阅读说明：本技术 接触板内具有气体膨胀腔的旁路晶闸管装置 (Bypass thyristor device with gas expansion cavity in contact plate ) 是由 T.维克斯特雷姆 R.鲍曼 S.波普洛 B.奥厄德加德 于 2018-07-03 设计创作，主要内容包括：旁路晶闸管装置(20)包括：半导体装置(22),其提供晶闸管(24),晶闸管(24)具有阴极侧(28)上的阴极电极(26)、阴极侧(28)上的被阴极电极(26)环绕的栅电极(30)和阳极侧(32)上的阳极电极(34)；导电盖元件(38),其设置在阴极侧(28)上并且在接触侧(50)上与阴极电极(26)电接触；以及栅极接触元件(46),其电连接到栅电极(30)并且设置在盖元件(38)的接触侧(50)中的栅极接触开口(48)中；其中盖元件(38)在面对阴极侧(28)的接触侧(50)中具有气体膨胀体积(56),该气体膨胀体积(56)与栅极接触开口(48)互连以便进行气体交换。(The bypass thyristor device (20) comprises: a semiconductor device (22) providing a thyristor (24), the thyristor (24) having a cathode electrode (26) on a cathode side (28), a gate electrode (30) on the cathode side (28) surrounded by the cathode electrode (26), and an anode electrode (34) on an anode side (32); an electrically conductive cover element (38) which is arranged on the cathode side (28) and which is in electrical contact with the cathode electrode (26) on a contact side (50); and a gate contact element (46) electrically connected to the gate electrode (30) and arranged in a gate contact opening (48) in a contact side (50) of the cap element (38); wherein the cover element (38) has a gas expansion volume (56) in the contact side (50) facing the cathode side (28), which gas expansion volume (56) is interconnected with the gate contact opening (48) for gas exchange.)

1. A bypass thyristor device (20) comprising:

a semiconductor device (22), the semiconductor device (22) comprising a thyristor (24), the thyristor (24) having a cathode electrode (26) on a cathode side (28), a gate electrode (30) on the cathode side (28) surrounded by the cathode electrode (26), and an anode electrode (34) on an anode side (32);

an electrically conductive cover element (38), the electrically conductive cover element (38) being arranged on the cathode side (28) and being in electrical contact with the cathode electrode (26) on a contact side (50);

a gate contact element (46), the gate contact element (46) being electrically connected to the gate electrode (30) and being arranged in a gate contact opening (48) in the contact side (50) of the cap element (38);

wherein the cover element (38) has a gas expansion volume (56) in the contact side (50) facing the cathode side (28), the gas expansion volume (56) being interconnected with the gate contact opening (48) for gas exchange.

2. The bypass thyristor device (20) of claim 1,

wherein the volume of the gas expansion volume (56) is greater than the volume of the gate contact opening (48).

3. The bypass thyristor device (20) of claim 1 or 2,

wherein the gas expansion volume (56) is divided into at least two independent chambers (58) interconnected for gas exchange.

4. The bypass thyristor device (20) of one of the preceding claims,

wherein the gas expansion volume (56) comprises at least one trench (58) surrounding the gate contact opening (48).

5. The bypass thyristor device (20) of claim 5,

wherein the at least one trench (58) is interconnected with the gate contact opening (48) via at least one channel (60) provided in the contact side (50).

6. The bypass thyristor device (20) of one of the preceding claims,

wherein the gas expansion volume (56) comprises at least two trenches (58) surrounding the gate contact opening (48).

7. The bypass thyristor device (20) of claim 6,

wherein the at least two trenches (58) are interconnected with each other via at least one trench (60) provided in the contact side (50).

8. The bypass thyristor device (20) of one of the preceding claims,

wherein the cover element (38) comprises a gate cable trench (54), a gate cable (52) being arranged in the gate cable trench (54), the gate cable (52) being electrically connected to the gate contact element (46);

wherein the gate contact cable (52) is guided outside the bypass thyristor device (20) through the gate cable trench (54).

