Bidirectional Phase Controlled Thyristor (BiPCT) - A New Semiconductor Device Concept

Abstract

A bidirectional thyristor device includes a semiconductor wafer with a number of layers forming pn junctions. A first main electrode and a first gate electrode are arranged on a first main side of the wafer. A second main electrode and a second gate electrode are arranged on a second main side of the wafer. First emitter shorts penetrate through a first semiconductor layer and second emitter shorts penetrate through a fifth semiconductor layer. In an orthogonal projection onto a plane parallel to the first main side, a first area occupied by the first semiconductor layer and the first emitter shorts overlaps in an overlapping area with a second area occupied by the fifth semiconductor layer and the second emitter shorts. The overlapping area, in which the first area overlaps with the second area, encompasses at least 50% of a total wafer area occupied by the semiconductor wafer.

Claims

1-14. (canceled)

15. A bidirectional thyristor device comprising: a semiconductor wafer having a first main side and a second main side opposite to the first main side, wherein the semiconductor wafer includes in an order from the first main side to the second main side the following layers: a first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type different from the first conductivity type, wherein the first semiconductor layer and the second semiconductor layer form a first p-n junction; a third semiconductor layer of the first conductivity type, the second semiconductor layer and the third semiconductor layer forming a second p-n junction; a fourth semiconductor layer of the second conductivity type, wherein the third semiconductor layer and the fourth semiconductor layer form a third p-n junction; and a fifth semiconductor layer of the first conductivity type, wherein the fourth semiconductor layer and the fifth semiconductor layer form a fourth p-n junction; a first main electrode arranged on the first main side and in direct contact with the first semiconductor layer; a first gate electrode is arranged on the first main side spaced from the first main electrode, the first gate electrode in direct contact with the second semiconductor layer; a second main electrode arranged on the second main side and in direct contact with the fifth semiconductor layer, and a second gate electrode arranged on the second main side spaced from the second main electrode, the second gate electrode in direct contact with the fourth semiconductor layer; a plurality of first emitter shorts, each first emitter short penetrating through the first semiconductor layer to electrically connect the second semiconductor layer with the first main electrode; and a plurality of second emitter shorts, each second emitter short penetrating through the fifth semiconductor layer to electrically connect the fourth semiconductor layer with the second main electrode; wherein, in an orthogonal projection onto a plane parallel to the first main side, a first area occupied by the first semiconductor layer and the first emitter shorts overlaps in an overlapping area with a second area occupied by the fifth semiconductor layer and the second emitter shorts; and wherein, in the orthogonal projection onto the plane parallel to the first main side, the overlapping area, in which the first area overlaps with the second area, encompasses at least 50% of a total wafer area occupied by the semiconductor wafer.

16. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts occupy at least 2% of the overlapping area.

17. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts occupy at least 5% of the overlapping area.

18. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts occupy at least 10% of the overlapping area.

19. The bidirectional thyristor device according to claim 15, wherein, in in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts are discrete.

20. The bidirectional thyristor device according to claim 19, wherein a distance between two neighboring first emitter shorts is varied in such way that an average distance between two neighboring first emitter shorts decreases with increasing distance from the first gate electrode.

21. The bidirectional thyristor device according to claim 19, wherein, in the orthogonal projection onto the plane parallel to the first main side, the second emitter shorts are discrete.

22. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts and the second emitter shorts have a lateral size in a range from 30 m to 500 m.

23. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first emitter shorts and the second emitter shorts have a lateral size in a range from 50 m to 200 m.

24. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first gate electrode has a rotational symmetry.

25. The bidirectional thyristor device according to claim 24, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first gate electrode and the second gate electrode have a rotational symmetry.

26. The bidirectional thyristor device according to claim 15, wherein, in the orthogonal projection onto the plane parallel to the first main side, the first gate electrode and the second gate electrode have the same shape.

27. The bidirectional thyristor device according to claim 15, wherein a density of deep levels acting as recombination centers in the third semiconductor layer has a first local maximum that is closer to the second p-n junction than to the third p-n junction.

28. The bidirectional thyristor device according to claim 27, wherein the density of deep levels acting as recombination centers in the third semiconductor layer has a second local maximum that is closer to the third p-n junction than to the second p-n junction.

29. The bidirectional thyristor device according to claim 27, wherein the first local maximum is less than 50 m from the second p-n junction.

30. The bidirectional thyristor device according to claim 15, wherein an excess carrier lifetime has a first local minimum in the third semiconductor layer that is closer to the second p-n junction than to the third p-n junction.

31. The bidirectional thyristor device according to claim 30, wherein the excess carrier lifetime has a second local minimum which is closer to the third p-n junction than to the second p-n junction.

