope them with dopants of opposite conductivity type, and we therefore in the following assume that the layer 3 is doped with acceptors, whereas the layer 4 is doped with donors. A metal contact 6, 7 connects the respective terminal to the diamond layer 3 and 4, respectively. The choice of the material of the metal contact is a very essential feature of the present invention and will be discussed further below. The metal contact has preferably vents of the type illustrated for the embodiment according to Fig 3 for allowing penetration of light from a light source through the metal contact and into the respective first layer 3, 4. It should be mentioned that characteristics of this device and the rest of the devices shown in the Figures having nothing to do with the present invention, such as passivation layers, have been omitted for the sake of clearness.

A device according to another possible type, to which the present invention may be applied, is shown in Fig 2, which differs from that illustrated in Fig 1 by the fact that the two first layers 3, 4 are made thicker and the intrinsic layer 5 is omitted, so that only the two first layers are arranged between the two metal contacts 6, 7.

The general function of a device of these types and also of the type shown in Fig 3 is described in said Swedish patent application No. 9903149-4 and reference is made thereto for further details and only a short explanation will be made here with reference to the embodiment illustrated in Fig 3. In this embodiment, the metal contacts 10 arranged on a first side are laterally displaced with respect to the metal contacts 1 1 arranged on the opposite side of the device and light sources 12 are adapted to illuminate a region under the respective metal contact for activation of dopants occupying deep energy levels there from the opposite side of the device, so that the light source shown to the right in the Figure will activate the acceptors in the layers 3 underneath the anodes through illumination through the intrinsic layer 5, whereas the light source shown to the left will illuminate and activate donors in the layers 4 underneath the cathodes of the device. A voltage source 13 and a load 14 for the circuit, to which the device is connected, are also indicated in this Figure. The acceptors in the layer 3 occupy levels at a considerable distance to the valence band of the diamond lattice, whereas the dopants of the first layer 4, which are assumed to be donors, occupy levels located at a substantial energy distance below the conduction band of the diamond lattice. Said energy distance is preferably above 0.5 eV, so that practically none of these dopants are thermally activated at room temperature. This means that the first layers 3 and 4 will without said illumination have practically no free charge carries for transport between the anode and the cathode and they will act as intrinsic layers, so that the switch is then able to block very high voltages applied thereacross in any direction as long as no charge carriers are injected at the contacts, and the present invention aims at taking care of that issue.

When the layers 3 and 4 are illuminated by light having an energy exceeding the activation energy of the dopants, these will be activated. When the activation energy of said dopants is lower than 2.5 eV a standard Xenon-lamp may be used as said irradiation sources 12, since it has only to deliver light with an energy above said activation energy, which for instance for donors of N is 1 .7 eV, which should be compared with energy needed to lift electrons from the valence band to the conduction band in diamond (5.4 eV). It is illustrated in Fig 4 what is happening when a metal having substantially the same work function φ, i.e. energy distance between the Fermi-level and the vacuum level, as the electron affinity (2.3 eV for diamond), i.e. the energy distance between the conduction band and the vacuum level, of the semiconductor material is selected as material for the contact layer 1 1 next to the layer 4 doped with donors. Candidates for the metal are Cs, Eu and Rd with the work functions 2.14 eV, 2.5 eV and 2.16 eV, respectively. The conduction band will at the interface to the metal be on Fermi-level and then rise into the bulk of the semiconductor layer forming a potential barrier therein due to electron transfer from the metal to the semiconductor. This potential barrier 15 has for diamond typically a height of 1 eV. The dopants 16 located on deep energy levels are here not activated. Accordingly, a potential barrier having a blocking function and preventing injection of electrons from the metal is formed in this way.

It is illustrated in Fig 5 what is happening when the region of the semiconductor layer 4 closest to the metal contact is illuminated with light having an energy sufficient to excite electrons of said deep energy level dopants. This will increase the free charge density in the semiconductor and thereby reduce the gap between the conduction band 17 and the Fermi-level 18 and thereby the height of said barrier. The lowering of the barrier to about 0.3 eV will result in an acceptably low resistance and thereby voltage drop and power losses at the contact in question in the conducting state of the device.