9. The bypass thyristor device (20) of claim 8,

wherein the cross-sectional area of the gate cable trench (54) is smaller than the cross-sectional area of one or more channels (60) interconnecting the gate contact opening (48) and the gas expansion volume (56).

10. The bypass thyristor device (20) of one of the preceding claims,

wherein the semiconductor device (22) has a circular shape and the gate electrode (30) is disposed in the center of the circular shape;

wherein the gas expansion volume (56) comprises one or more circular trenches (58), the circular trenches (58) surrounding the gate contact opening (48) disposed above the gate electrode (30).

11. The bypass thyristor device (20) of one of the preceding claims, further comprising:

an anode-side cover element (40) electrically connected to the anode electrode (34);

an electrically insulating flange (42) surrounding said semiconductor device (22), said cathode-side cover element (38) and said anode-side cover element (40) being attached to said flange (42).

12. The bypass thyristor (20) of one of the preceding claims,

wherein the thyristor (24) is adapted to form a permanent conduction path through an active region (70) of the semiconductor device (22) when a current flowing through the thyristor (24) exceeds a maximum current.

13. The bypass thyristor device (20) of one of the preceding claims,

wherein, for forming the thyristor (24), the semiconductor device (22) comprises the following layers from the cathode side (28) to the anode side (32):

a cathode layer (62) of a first conductivity type (n +) connected to the cathode electrode (26);

a base layer (64) connected to the gate electrode (30) having a second conductivity type (p) different from the first conductivity type;

a drift layer (66) of the first conductivity type (n-);

an anode layer (68) of the second conductivity type (p) connected to the anode electrode (34);

wherein in the active region (70), the semiconductor device (22) comprises: a first region (70 a) in which an average value of carrier lifetimes between the cathode side (28) and the anode side (32) has a first value; and a second region (70 b), wherein the mean value of the carrier lifetime between the cathode side (28) and the anode side (32) has a second value, which is smaller than the second value.

14. The bypass thyristor device (20) of one of the preceding claims,

wherein the first region (70 a) is disposed between the gate electrode (30) and the anode electrode (34).

15. A converter cell (10) comprising:

at least two power semiconductor switches (16) for connecting a cell capacitor (14) with an output (18) of the converter cell (10) and for disconnecting the cell capacitor (14) from the output (18);

the bypass thyristor device (20) connected in parallel to the cell capacitor (14) according to one of the preceding claims.

Technical Field

The present invention relates to a bypass thyristor device and a converter unit having such a bypass thyristor device.

Background

Modular Multilevel (MMC) converters may convert electrical power from high voltage Alternating Current (AC) to Direct Current (DC), or vice versa. An example of an application is High Voltage Direct Current (HVDC) power transmission systems, which transmit electrical energy over long distances or between AC power systems of different frequencies. For converting high voltages (such as several hundred kV) and high powers (e.g. in the GW range), the converter station may comprise a plurality of such MMCs.

Each MMC may utilize a cascade of individual inverter sub-modules or converter cells, each having its own storage capacitor, which may also be referred to as a cell capacitor. In the phase leg of the MMC the converter cells are typically connected in series. The converter cells may be controlled independently of each other such that AC current may be generated from DC current in as many stages as the converter cells are available.

MMC converters are typically designed to include redundancy by increasing the number of converter cells to more converter cells than necessary. In case of a failure, the failed converter cell may have to be discharged and bypassed to avoid overcharging the DC link capacitor. This function is typically provided by a bypass switch. The energy stored in the converter cells can be very large, even exceeding 100kJ, making it challenging to discharge the converter cells without causing more damage to the converter cells themselves or surrounding cells.

One possibility of such a bypass switch is a mechanical switch driven by an explosive. Such a pyrotechnic device may be expensive and may require efforts to adapt it structurally to the converter so that it can be easily replaced and repaired while keeping volume, stray impedance, and capacitance to a minimum. In addition, explosive charges may require routine replacement to ensure function, and mechanical components may require maintenance. In the case of a failure of the bypass switch itself, it fails in the off state so that the capacitor may be destroyed due to overcharge.