32. The bidirectional thyristor device according to claim 30, wherein the first local minimum is less than 50 m from the second p-n junction.

33. A bidirectional thyristor device comprising: a semiconductor wafer having a first main side and a second main side opposite to the first main side, wherein the semiconductor wafer includes in an order from the first main side to the second main side the following layers: a first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type different from the first conductivity type, wherein the first semiconductor layer and the second semiconductor layer form a first p-n junction; a third semiconductor layer of the first conductivity type, the second semiconductor layer and the third semiconductor layer forming a second p-n junction; a fourth semiconductor layer of the second conductivity type, wherein the third semiconductor layer and the fourth semiconductor layer form a third p-n junction; and a fifth semiconductor layer of the first conductivity type, wherein the fourth semiconductor layer and the fifth semiconductor layer form a fourth p-n junction; a first main electrode arranged on the first main side and in direct contact with the first semiconductor layer; a first gate electrode is arranged on the first main side spaced from the first main electrode, the first gate electrode in direct contact with the second semiconductor layer; a second main electrode arranged on the second main side and in direct contact with the fifth semiconductor layer, and a second gate electrode arranged on the second main side spaced from the second main electrode, the second gate electrode in direct contact with the fourth semiconductor layer; a plurality of first discrete emitter shorts, each first emitter short penetrating through the first semiconductor layer to electrically connect the second semiconductor layer with the first main electrode, wherein a distance between two neighboring first emitter shorts is varied in such way that an average distance between two neighboring first emitter shorts decreases with increasing distance from the first gate electrode; and a plurality of second discrete emitter shorts, each second emitter short penetrating through the fifth semiconductor layer to electrically connect the fourth semiconductor layer with the second main electrode, wherein a distance between two neighboring second emitter shorts is varied in such way that an average distance between two neighboring second emitter shorts decreases with increasing distance from the second gate electrode; wherein, in an orthogonal projection onto a plane parallel to the first main side, a first area occupied by the first semiconductor layer and the first emitter shorts overlaps in an overlapping area with a second area occupied by the fifth semiconductor layer and the second emitter shorts; and wherein, in the orthogonal projection onto the plane parallel to the first main side, the overlapping area, in which the first area overlaps with the second area, encompasses at least 50% of a total wafer area occupied by the semiconductor wafer.

34. A bidirectional thyristor device comprising: a semiconductor wafer having a first main side and a second main side opposite to the first main side, wherein the semiconductor wafer includes in an order from the first main side to the second main side the following layers: a first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type different from the first conductivity type, wherein the first semiconductor layer and the second semiconductor layer form a first p-n junction; a third semiconductor layer of the first conductivity type, the second semiconductor layer and the third semiconductor layer forming a second p-n junction; a fourth semiconductor layer of the second conductivity type, wherein the third semiconductor layer and the fourth semiconductor layer form a third p-n junction; and a fifth semiconductor layer of the first conductivity type, wherein the fourth semiconductor layer and the fifth semiconductor layer form a fourth p-n junction; a first main electrode arranged on the first main side and in direct contact with the first semiconductor layer; a first gate electrode is arranged on the first main side spaced from the first main electrode, the first gate electrode in direct contact with the second semiconductor layer; a second main electrode arranged on the second main side and in direct contact with the fifth semiconductor layer, and a second gate electrode arranged on the second main side spaced from the second main electrode, the second gate electrode in direct contact with the fourth semiconductor layer; a plurality of first emitter shorts, each first emitter short penetrating through the first semiconductor layer to electrically connect the second semiconductor layer with the first main electrode; and a plurality of second emitter shorts, each second emitter short penetrating through the fifth semiconductor layer to electrically connect the fourth semiconductor layer with the second main electrode; wherein, in an orthogonal projection onto a plane parallel to the first main side, a first area occupied by the first semiconductor layer and the first emitter shorts overlaps in an overlapping area with a second area occupied by the fifth semiconductor layer and the second emitter shorts; wherein, in the orthogonal projection onto the plane parallel to the first main side, the overlapping area, in which the first area overlaps with the second area, encompasses at least 50% of a total wafer area occupied by the semiconductor wafer; wherein a density of deep levels acting as recombination centers in the third semiconductor layer has a first local maximum that is closer to the second p-n junction than to the third p-n junction, the first local maximum is less than 50 m from the second p-n junction; and wherein the density of deep levels acting as recombination centers in the third semiconductor layer has a second local maximum that is closer to the third p-n junction than to the second p-n junction, the second local maximum is less than 50 m from the third p-n junction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Detailed embodiments of the invention will be explained below with reference to the accompanying figures, in which:

[0025] FIG. 1 shows a cross-sectional view of a bidirectional thyristor device according to an embodiment of the invention;

[0026] FIG. 2 shows a top view of the bidirectional thyristor device of FIG. 1;

[0027] FIG. 3 shows a bottom view of the bidirectional thyristor device of FIG. 1;

[0028] FIG. 4 shows an I-V curve of bidirectional thyristor devices having different patterns of first and second emitter shorts;

[0029] FIG. 5 shows a cross-sectional view of a bidirectional thyristor device of FIG. 1 together with a graph illustrating the spatial distribution of deep energetic levels in an energy band gap (point defects);

[0030] FIG. 6a shows a result of spreading resistance profiling for a specific example of the bidirectional thyristor device according to the embodiment; and

[0031] FIG. 6b shows a result of spreading resistance profiling for another specific example of the bidirectional thyristor device according to the embodiment.

[0032] The reference signs used in the figures and their meanings are summarized in the list of reference signs. Generally, similar elements have the same reference signs throughout the specification. The described embodiments are meant as examples and shall not limit the scope of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0033] FIG. 1 shows a vertical cross-section of a bidirectional thyristor device 100 according to an embodiment of the invention, FIG. 2 shows a top view of the bidirectional thyristor device 100 and FIG. 3 shows a bottom view of the bidirectional thyristor device 100. The bidirectional thyristor device 100 comprises a semiconductor wafer having a first main side 102 and a second main side 104 opposite and parallel to the first main side 102. The plane of the drawing in FIG. 1 is a plane perpendicular to the first main side 102. The cross-section shown in FIG. 1 is taken along a line A-A in FIGS. 2 and 3, respectively.

[0034] In an order from the first main side 102 of the semiconductor wafer, the semiconductor wafer comprises an n.sup.+-doped first semiconductor layer 106, a p-doped second semiconductor layer 108, an n.sup.-doped third semiconductor layer 110, a p-doped fourth semiconductor layer 112, and an n.sup.+-doped fifth semiconductor layer 114. The n.sup.+-doped first semiconductor layer 106 and the p-doped second semiconductor layer 108 form a first p-n junction J.sub.1, the p-doped second semiconductor layer 108 and the n.sup.-doped third semiconductor layer 110 form a second p-n junction J.sub.2, the n.sup.-doped third semiconductor layer 110 and the p-doped fourth semiconductor layer 112 form a third p-n junction J.sub.3, and the p-doped fourth semiconductor layer 112 and the n.sup.+-doped fifth semiconductor layer 114 form a fourth p-n junction J.sub.4. A plurality of first emitter shorts 128 is provided at the first main side 102, wherein each first emitter short 128 is a p-type semiconductor region penetrating through the first semiconductor layer 106 to electrically connect the p-type second semiconductor layer 108 with the first main electrode 115. The doping level of the first emitter shorts 128 may exemplarily be the same as that of the p-doped second semiconductor layer 108 or may be higher than that of the p-doped second semiconductor layer 108. Likewise, a plurality of second emitter shorts 138 is provided at the second main side 104, wherein each second emitter short 138 is a p-type semiconductor region penetrating through the n.sup.+-doped fifth semiconductor layer 114 to electrically connect the p-type fourth semiconductor layer 112 with the second main electrode 116. The doping level of the second emitter shorts 138 may exemplarily be the same as that of the p-doped fourth semiconductor layer 112 or may be higher than that of the p-doped fourth semiconductor layer 112. In the embodiment the first and the second emitter shorts 128, 138 are discrete. For example, the first and the second emitter shorts 128, 138 may be dot-shaped in the orthogonal projection onto the plane parallel to the first main side 102 and may have a lateral size in a range from 30 m to 500 m, exemplarily in a range from 50 m to 200 m. Therein, the lateral size is defined as the maximal lateral dimension in the orthogonal projection onto the plane parallel to the first main side 102. Alternatively, the first and second emitter shorts 128, 138 may be polygon-shaped.