Although the blocking capacity of a device having a contact with the properties illustrated in Figs 4 and 5 sometimes are acceptable, it will in some cases be desired to reduce the leakage current in the blocking state further, and this is done by selecting a metal for the contact layer with a work function ψ being substantially higher than the electron affinity χ of the semiconductor material. We assume that we speak about diamond also in this case. This means that the conduction band 17 of the semiconductor material will directly at the interface form a potential barrier of Schottky-type preventing injections of electrons from the metal. This means that an illumination through the light source 12 of the region of the semiconductor material with an energy high enough for exciting electrons of the deep energy level donors will result in a curve of the conduction band illustrated in Fig 7, with a Schottky-barrier extending a distance 19 into the semiconductor layer. It is desired to obtain a width 19 of the barrier being in the region of 0.5 μm for obtaining tunnelling of electrons therethrough and sufficiently low forward losses in the conducting state of the device. Candidates for the metal in the case of diamond as said semiconductor material are Nd and Sc with a work function of 3.2 eV and 3.5 eV, respectively. The blocking function (Fig 6) of this contact will be improved with respect to the contact according to Figs 4 and 5. However, it is necessary to use an intensity of the light source 12 being high enough for making the width 19 of the Schottky-barrier small enough for enabling a high probability of tunnelling therethrough. This is addressed by the embodiment illustrated in Fig 8, which differs from that according to Figs 6 and 7 by the fact that defects 20 promoting tunnelling of charge carriers through said Schottky-barrier have been introduced close to the interface 21 between the metal and the semiconductor material. This may be done by arranging a very thin layer of for instance TiC at said interface 21 . An alternative is to expose the region of the semiconductor layer closest to the contact layer for radiation for damaging it and introduce defects thereby. This means that the probability of tunnelling through a barrier with a given width will increase and it will accordingly be possible to use a light source requiring less power. A concentration of the dopants of at least 10¹⁷cm^"3, preferably 10¹⁸cm^"3, is preferred, since that makes it possible to create sufficient free charge carriers (about 10¹⁵cm^"2) through said illumination for obtaining a Schottky-barrier being thin enough. A possible defect layer 25 of TiC is schematically indicated in Fig 8.

The inventional idea is of course also applicable to a contact between a metal contact layer and a semiconductor layer doped according to p-type, i.e. with acceptors, and the work function of the metal is in such a case to be compared with the sum of on one hand the band gap between the conduction band a the valence band and on the other the electron affinity of the semicon- ductor material, and it has to be substantially as high as or lower than that sum. It is schematically illustrated in Figs 9 and 10 what is happening on the p-side, accordingly close to the interface between the contact layer 10 and the layer 3 in the device according to Fig 3, in the blocking state and in the con- ducting state of the device when selecting a metal for said contact layer having a work function being substantially the same as said sum. It is illustrated how the valence band 22 will be at the Fermi-level 23 of diamond at the interface 21 and at the Fermi- level of the metal there. However, a barrier will be formed in the direction of the bulk being approximately 1 .0 eV. Irradiation of this region of the semiconductor material will result in a lowering of this barrier through activation of the acceptors 24 as illustrated in Fig 10, and this will have the same consequence for this contact as for the contact in the state of Fig 5. For diamond of p-type a metal having a work function of 7.5 eV-8.0 eV is advantageous for the contact layer, whereas it should be 6.2-6.6 eV for SiC of p-type and the embodiment according to Figs 4 and 5.

It would be preferred to use the same light source 12 for both illumination of the active layers of the device and the region close to each contact as illustrated in Fig 3.