As a further example, WO 2013/044961 a1 describes the use of thyristors as bypass thyristors.

EP 0220698 a2 relates to a thyristor with a circular housing, in which the gate contacts are guided through slots (slips) in the cover plate and through radial openings in the insulating ring to the outside.

Disclosure of Invention

It is an object of the invention to provide a reliable, easy to integrate, easy to maintain and economical bypass switch for a converter cell.

This object is achieved by the subject matter of the independent claims. Further exemplary embodiments are apparent from the dependent claims and the following description.

Aspects of the present invention relate to a bypass thyristor device. Basically, the bypass device may be any device suitable for bypassing another electrical device, which may be performed by triggering the bypass switch into a conducting state. The present bypass device is based on a thyristor as a bypass switch and may comprise a semiconductor device together with a housing enclosing the semiconductor device and providing electrical contacts. The semiconductor device may be or may include a semiconductor wafer or semiconductor chip. It has to be noted that the present disclosure may relate to a power bypass thyristor device, which is adapted to handle voltages above 600V and/or above 10A.

According to an embodiment of the invention, the bypass thyristor device comprises a semiconductor device comprising a thyristor having a cathode electrode on the cathode side, a gate electrode (gateelectrode) on the cathode side surrounded by the cathode electrode, and an anode electrode on the anode side. The cathode side and the anode side may be opposite sides on the semiconductor device. The semiconductor device may be a generally circular plate in which elements (elements) of the thyristor have been doped. The cathode electrode, the gate electrode and the anode electrode may be provided by a metallization layer on a substrate of the semiconductor device.

Further, the bypass thyristor device includes: a conductive cap (cover) element disposed on the cathode side and in electrical contact with the cathode electrode on the contact side; and a gate contact member electrically connected to the gate electrode and disposed in the gate contact opening in the contact side of the cap member. The gate contact opening may be a hole in the cap member in which the gate contact member is received. The gate contact element may be electrically connected with a cable led through the cover element to the outside of the bypass thyristor device.

Furthermore, the bypass thyristor device may comprise a further cover element which is arranged on the anode side and which is in electrical contact with the anode electrode. The further cover element may be regarded as an anode cover element and the further cover element as a cathode cover element. The two cover elements may be mechanically interconnected with an electrically insulating flange (flange) which may surround the semiconductor device. The cover element and the insulating flange (which may be made of ceramic) may be considered as a housing of the bypass thyristor device. The insulating flange and the cover member constitute a hermetic seal of the interior of the housing.

The cathode cover element has a gas expansion volume in the contact side facing the cathode side, which gas expansion volume is interconnected with the gate contact opening for gas exchange. The gas expansion volume and the semiconductor device form a cavity in the bypass thyristor device that is adapted to contain a gas and/or plasma that vaporizes from the semiconductor device in the event of thermal destruction of the thyristor and/or semiconductor device. This may be the case when the current through the thyristor becomes higher than the maximum inrush current of the thyristor.

Such an event may occur, for example, when the charge of a cell capacitor shorted by the bypass switching device is strongly charged. Further, such an event may be expected to form a permanent conductive path through the semiconductor device. Due to the high current, the material of the semiconductor device may melt and a conduction path may be formed. Without the gas expansion volume, the plasma and/or gas generated within the semiconductor device may generate high pressure on the cover element, which may cause the housing of the bypass thyristor device to rupture. In addition, the walls of the gas expansion volume may cool the gas and may promote condensation.

A bypass thyristor device having such a gas expansion volume may have an improved non-rupture capability without increasing production costs. This may enable the design of a higher power MMC converter cell without increasing the cost of the bypass solution.

According to an embodiment of the invention, the volume of the gas expansion volume is larger than the volume of the gate contact opening. This may be the case: the volume of the gas expansion volume is at least 10 times the volume of the gate contact opening. The volume of the gas expansion volume may depend on the average amount of gas generated during the bypass operation. The optimal volume can be determined experimentally.