[0035] On the first main side 102 of the semiconductor wafer there is arranged a first main electrode 115 in direct contact with the n.sup.+-doped first semiconductor layer 106 to form an ohmic contact to the n.sup.+-doped first semiconductor layer 106. Likewise, on the second main side 104 of the semiconductor wafer there is arranged a second main electrode 116 in direct contact with the n.sup.+-doped fifth semiconductor layer 114 to form an ohmic contact to the n.sup.+-doped fifth semiconductor layer 114. A first amplifying gate electrode 135 (which is an example of the first gate electrode in the claims) is arranged on the first main side 102. The first amplifying gate electrode 135 is electrically separated from the first main electrode 115 and is in direct contact with the p-doped second semiconductor layer 108 to form an ohmic contact with the p-doped second semiconductor layer 108 at a position lateral to the first main electrode 115. As can be seen best from FIG. 2, the first amplifying gate electrode 135 comprises a first ring-shaped electrode portion 135a, which is concentric with a center of the semiconductor in top view, first finger electrode portions 135b extending from the first ring-shaped electrode portion 135a towards an outer edge termination region 191 of the semiconductor wafer in top view, and second finger electrode portions 135c that bifurcate from the first finger electrode portions 135b and extend towards the outer edge termination region 191 of the semiconductor wafer.

[0036] Similarly, a second amplifying gate electrode 145 (which is an example of the second gate electrode in the claims) is arranged on the second main side 104 as can be seen in FIG. 1 or in FIG. 3. The second amplifying gate electrode 145 is electrically separated from the second main electrode 116 and is in direct contact with the p-doped fourth semiconductor layer 112 to form an ohmic contact with the p-doped fourth semiconductor layer 112 at a position lateral to the second main electrode 116. As can be seen best from FIG. 3, the second amplifying gate electrode 145 comprises a second ring-shaped electrode portion 145a, which is concentric with a center of the semiconductor in top view, third finger electrode portions 145b extending from the first ring-shaped electrode portion 145a towards the outer edge termination region 191 of the semiconductor wafer in top view, and fourth finger electrode portions 145c that bifurcate from the third finger electrode portions 145b towards the edge termination region 191 of the semiconductor wafer indicated in FIG. 2 or 3.

[0037] In an orthogonal projection onto a plane parallel to the first main side 102 an area occupied by the first semiconductor layer 106 and the first emitter shorts 128 will be referred to as a first area. Similarly, an area occupied by the fifth semiconductor layer 114 and the second emitter shorts 138 in the orthogonal projection onto the plane parallel to the first main side 102 will be referred to as a second area. The area in which the first area overlaps with the second area in the orthogonal projection onto the plane parallel to the first main side 102 is referred to as an overlapping area. In the orthogonal projection onto the plane parallel to the first main side 102 the first emitter shorts 128 and the second emitter shorts 138 are located within the overlapping area. In the bidirectional thyristor device 100 according to the embodiment the first area is identical to the second area, i.e. there is a perfect overlap between the first and second area.

[0038] A first thyristor comprising four semiconductor layers having alternating conductivity types, i.e. an n-p-n-p layer stack structure is formed in the bidirectional thyristor device 100 by the n.sup.+-doped first semiconductor layer 106, the p-doped second semiconductor layer 108, the n.sup.-doped third semiconductor layer 110, the p-doped fourth semiconductor layer 112 and the second emitter shorts 138. The n.sup.+-doped first semiconductor layer 106 is a cathode emitter layer of the first thyristor, the p-doped second semiconductor layer 108 is a p-doped base layer of the first thyristor, the n.sup.-doped third semiconductor layer 110 is a n.sup.-doped base layer of the first thyristor, and the p-doped fourth semiconductor layer 112 and the second emitter shorts 138 form together an anode layer of the first thyristor. The first main electrode 115 is a cathode electrode of the first thyristor, and the second main electrode 116 is an anode electrode of the first thyristor.

[0039] A second thyristor comprising four semiconductor layers having alternating conductivity types, i.e. an n-p-n-p layer stack structure is formed in the bidirectional thyristor device 100 by the n.sup.+-doped fifth semiconductor layer 114, the p-doped fourth semiconductor layer 112, the n.sup.-doped third semiconductor layer 110, the p-doped second semiconductor layer 108 and the first emitter shorts 128. The n.sup.+-doped fifth semiconductor layer 114 is a cathode emitter layer of the second thyristor, p-doped fourth semiconductor layer 112 is a p-doped base layer of the second thyristor, the n.sup.-doped third semiconductor layer 110 is an n.sup.-doped base layer of the second thyristor, and the p-doped second semiconductor layer 108 and the first emitter shorts 128 form together an anode layer of the second thyristor. The second main electrode 116 is a cathode electrode of the second thyristor, and the first main electrode 115 is an anode electrode of the second thyristor.

[0040] Accordingly, the first and the second thyristor are integrated in the bidirectional thyristor device 100 in an anti-parallel configuration between the first and second main electrode 115 and 116.

