Patent Publication Number: US-9899478-B2

Title: Desaturable semiconductor device with transistor cells and auxiliary cells

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Application Serial No. 102015111347.3 filed Jul. 14, 2015 and entitled “Desaturable Semiconductor Device with Transistor Cells and Auxiliary Cells.” 
     BACKGROUND 
     In semiconductor devices including both transistor cells and a diode functionality such as MCDs (MOS controlled diodes) and RC-IGBTs (reverse conducting insulated gate bipolar transistors), mobile charge carriers flood a semiconductor region along a forward biased pn junction and form a dense charge carrier plasma resulting in a low forward resistance of the diode. When the concerned pn junction commutates thereby changing from forward biased to reverse biased, a reverse recovery current removes the charge carrier plasma. The reverse recovery current contributes to dynamic switching losses of the semiconductor device. Typically, in a desaturation period preceding the change of the pn junction from forward biased to reverse biased a gated MOS channel attenuates the charge carrier plasma in order to reduce the dynamic switching losses. A safety period between the end of the desaturation period and the beginning of the commutation secures that the semiconductor device is in a blocking mode with closed MOS channel before commutation starts. During the safety period the charge carrier plasma partially recovers and foils to some degree the desaturation mechanism. 
     It is desirable to improve the switching characteristics of semiconductor devices including transistor cells as well as a diode functionality. 
     SUMMARY 
     According to an embodiment a semiconductor device includes transistor cells configured to connect a first load electrode with a drift structure forming first pn junctions with body zones when a gate voltage applied to a gate electrode exceeds a first threshold voltage. First auxiliary cells in a vertical projection of and electrically connected with the first load electrode are configured to inject charge carriers into the drift structure at least in a forward biased mode of the first pn junctions. Second auxiliary cells are configured to inject charge carriers into the drift structure at high emitter efficiency when in the forward biased mode of the first pn junctions the gate voltage is below a second threshold voltage lower than the first threshold voltage and at low emitter efficiency when the gate voltage exceeds the second threshold voltage. 
     According to an embodiment, a semiconductor device includes a semiconductor body that includes a drift structure and cell mesas formed between gate structures that extend from a first surface of the semiconductor body into the drift structure. The cell mesas include bottleneck sections and wide sections between the bottleneck sections and the first surface, wherein the wide sections are wider than narrow portions of the bottleneck sections. Transistor cells include body zones forming first pn junctions with the drift structure and second pn junctions with source zones. First auxiliary cells are electrically connected in parallel to the transistor cells and second auxiliary cells are electrically connected in parallel to the transistor cells, wherein the narrow portions of the bottleneck sections in the first auxiliary cells are wider than the narrow portions of the bottleneck sections in the second auxiliary cells. 
     Those skilled in the art will recognize additional features and advantages upon reading the following detailed description and on viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain principles of the invention. Other embodiments of the invention and intended advantages will be readily appreciated as they become better understood by reference to the following detailed description. 
         FIG. 1A  combines schematic vertical cross-sectional views of portions of a semiconductor device with transistor cells, first auxiliary cells and second auxiliary cells according to an embodiment. 
         FIG. 1B  is a schematic diagram illustrating characteristics of the transistor cells, first auxiliary cells and second auxiliary cells of  FIG. 1A  for discussing effects of the embodiments. 
         FIG. 1C  is a schematic timing diagram illustrating a method of operating the semiconductor device of  FIG. 1A . 
         FIG. 2A  is a schematic vertical cross-sectional view of a portion of a semiconductor device according to an embodiment related to RC-IGBTs. 
         FIG. 2B  is a schematic vertical cross-sectional view of a portion of a semiconductor device according to an embodiment related to MCDs. 
         FIG. 3A  is a schematic horizontal cross-sectional view of an RC-IGBT according to an embodiment with evenly distributed first auxiliary cells. 