According to an embodiment of the invention, the gas expansion volume is divided into at least two separate chambers interconnected for gas exchange. Expansion of the gas into multiple chambers may be beneficial. On the one hand, the mechanical stability of the cover element can be enhanced by more but smaller cavities. On the other hand, the surface of the gas expansion volume can be increased, which can cool the boil-off gas.

According to an embodiment of the invention, the gas expansion volume comprises at least one trench (trench) surrounding the gate contact opening. For example, such a groove may be machined into the cover element. Such trenches may also have a height greater than the height of the gate contact opening.

According to an embodiment of the invention, the at least one trench is interconnected with the gate contact opening via at least one channel provided in the contact side. Such a channel may have a smaller cross-sectional area than the at least one trench. Further, the gate contact opening and the at least one trench may be interconnected with a plurality of channels.

According to an embodiment of the invention, the gas expansion volume comprises at least two trenches surrounding the gate contact opening. The trenches may be linked to each other as the gate contact opening is linked to one of the trenches.

According to an embodiment of the invention, the at least two trenches are interconnected with each other via at least one trench provided in the contact side, which may have a smaller cross-sectional area than each trench. Two trenches may interconnect the plurality of trenches.

According to an embodiment of the invention, the cover element comprises a gate cable trench in which a gate cable is arranged, which gate cable is electrically connected to the gate contact element. The gate contact cable may be guided to the outside of the bypass thyristor device through the gate cable trench. The gate cable trench may be smaller than the gas expansion volume and/or its trench. It may only have a cross-section suitable for accommodating the grid contact cable. The amount of air that is forced through the gate cable trench and reaches the interior air of the bypass thyristor device can be significantly reduced or even eliminated with a gas expansion volume. The need for a cover member and an insulating flange of the protective housing may be reduced or eliminated.

According to an embodiment of the invention, the cross-sectional area of the gate cable trench is smaller than the cross-sectional area of the one or more channels interconnecting the gate contact opening and the gas expansion volume. The channel may be adapted to equalize pressure between the gate contact opening and the gas expansion volume and/or between different cavities of the gas expansion volume. The gate cable trench may only accommodate the gate cable and may be quite small.

According to an embodiment of the present invention, the semiconductor device has a circular shape and the gate electrode is disposed at the center of the circular shape. The gas expansion volume may include one or more circular trenches surrounding a gate contact opening, which may be disposed over the gate electrode. Furthermore, the channel interconnecting the gate contact opening and the one or more circular trenches is arranged in a radial direction with respect to the center. This may provide a mechanically stable cover element with a large gas expansion volume, which takes into account the geometry of the circular semiconductor device.

According to an embodiment of the present invention, the bypass thyristor device further comprises an anode-side cover element electrically connected to the anode electrode and an electrically insulating flange surrounding the semiconductor device, the cathode-side cover element and the anode-side cover element being attached to the flange. As already mentioned, the housing of the bypass thyristor device may be made of two cover elements interconnected with an insulating flange (which may be made of ceramic).

According to an embodiment of the invention, the thyristor is adapted to form a permanent conduction path through the active region of the semiconductor device when the current flowing through the semiconductor switch exceeds a maximum current. Such a path may be formed when the current through the thyristor melts the material of the semiconductor device. In this case, a gas may be generated that is contained by the gas expansion volume.

According to an embodiment of the invention, for forming a thyristor, the semiconductor arrangement comprises the following layers from the cathode side to the anode side: a cathode layer of a first conductivity type (such as n +) connected to the cathode electrode; a base layer having a second conductivity type (such as p) different from the first conductivity type connected to the gate electrode; a drift layer having a first conductivity type (such as n-); and an anode layer of a second conductivity type (such as p) connected to the anode electrode. In an active region (in which a cathode electrode contacts a cathode layer), a semiconductor device includes: a first region in which an average value of carrier lifetimes between the cathode side and the anode side has a first value; and a second region, wherein an average value of the lifetime of carriers between the cathode side and the anode side has a second value, which is smaller than the second value.