[0041] To facilitate triggering of the first thyristor in the bidirectional thyristor device 100, a first auxiliary thyristor is provided. The first auxiliary thyristor may also be referred to as a first pilot thyristor and is arranged laterally next to the first thyristor in the semiconductor wafer. In the orthogonal projection onto the plane parallel to the first main side 102 the first auxiliary thyristor is positioned in the center region of the wafer. The first auxiliary thyristor comprises four semiconductor layers having alternating conductivity types, i.e. an n-p-n-p layer stack structure like the first thyristor. In an order from the first main side 102 of the semiconductor wafer to the second main side 104 of the semiconductor wafer, the first auxiliary thyristor is formed by an n.sup.+-doped first auxiliary cathode emitter layer 152, the p-doped second semiconductor layer 108, the n.sup.-doped third semiconductor layer 110, and the p-doped fourth semiconductor layer 112 together with the second emitter shorts 138. In the orthogonal projection onto the plane parallel to the first main side 102, the n.sup.+-doped first auxiliary cathode emitter layer 152 is ring-shaped around a lateral center of the semiconductor wafer. The n.sup.+-doped first auxiliary cathode emitter layer 152 is overlapped and electrically contacted by an inner portion of the ring-shaped electrode portion 135a formed on the first main side 102 of the semiconductor wafer. In the embodiment the first amplifying gate electrode 135 is an amplifying gate for the first thyristor. In the center of the semiconductor wafer a first main gate electrode 175 is formed on the first main side 102 to be in direct contact with the p-doped second semiconductor layer 108.

[0042] To facilitate triggering of the second thyristor in the bidirectional thyristor device 100, a second auxiliary thyristor is provided. The second auxiliary thyristor may also be referred to as a second pilot thyristor and is arranged laterally next to the second thyristor in the semiconductor wafer. In the orthogonal projection onto the plane parallel to the first main side 102 the second auxiliary thyristor is positioned in the center region of the semiconductor wafer. The first auxiliary thyristor comprises four semiconductor layers having alternating conductivity types, i.e. an n-p-n-p layer stack structure like the second thyristor. In an order from the second main side 104 of the semiconductor wafer to the first main side 102 of the semiconductor wafer, the second auxiliary thyristor is formed by an n.sup.+-doped second auxiliary cathode emitter layer 162, the p-doped fourth semiconductor layer 112, the n.sup.-doped third semiconductor layer 110, and the p-doped second semiconductor layer 112 together with the first emitter shorts 128. In the orthogonal projection onto the plane parallel to the first main side 102, the n.sup.+-doped second auxiliary cathode emitter layer 162 is ring-shaped around the lateral center of the semiconductor wafer. The n.sup.+-doped second auxiliary cathode emitter layer 162 is overlapped and electrically contacted by an inner portion of the ring-shaped electrode portion 145a formed on the second main side 104 of the semiconductor wafer. In the embodiment, the second amplifying gate electrode 145 is an amplifying gate for the second thyristor. In the center of the semiconductor wafer a second main gate electrode 185 is formed on the second main side 104 to be in direct contact with the p-doped fourth semiconductor layer 112.

[0043] The first main gate electrode 175 may be connected to a gate unit (not shown in the Figures) via a first thin wire (not shown in the Figures), whereas the first main electrode 115 may be contacted by pressing a first molybdenum disk (not shown in the Figures) thereon. Likewise the second main gate electrode 185 may be connected to a gate unit (not shown in the Figures) via a second thin wire (not shown in the Figures), whereas the second main electrode 116 may be contacted by pressing a second molybdenum disk (not shown in the Figures) thereon.

[0044] In operation the plasma formation will spread during triggering of the first thyristor in the p-doped second semiconductor layer 108, in the n.sup.-doped third semiconductor layer 110 and in the p-doped fourth semiconductor layer 112 in a direction away from the first amplifying gate electrode 135, wherein the ignition process is speeded up by the distributed gate structure provided by the first amplifying gate electrode 135. Likewise, the plasma formation will spread during triggering of the second thyristor in the p-doped fourth semiconductor layer 112, in the n.sup.-doped third semiconductor layer 110 and the p-doped second semiconductor layer 108, in a direction away from the second amplifying gate electrode 145 wherein the ignition process is speeded up by the distributed gate structure provided by the second amplifying gate electrode 145.

[0045] When being viewed in the orthogonal projection onto the plane parallel to the first main side 102, the first emitter shorts 128 may occupy at least 2%, exemplarily at least 5%, exemplarily at least 8%, more exemplarily at least 10% of the overlapping area in which the first and the second area overlap. Likewise, when being viewed in the orthogonal projection onto the plane parallel to the first main side, the second emitter shorts 138 may occupy at least 2%, exemplarily at least 5%, exemplarily at least 8%, more exemplarily at least 10% of the overlapping area in which the first and the second area overlap.