Furthermore, it may be mentioned that for a semiconductor de- vice having SiC of the 6H-polytype having an electron affinity of

3.2 eV Nd and Sc are candidates for the embodiment shown in Figs 4 and 5 where the work function is 3.0-3.5 eV and Ag and Al having both a work function of 4.3 eV are candidates for the embodiment shown in Figs 6-8 where the work function shall exceed 3.8 eV, while Pt, Pd and Ni having work functions of 5.63 eV, 5.6 eV and 5.2 eV, respectively, are candidates for the embodiment according to Figs 6-8 with a p-type semiconductor layer where the work function shall be lower than 5.8 eV. For diamond Pt and Pd may be used for p-type dopants and the embodiment according to Figs 6-8.

A device according to the invention is preferably adapted to hold voltages above 1 kV, especially above 10 kV, in absence of irradiation through said source. This device may advantageously be used in a high voltage converter operating at a voltage in the region of 10 kV-500 kV.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.

It would also be possible to create said doping on deep energy levels by treating a surface of the layer, such as the diamond layer, by hydrogen at a high temperature for creating a hydro- genated surface with charge carriers on deep energy levels corresponding to a doping of p-type.

All definitions of materials in this disclosure of course also in- elude inevitable impurities except for intentional doping, so that the layers provided with deep level dopants may also contain a low concentration of thermally activated dopants unintentionally built into the structure during the processing thereof.

"Semiconductor material" as used in the claims is to be interpreted broadly and also include an insulator as diamond being doped with deep level dopants, which may be activated by irradiation.

It is also pointed out that irradiation does not necessarily have to be illumination, but it is also conceivable to use for instance beams of electrons.

It is pointed out that "metal" is in this disclosure defined as covering all materials being electric conducting. This has to be considered when interpreting the claims.

Claims

1. A semiconductor device comprising a first semiconductor layer (3, 4) doped according to either a) n-type or b) p-type and a metal layer (6, 7, 10, 1 1 ) forming a contact thereto, said semiconductor layer being doped by dopants (16, 24) assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature, the contact layer being of a metal having a work function being for a) substantially as high as or higher than the electron affinity of said semiconductor material and for b) substantially as high as or lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material, the device also comprising an irradiation source (12) adapted to emit radiation of an energy being high enough for activating said dopants and thereby controlling the barrier against charge transport between the contact layer and the semiconductor layer through irradiation of the region of the semiconductor layer closest to the contact layer, characterized in that the metal of the contact layer (6, 7, 10, 1 1 ) has a work function being for a) substantially higher than the electron affinity of the material of the semiconductor layer ( 3, 4) and for b) substantially lower than said sum for forming a Schottky-barrier at the interface between the contact layer and the semiconductor layer.

2. A device according to claim 1 , characterized in that substantially higher and substantially lower means a difference of at least 0.6 eV.

3. A device according to claim 1 or 2, characterized in that it comprises means (25) promoting tunnelling of charge carriers through said Schottky-barrier when said region of the semicon- ductor layer is irradiated.

4. A device according to claim 3, characterized in that said means is formed by defects (20) introduced close to the contact layer.

5. A device according to claim 4, characterized in that it comprises a third thin layer (25) arranged between said semiconductor layer (4) and the contact layer (1 1 ) and being of a material introducing said defects for promoting tunnelling of charge carriers between the contact layer and said semiconductor layer.

6. A device according to claim 4, characterized in that said defects are formed by radiation damage in a thin region of the semiconductor layer next to the contact layer.

7. A device according to any of the preceding claims, characterized in that said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV.

8. A device according to any of the preceding claims, characterized in that said semiconductor material is diamond.

9. A device according to any of the preceding claims, characterized in that said semiconductor layer is SiC.

10. A device according to claim 8, characterized in that said semiconductor layer is doped according to n-type and a metal having a work function exceeding 2.9 eV forms the contact layer.

1 1 . A device according to claim 10, characterized in that the metal of the contact layer is Nd or Sc.

12. A device according to claim 8, characterized in that said se- miconductor layer is doped according to p-type and a metal having a work function lower than 7.2 eV forms the contact layer.