In other words, the active region of the semiconductor device is divided into a first longer carrier lifetime region and a second shorter carrier lifetime region.

With such a thyristor, a short-circuit failure mode of the converter cell may be provided, wherein the thyristor may act as a bypass switch for the cell capacitor. The first region with longer carrier lifetime can trigger the thyristor and conduct current. The second region with a shorter carrier lifetime may not contribute significantly in terms of on-current, forcing current to flow mainly through the longer lifetime region. In case less energy is stored on the cell capacitor, the switch may be reliably disabled, since the first region is only a part of the active region of the semiconductor device. The second, reduced carrier lifetime region may increase the on-state voltage to such regions where little current flows and may increase the semiconductor volume where heat may diffuse in the event of a short circuit failure.

According to an embodiment of the invention, the second value is at most 75% of the first value. This may ensure reliable triggering of the thyristor.

According to an embodiment of the present invention, the first region is disposed between the gate electrode and the anode electrode. In other words, the second region may be a central region, which is surrounded by the first region. In the short failure mode, all material around the first region may contribute to forming a conductive path.

However, other types of thyristors that can be triggered into a short circuit failure mode may also be used to bypass the thyristor arrangement.

Further aspects of the invention relate to a converter cell comprising a bypass thyristor device as described above and below. Such converter cells may be interconnected to form a converter, such as a modular multilevel converter. In case of a failure of the converter cell, the bypass thyristor may be triggered to discharge the cell capacitor of the converter cell. In this way, the redundant converter cells can be disconnected.

It has to be noted that the characteristics of the bypass thyristor device may be the characteristics of the converter cell and vice versa.

According to an embodiment of the invention, the converter unit comprises: at least two power semiconductor switches for connecting the cell capacitor with the output of the converter cell and for disconnecting the cell capacitor from the output; and a bypass thyristor device connected in parallel to the cell capacitor as described above and below.

The converter cell may comprise one or more half bridges consisting of two series-connected power semiconductor switches. The half-bridge may be connected in parallel with the cell capacitor and the bypass thyristor. The output of the converter cell may be provided by the midpoint of the half bridge.

The power semiconductor switches for switching the current through the converter cells may be transistors or thyristors, such as Integrated Gate Bipolar Transistors (IGBTs) or Insulated Gate Commutated Thyristors (IGCTs). However, these power semiconductor switches, although thyristors at this time, may have a different design than the bypass thyristor devices. The requirements on the bypass thyristor device may be very different from the requirements on the other switches of the converter unit, since the bypass thyristor device is blocking during normal operation of the converter unit. In the absence of conduction or switching losses in normal operation, the bypass thyristor device may exhibit approximately the cooling medium temperature. Thus, the requirements for load-and temperature cycling can be completely eliminated compared to the switching of the converter cells. The reduction in requirements provides more design freedom in terms of thermal resistance and load cycle resilience. Thus, a bypass thyristor device as described above and below may be used, e.g. a bypass thyristor device based on a thyristor as described above and below and/or having an active region as described above and below divided into a first region and a second region having different carrier lifetimes.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Drawings

The subject matter of the invention will be explained in more detail hereinafter with reference to exemplary embodiments illustrated in the drawings.

Fig. 1 schematically shows a circuit diagram of a full bridge converter cell according to an embodiment of the invention.

Fig. 2 schematically shows a cross-sectional view of a bypass thyristor device according to an embodiment of the invention.

Fig. 3 shows a perspective view of a cover element for a bypass thyristor device according to an embodiment of the invention.

Fig. 4 schematically shows a cross-sectional view of a semiconductor device for a bypass thyristor device according to an embodiment of the invention.