[0046] Further, in the bidirectional thyristor device 100 of the embodiment the first emitter shorts 128 are distributed in the orthogonal projection onto the first main side 102 in a manner that a distance between two neighbouring discrete first emitter shorts 128 is decreasing with increasing distance from the first amplifying gate electrode 135. Such variation of the density of the first emitter shorts 128 allows the second thyristor to have a relatively low on-state voltage at a high anode current while the first thyristor can be triggered at a relatively low anode current (i.e. the first thyristor has a high 20 di/dt capability). Therein, the average distance between two neighbouring first emitter shorts 128 at a certain distance d from the first amplifying gate electrode 135 means the arithmetic mean of all pairs of neighbouring first emitter shorts 128 in an area including all positions that have a distance in a range from d to d+d, wherein d is constant for the calculation of the average distance at all distances d, for example d=5 mm. The density of the first emitter shorts 128 may increase either continuously with increasing distance from the first amplifying gate electrode 135 or may increase stepwise, i.e. there is a first region close to the first gate region, in which the density of the first emitter shorts 128 is relatively low (i.e. the average distance between neighbouring first emitter shorts 128 is relatively high) and a second region farther away from the first amplifying gate electrode 135 than the first region (i.e. the second region is separated from the first amplifying gate electrode 135 by the first region), in which the density of the first emitter shorts 128 is higher compared to the density of the first emitter shorts 128 in the first region (i.e. the average distance between neighbouring first emitter shorts 128 is lower compared to the average distance in the first region).

[0047] Likewise, in the bidirectional thyristor device 100 of the embodiment the second emitter shorts 138 are distributed in the orthogonal projection onto the first main side 102 in a manner that a distance between two neighbouring discrete second emitter shorts 138 is decreasing with increasing (lateral) distance from the second amplifying gate electrode 145. Such variation of the density of the second emitter shorts 138 allows the first thyristor to have a relatively low on-state voltage at a high anode current while the second thyristor can be triggered at a relatively low anode current (i.e. the second thyristor has a high didt capability). Therein, the average distance between two neighbouring second emitter shorts 138 at a certain distance d from the second amplifying gate electrode 145 means the arithmetic mean of a distance between all pairs of neighbouring second emitter shorts 138 in an area including all positions that have a distance in a range from d to d+d, wherein d is constant for the calculation of the average distance at all distances d, for example d=5 mm. The density of the second emitter shorts 138 may increase either continuously with increasing distance from the second amplifying gate electrode 145 or may increase stepwise, i.e. there is a first region dose to the second amplifying gate region 145, in which the density of the second emitter shorts 138 is relatively low (i.e. the average distance between neighbouring second emitter shorts 138 is relatively high) and a second region farther away from the second amplifying gate electrode 145 than the first region (i.e. the second region is separated from the second amplifying gate electrode 145 by the first region), in which the density of the second emitter shorts 138 is higher compared to the density of the second emitter shorts 138 in the first region (i.e. the average distance between neighbouring second emitter shorts 138 is lower compared to the average distance in the first region).

[0048] The effect of the variation of the density of the first and second emitter shorts 128 and 138 can be seen best from FIG. 4. There are shown the I-V curves for three different bidirectional thyristor devices. The three different bidirectional thyristor devices were identical with each other except for the density and pattern of the first and second emitter shorts 128 and 138. In all three different bidirectional thyristor devices the pattern of the first emitter shorts 128 were identical to the pattern of the second emitter shorts 138. The first curve A was measured for a bidirectional thyristor device having a constant low density of first and second emitter shorts 128 and 138, the second curve B was measured for a bidirectional thyristor device having a constant but relatively high density of first and second emitter shorts 128 and 138, and the third curve C was measured for a bidirectional thyristor device according to the embodiment, in which the density of the first and second emitter shorts 128 and 138 is varied to increase with increasing distance from the first and second amplifying gate electrodes 135 and 145, respectively. It can be seen that in curve A the bidirectional thyristor device is triggered at relatively low anode current but has a relatively high on-state voltage at high anode currents, whereas according to curve B the bidirectional thyristor device is triggered only at relatively high anode current but has a relatively low on-state voltage at high anode current. Finally, the curve C which was measured at the bidirectional thyristor with a variation of the density of first and second emitter shorts as explained above is triggered at a relatively low anode current and has a relatively low on-state voltage at high anode currents. The area of thyristors discussed above allows one to specify a typical rating current of about 2.5 kA. FIG. 4 shows that the application of the dense emitter short pattern provides a relatively low ON-state voltage even under overload conditions (short circuit operation) well above 2.5 kA.