13. A device according to claim 12, characterized in that the metal of the contact layer is Pt or Pd.

14. A device according to claim 5, characterized in that said semiconductor material is diamond and that said thin layer is made of TiC.

15. A device according to claim 9, characterized in that said semiconductor layer is doped according to n-type, and that a metal having a work function exceeding 3.8 eV forms the contact layer.

16. A device according to claim 15, characterized in that the metal of the contact layer is Ag or Al.

17. A device according to claim 9, characterized in that said semiconductor layer is doped according to p-type, and that a metal having a work function lower than 5.8 eV forms the con- tact layer.

18. A device according to claim 17, characterized in that the metal of the contact layer is Pt, Pd or Ni.

19. A device according to any of the preceding claims, characterized in that it is adapted to switch between a state of conducting current and a state of blocking transport of charge carriers between two terminals (1 , 2) of the device upon applying a voltage thereacross, and that said irradiation source (12) is adapted to irradiate said region of the semiconductor layer closest to the contact layer when the device is in the conducting state and interrupt such irradiation when it is in the blocking state.

20. A device according to claim 19, characterized in that it comprises means (12) for switching between the conducting state and the blocking state by irradiating material layers (3, 4, 10, 1 1 ) between said two terminals.

21 . A device according to claim 20, characterized in that said means for said switching are constituted by the irradiation source (12) for irradiating said region.

22. A device according to any of the proceeding claims, characterized in that said irradiation source (12) is a light source adapted to emit light.

23. A device according to any of the preceding claims, charac- terized in that the activation energy of said dopants is higher than 0.3 eV.

24. A device according to any of the preceding claims, charac- erized in that the activation energy of said dopants is lower than 2.5 eV.

25. A device according to any of the preceding claims, characterized in that at least the region of the semiconductor layer closest to the contact layer is highly doped with dopants (16, 24) on deep energy levels.

26. A device according to claim 25, characterized in that said semiconductor material is diamond and that the concentration of dopants on deep energy levels in said region of the semicon- ductor layer is above 10¹⁷cm^"3, preferably above 10¹⁸cm^"3.

27. A device according to any of the preceding claims, characterized in that it comprises two first semiconductor layers (3, 4) doped with dopants on deep energy levels connected in series between two terminals of the device.

28. A device according to claim 27, characterized in that it comprises a second layer (5) of a substantially intrinsic semiconductor material arranged between said two doped semiconductor layers (3, 4).

29. A device according to claim 27 or 28, characterized in that one of said first semiconductor layers is doped by acceptors as said dopants occupying deep energy levels and the other first layer is doped with donors as dopants occupying said deep en- ergy levels, and that a said contact layer is arranged next to each of said first layers.

30. A device according to claim 28, characterized in that said two first layers are doped by dopants of the same conductivity type, n or p, occupying said deep energy levels.

31 . A device according to any of the preceding claims, characterized in that it is adapted to hold voltages above 1 kV, preferably above 10 kV, when said semiconductor layer is not irradi- ated by said irradiation source.

32. A semiconductor device comprising a first semiconductor layer (3, 4) doped according to either a) n-type or b) p-type and a metal layer (6, 7, 10, 1 1 ) forming a contact thereto, said semi- conductor layer being doped by dopants (16, 24) assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature, said contact layer being of a metal having a work function being for a) substantially as high as or higher than the electron affinity of said semiconductor material and for b) substantially as high as or lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material, said device further comprising an irradiation source (12) adapted to emit radiation of an energy being high enough for activating said dopants and thereby controlling the barrier against charge transport between the contact layer and the semiconductor layer through irradiation of the region of the semiconductor layer closest to the contact layer, characterized in that the metal of the contact layer has a work function being for a) substantially as high as the electron affinity of the material of the semiconductor layer and for b) substantially as high as said sum, and that substantially means a difference of less than +0.3 eV.