The reference symbols used in the figures and their meanings are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

Detailed Description

Fig. 1 shows a converter cell 10 comprising a full bridge 12 interconnected in parallel with a cell capacitor 14. The full bridge 12 is composed of four reverse conducting semiconductor switches 16, here IGCTs, which reverse conducting semiconductor switches 16 are used to attach or disconnect the cell capacitors directly or reversely to the output terminals (18). The output 18 of the converter cell 10 is provided by the midpoint of the half bridge 12, said output 18 being disconnectable from the cell capacitor 14 and connectable to the cell capacitor in both directions, i.e. the converter cell 10 is a bipolar cell. It may also be such that: the converter cell 10 is a unipolar cell or has another switching topology. Converter cells like the converter cell 10 shown in fig. 1 may be connected in series to form an arm of a Modular Multilevel Converter (MMC).

In addition, the converter cell 10 comprises a bypass thyristor device 20 adapted to short-circuit the cell capacitor 14.

Fig. 2 shows the bypass thyristor device 20 in more detail. The bypass thyristor device 20 comprises a semiconductor device 22 which comprises or provides a thyristor 24. The semiconductor arrangement 22 is provided with a cathode electrode 26 and a gate electrode 30 on a cathode side 28, which gate electrode 30 is surrounded by the cathode electrode 26. On the other anode side 32, the semiconductor arrangement 22 is provided with an anode electrode 34.

The semiconductor device 22 may be made of doped Si or doped SiC as a substrate. The cathode electrode 26, the gate electrode 30, and the anode electrode 34 may be metallization layers on the substrate of the semiconductor device 22. The semiconductor arrangement 22, which is shown in cross section from the side in fig. 2, may have a circular shape when viewed from above.

The semiconductor device 22 is enclosed in a housing 36 of the bypass thyristor device 20, the housing 36 including a cathode-side cover element 38 and an anode-side cover element 40, the cathode-side cover element 38 and the anode-side cover element 40 sandwiching the semiconductor device 22 therebetween. The cover members 38, 40 are electrically conductive and in direct electrical contact with the cathode electrode 26 and the anode electrode 34, respectively. For example, the cover member may be made of metal such as Cu or Al.

The cover elements 38, 40 are separated by an electrically insulating flange 42, the electrically insulating flange 42 surrounding the semiconductor device 22 and/or the electrically insulating flange 42 may be made of ceramic. Between the cover elements 38, 40 and inside the flange 42, radially outside the semiconductor device 22, there is an outer interior space 44, which may be filled with air.

The gate electrode 30 may be located in the center of the semiconductor device 22. A gate contact element 46 directly electrically connected to the gate electrode 30 is provided in a gate contact opening 48 in a contact side 50 of the cap element 38. For example, the gate contact element is soldered or sintered to the gate electrode 30.

The gate contact element 46 is electrically connected to a gate contact cable 52 which is led outside the bypass thyristor device 20 through a gate cable trench 54. A gate cable trench 54 is provided in the contact side 50 of the cover element 38. The grid contact cable 52 may be guided through the flange 42 through a further opening.

In addition, the cover element 38 has a gas expansion volume 56 provided in the contact side 50, which gas expansion volume 56 may consist of one or more cavities 58, which cavities 58 are interconnected for gas exchange with the gate contact openings 48. Such gas exchange may take place through the channels 60, the channels 60 being provided in the contact side 50 and the channels 60 interconnecting the cavities 58 with each other and with the gate contact openings 48.

The gas expansion volume 56 is used to mitigate the effects of gases and/or plasmas generated by the semiconductor device 22 through high current events. Triggering the bypass thyristor device 20 with the high energy stored in the cell capacitor 14 may discharge a current that exceeds the inrush current capability of the bypass thyristor device 20 by orders of magnitude. The active region of the semiconductor device 22 may melt and may vaporize into a gaseous state. This gas may be contained in a gas expansion volume 56, its pressure may be reduced and/or it may be cooled.

In contact elements without a gas expansion volume 56, there may be little volume available for the gas generated. Hot gas may travel along the grid cable trench 54 and into the interior space 44, where it compromises the integrity of the ceramic flange 42 and the sealing of the cover elements 38, 40. It can also generate sufficient pressure to force the surfaces apart and escape through the sealing ring.