[0049] As illustrated in FIG. 5, in the bidirectional thyristor device 100 according to the embodiment a (spatial) density of deep levels (radiation defects) acting as recombination centers in the n.sup.-doped third semiconductor layer 110 has along a line extending perpendicular to the first main side 102, a first local maximum which is closer to the second p-n junction J.sub.2 than to the third p-n junction J.sub.3, and has a second local maximum which is closer to the third p-n junction J.sub.3 than to the second p-n junction J.sub.2. In FIG. 5 the position of the first local maximum of the density of deep levels is indicated by a dotted line P.sub.1 and the position of the second local maximum of the density of deep levels is indicated by a dotted line P.sub.2 in FIG. 5. On the right side of FIG. 5 there is shown the density of deep levels (i.e. density of radiation defects which is referred to as a defect concentration in FIG. 5) as a function of the depthfrom the second main side 104. The first local maximum may exemplarily be less than 50 m from the second p-n junction J.sub.2, and the second local maximum may be exemplarily less than 50 m from the third p-n junction J.sub.3. The local maximum of the density of deep levels acting as recombination centers in the third semiconductor layer close to the second and/or third p-n junction can improve the turn-off capability of the bidirectional thyristor device. With growing distance of P.sub.1 (P.sub.2) from the adjacent p-n junction J.sub.2 (J.sub.3) the turn-off capability improvement grows at the penalty of a higher ON-state voltage drop (losses). An optimal distance of P.sub.1 (P.sub.2) from the adjacent junction J.sub.2 (J.sub.3) exists for a given thyristor structure and application (commutation turn-off) conditions.

[0050] The first local maximum of the density of deep levels in the third semiconductor layer 110 can, for example, be generated by proton irradiation with an appropriate energy that depends on the material and thickness of the layers through which the protons shall go through to form the deep level centers in the desired position relative to the second and third p-n junction J.sub.2 and J.sub.3. The deep levels may also be generated by irradiation with other particles such as electron irradiation or helium irradiation.

[0051] In accordance with the first and second local maximum of the density of deep levels acting as recombination centers in the n.sup.-doped third semiconductor layer 110, an excess carrier lifetime has a first and second local minimum at the same position as the first and second local maximum of the density of deep levels. The position of local minima of the excess carrier lifetime can, for example, be measured by spreading resistance profiling, which can show a local deviation from the background doping concentration in the n.sup.-doped third semiconductor layer 110 as a result of doping compensation by the acceptor type deep energetic levels formed by the radiation defects. In FIG. 6a there is shown a measurement result of spreading resistance profiling for a specific example of the bidirectional thyristor device according to the embodiment, in which the density of deep levels had a local maximum at a distance of about 10 m from the p-n junction J.sub.2/J.sub.3, and in FIG. 6b there is shown a measurement result of spreading resistance profiling for another specific example of the bidirectional thyristor device according to the embodiment, in which the density of deep levels had a local maximum at a distance of about 80 m from the p-n junction J.sub.2/J.sub.3. The locally reduced excess carrier lifetime facilitates the turn-off capability of the bidirectional thyristor device for both polarities.

[0052] It will be apparent for persons skilled in the art that modifications of the above described embodiment are possible without departing from the scope of the invention as defined by the appended claims.

[0053] In the bidirectional thyristor device 100 according to the embodiment the first area is identical to the second area, i.e. there is a perfect overlap between the first and second area. However, the bidirectional thyristor device of the invention may not have a perfect overlap between the first and the second area. It is sufficient if there exists an overlapping area between the first and second area in the orthogonal projection on the plane parallel to the first main side. In an exemplary embodiment of the bidirectional thyristor device, when being viewed in the orthogonal projection onto the plane parallel to a first main side of the semiconductor wafer, an overlapping area, in which the first area overlaps with the second area, encompasses at least 50% of a total wafer area occupied by the semiconductor wafer.

[0054] In the above embodiment of the bidirectional thyristor device some or all of the first to fourth finger electrode portions 135b, 135c, 145b, 145c may be omitted. Likewise the first and second amplifying gate electrodes 135, 145 may include additional finger electrode portions.

[0055] Also while the embodiment was described with the first pilot thyristor for facilitating triggering of the first thyristor and with the second pilot thyristor for facilitating triggering of the second thyristor, the bidirectional thyristor device of the invention does not necessarily include any pilot thyristor for triggering the first and second thyristor. That means that the whole first amplifying gate electrode 135 and the n.sup.+-doped first auxiliary cathode emitter layer 152 can be omitted. In this case the first main gate electrode would correspond to the first gate electrode in the claims. Likewise, the whole second amplifying gate electrode 145 and the n.sup.+-doped second auxiliary cathode emitter layer 162 could be omitted. In this case the second main gate electrode would correspond to the second gate electrode in the claims.