33. A device according to claim 32, characterized in that said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV.

34. A device according to any claim 32 or 33, characterized in that said semiconductor material is diamond.

35. A device according to any of claims 32-34, characterized in that said semiconductor layer is SiC.

36. A device according to claim 34, characterized in that the semiconductor is doped according to n-type and that a metal having a work function of 2.1 eV-2.5 eV forms the contact layer.

37. A device according to claim 36, characterized in that the metal of the contact layer is Cs, Eu or Rd.

38. A device according to claim 34, characterized in that the semiconductor is doped according to p-type and that a metal having a work function of 7.5 eV-8.0 eV forms the contact layer.

39. A device according to claim 35, characterized in that the semiconductor layer is doped according to n-type and that the work function of the metal of the contact layer is 3.0-3.5 eV.

40. A device according to claim 39, characterized in that the metal of the contact layer is Nd or Sc.

41 . A device according to claim 35, characterized in that the semiconductor layer is doped according to p-type and that the work function of the metal of the contact layer is 6.2-6.6 eV.

42. A device according to any of claims 32-41 , characterized in that it is adapted to switch between a state of conducting current and a state of blocking transport of charge carriers between two terminals (1 , 2) of the device upon applying a voltage thereacross, and that said irradiation source (12) is adapted to irradiate said region of the semiconductor layer closest to the contact layer when the device is in the conducting state and inter- rupt such irradiation when it is in the blocking state.

43. A device according to claim 42, characterized in that it comprises means (12) for switching between the conducting state and the blocking state by irradiating material layers (3, 4, 10, 1 1 ) between said two terminals.

44. A device according to claim 43, characterized in that said means for said switching are constituted by the irradiation source (12) for irradiating said region.

45. A device according to any of claims 32-44, characterized in that said irradiation source (12) is a light source adapted to emit light.

46. A device according to any of claims 32-45, characterized in that the activation energy of said dopants is higher than 0.3 eV.

47. A device according to any of claims 32-46, characerized in that the activation energy of said dopants is lower than 2.5 eV.

48. A device according to any of claims 32-47, characterized in that at least the region of the semiconductor layer closest to the contact layer is highly doped with dopants (16, 24) on deep energy levels.

49. A device according to claim 48, characterized in that said semiconductor material is diamond and that the concentration of dopants on deep energy levels in said region of the semiconductor layer is above 10¹⁷cm^"3, preferably above 10¹⁸cm^"3.

50. A device according to any of claims 32-49, characterized in that it comprises two first semiconductor layers (3, 4) doped with dopants on deep energy levels connected in series between two terminals of the device.

51 . A device according to claim 50, characterized in that it comprises a second layer (5) of a substantially intrinsic semiconductor material arranged between said two doped semiconductor layers (3, 4).

52. A device according to claim 50 or 51 , characterized in that one of said first semiconductor layers is doped by acceptors as said dopants occupying deep energy levels and the other first layer is doped with donors as dopants occupying said deep en- ergy levels, and that a said contact layer is arranged next to each of said first layers.

53. A device according to claim 51 , characterized in that said two first layers are doped by dopants of the same conductivity type, n or p, occupying said deep energy levels.

54. A device according to any of claims 32-53, characterized in that it is adapted to hold voltages above 1 kV, preferably above 10 kV, when said semiconductor layer is not irradiated by said irradiation source.

55. A use of a device according to any of the preceding claims as a switch in a converter.

56. A use of a device according to any of claims 1 -54 as a switch for switching high powers and/or high voltages and/or high currents.

57. A use according to claim 55 or 56 of the device as a switch in a high voltage converter operating at a voltage in the region of 10 kV-500 kV.

58. A use according to claim 56 of the device as a switch in a current valve of a converter.

59. A use according to claim 58, in which a plurality of said devices are connected in series for together holding the voltage across said current valve in the blocking state thereof.

60. A use according to claim 56 in an arrangement for protection of an equipment for electric power applications.