As shown in fig. 2, the volume of the gas expansion volume 56 may be 2 times or more the volume of the gate contact opening 48. This may be achieved by a deeper and/or wider cavity 58 than the gate contact opening 48.

As with the gate contact openings 48 and the gate cable trenches 54, the cavities 58 and channels 60 may be machined (machined) into the cover member 38.

Fig. 3 shows an embodiment of the cover element 38 in a perspective view from the direction towards the contact side 50. The gate contact opening 48 may be a central hole surrounded by a coaxial annular cavity 58 in the form of a trench. The trenches 58 and the gate contact openings 48 are interconnected with radially extending channels 60, which radially extending channels 60 may have a smaller cross-sectional area than the trenches 58. The gate cable trench 54, which has a smaller cross-sectional area than the channel 60, may also run in a radial direction.

It can be seen that the only possibility for gas exchange between the gas expansion volume 56 and the inner space 44 may be via the grid cable trench 54, which may have a cross-sectional area substantially like the grid contact cable 52. Thus, a majority of the gas generated by the above-mentioned events may reside within the gas expansion volume 56.

It may also be such that: the semiconductor device 22 is intentionally melted during a high energy event to form a conduction path through the bypass thyristor device 20. Such a semiconductor device 22 is shown in fig. 4.

From the cathode side 28 to the anode side 32 opposite the cathode side 28, the semiconductor device 22 includes an n + doped cathode layer 62, a p doped base layer 64, an n-doped drift layer 66, and a p doped anode layer 68. Cathode electrode 26 is provided on cathode layer 62 and anode electrode 34 is provided on anode layer 68.

The gate electrode 30 is provided on the base layer 64, and the cathode layer 62 is embedded in the base layer 64. The base layer 64 may be a continuous layer below the cathode layer 62. The bottom and lateral sides of the cathode layer of cathode layer 62 may contact base layer 64.

The active area 70 of the semiconductor device 22, which may be considered as the area where the cathode electrode 26 contacts the cathode layer 62, is divided into: a first region 70a extending from the cathode side 28 to the anode side 32, wherein an average of carrier lifetimes between the cathode side 28 and the anode side 32 has a first value; and a second region 70b in which the average value of the carrier lifetime between the cathode side 28 and the anode side 42 has a second value, which may be smaller than the first value. The second value may be at most 75% of the first value.

For example, in all planes parallel to the cathode side 28, the average carrier lifetime in the second region 70b may be less than the average carrier lifetime in the first region 70a in the same plane, such as at most 75% of the average carrier lifetime in the first region 70a in the same plane.

In further examples, the average carrier lifetime in the second region 70b between the first and second pn junctions 72, 74 may be less than the average carrier lifetime in the first region, such as at most 75% of the average carrier lifetime in the first region.

In a plane parallel to the cathode side 28, the area of the first region 70a may be at least 0.1% of the area of the active region 70. Such a small area (or more precisely, the area in the projection of this area into the cathode side 28 and the anode side 32) may be sufficient to fire the bypass thyristor 24 in the case of a Short Circuit Failure Mode (SCFM). However, the area of the first region 70a may also have a size of at most 50% or at most 5% of the area of the active region 70. This means that in an exemplary embodiment, the second region 70b (in a plane parallel to the cathode side 28) may have a size of between 50% and 99.9% of the area of the active region 70.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims shall not be construed as limiting the scope.

REFERENCE SIGNS LIST

10 converter unit

12 half bridge

14 cell capacitor

16 semiconductor switch

18 output

20 bypass thyristor device

22 semiconductor device

24 thyristor

26 cathode electrode

28 cathode side

30 gate electrode

32 anode side

34 anode electrode

36 outer cover

38 cover element

40 cover element

42 flange

44 inner space

46 gate contact element

48 gate contact opening

50 contact side

52 grid contact cable

54 grid cable trench

56 gas expansion volume

58 cavities, grooves

60 channel

62 cathode layer

64 base layer

66 drift layer

68 anode layer

70 active region

70a first region

70b second region

72 first pn junction

74 second pn junction