[0056] In the Figures of the above described embodiment the semiconductor wafer is shown in FIGS. 2 and 3 as a circular wafer. However, the invention can also be applied to other geometries of the semiconductor wafer. For example the semiconductor wafer may also have a rectangular shape or a polygonal shape.

[0057] The first emitter shorts 128 were described to be p-type semiconductor regions. However, they may also be formed of another conductive material which forms an ohmic contact to the p-doped second semiconductor layer 108. Likewise, the second emitter shorts 138 were described to be p-type semiconductor regions. However, they may also be formed of another conductive material which forms an ohmic contact to the p-doped fourth semiconductor layer 112.

[0058] The above embodiment was described without any emitter shorts in the n.sup.+-doped first cathode emitter layer 152 or in the n.sup.+-doped second cathode emitter layer 162. However, there may be provided first auxiliary emitter shorts penetrating through the n.sup.+-doped first cathode emitter layer 152 for connecting the p-type first semiconductor layer 108 with the first ring-shaped electrode portion 135a. Likewise there may be formed second auxiliary emitter shorts penetrating through the n.sup.+-doped second cathode emitter layer 162 for connecting the p-type fourth semiconductor layer 112 with the second ring-shaped electrode portion 145a.

[0059] In the above embodiment of the bidirectional thyristor device the first and fifth semiconductor layers 106 and 116 extend up to the edge termination region 191. However, a p-anode ring of the second thyristor (i.e. a cathode short ring of the first thyristor) contacted by the first main electrode 115 may be formed at the first main side 102 to laterally surround the outer edge of the first semiconductor layer 106. Likewise, a p-anode ring of the first thyristor (i.e. a cathode short ring of the second thyristor) contacted by the second main electrode 116 may be formed at the second main side 104 to laterally surround the outer edge of the fifth semiconductor layer 114. While the existence of this p-anode ring at the cathode side improves blocking stability, it provides at the same time a larger anode area at the opposite side.

[0060] In addition or alternatively to the before explained variation of the average distance between neighbouring first emitter shorts 128 the average lateral size of the first emitter shorts 128 may increase with increasing distance from the first amplifying gate electrode 135. Likewise, in addition or alternatively to the before explained variation of the average distance between neighbouring second emitter shorts 138 the average lateral size of the second emitter shorts 138 may increase with increasing distance from the second amplifying gate electrode 145.

[0061] In the above described embodiment the second and third p-n junctions J.sub.2 and J.sub.3 were respectively plane and parallel to the first main side. However, the distance between the second and third p-n junctions J.sub.2 and J.sub.3 may vary, for example in such way that the distance between the second and third p-n junctions J.sub.2 and J.sub.3 is smaller in the edge termination region than in an active region of the device.

[0062] In the above described embodiment the junction termination was formed by a negative bevel. However, the junction termination may be formed by positive bevel, combination of positive-negative bevel, guard rings, variation of lateral doping (VLD) structure, junction termination extension (JTE) or another semiconductor structure appropriate for this purpose.

[0063] It should be noted that the term comprising does not exclude other elements or steps and that the indefinite article a or an does not exclude the plural. Also elements described in association with different embodiments may be combined.

LIST OF REFERENCE SIGNS

[0064] 100 bidirectional thyristor device [0065] 102 first main side [0066] 104 second main side [0067] 106 n.sup.+-doped first semiconductor layer [0068] 108 p-doped second semiconductor layer [0069] 110 n.sup.-doped third semiconductor layer [0070] 112 p-doped fourth semiconductor layer [0071] 114 n.sup.+-doped fifth semiconductor layer [0072] 115 first main electrode [0073] 116 second main electrode [0074] 128 first emitter shorts [0075] 135 first amplifying gate electrode [0076] 135a first ring-shaped electrode portion [0077] 135b first finger electrode portion [0078] 135c second finger electrode portion [0079] 138 second emitter shorts [0080] 145 second amplifying gate electrode [0081] 145a second ring-shaped electrode portion [0082] 145b third finger electrode portion [0083] 145c fourth finger electrode portion [0084] 152 n.sup.+-doped first auxiliary cathode emitter layer [0085] 162 n.sup.+-doped second auxiliary cathode emitter layer [0086] 175 first main gate electrode [0087] 185 second main gate electrode [0088] J.sub.1 first p-n junction [0089] J.sub.2 second p-n junction [0090] J.sub.3 third p-n junction [0091] J.sub.4 fourth p-n junction [0092] P.sub.1 first local maximum of the density of deep levels [0093] P.sub.2 second local maximum of the density of deep levels