         FIG. 3B  is a schematic horizontal cross-sectional view of an RC-IGBT according to an embodiment with first auxiliary cells arranged in the center of the diode region. 
         FIG. 3C  is a schematic horizontal cross-sectional view of an RC-IGBT according to an embodiment with one or more first auxiliary cells arranged in a center of a pilot region, which is surrounded by a bipolar region. 
         FIG. 3D  is a schematic horizontal cross-sectional view of an RC-IGBT according to an embodiment with auxiliary cells arranged in peripheral portions of a pilot region, which is surrounded by a bipolar region. 
         FIG. 4  is a schematic vertical cross-sectional view of an RC-IGBT for illustrating the arrangement of auxiliary cells according to embodiments related to wide collector channels. 
         FIG. 5A  is a schematic horizontal cross-sectional view of a portion of an RC-IGBT according to an embodiment related to auxiliary cells defined by openings in a barrier structure. 
         FIG. 5B  is a schematic planar projection of a vertical cross-section of the semiconductor device portion of  FIG. 5A  along line B-B. 
         FIG. 5C  is a schematic vertical cross-sectional view of the semiconductor device portion of  FIG. 5A  along line C-C. 
         FIG. 6A  is a schematic vertical cross-sectional view of a portion of an RC-IGBT along a longitudinal mesa axis according to an embodiment related to auxiliary cells defined by a barrier structure with locally attenuated portions. 
         FIG. 6B  is a schematic horizontal cross-sectional view of a portion of an RC-IGBT according to an embodiment related to auxiliary cells defined by a variation of a cell mesa width. 
         FIG. 6C  is a schematic horizontal cross-sectional view of a portion of an RC-IGBT according to an embodiment related to auxiliary cells defined in mesas of different widths. 
         FIG. 7A  is a schematic horizontal cross-sectional view of a portion of an RC-IGBT according to another embodiment related to auxiliary cells defined by locally widened cell mesas. 
         FIG. 7B  is a schematic planar projection of a vertical cross-section of the semiconductor device portion of  FIG. 7A  along line B-B. 
         FIG. 7C  is a schematic vertical cross-sectional view of the semiconductor device portion of  FIG. 7A  along line C-C. 
         FIG. 7D  is a schematic horizontal cross-sectional view of a portion of an RC-IGBT according to another embodiment related to first auxiliary cells in wide cell mesas and second auxiliary cells in narrow cell mesas. 
         FIG. 8A  is a schematic vertical cross-sectional view of a portion of a semiconductor device with injection cells based on cell mesas including bottleneck sections for illustrating effects of the embodiments. 
         FIG. 8B  is a schematic diagram plotting collector-to-emitter voltage VCE and storage charge QF of the injection cells of  FIG. 8A  against the gate voltage for different vertical extensions of narrow portions of the bottleneck sections. 
         FIG. 8C  shows a section of the diagram of  FIG. 8B  around a gate voltage of 0V in detail. 
         FIG. 9A  combines schematic vertical cross-sectional views of portions of a semiconductor device according to an embodiment related to meta cells. 
         FIG. 9B  is a schematic diagram for illustrating the effect of the meta cells in  FIG. 9A . 
         FIG. 10A  is a schematic horizontal cross-sectional view of a portion of a semiconductor device in accordance with an embodiment concerning RC-IGBTs with meta cells. 
         FIG. 10B  is a schematic planar projection of a vertical cross-section of the semiconductor device portion of  FIG. 10A  along line B-B. 
         FIG. 10C  is a schematic vertical cross-sectional view of the semiconductor device portion of  FIG. 10A  along line C-C. 
         FIG. 11  is a schematic planar projection of a vertical cross-section of a semiconductor device according to an embodiment combining barrier structures with bottle shaped gate structures. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and in which are shown by way of illustrations specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. For example, features illustrated or described for one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present invention includes such modifications and variations. The examples are described using specific language, which should not be construed as limiting the scope of the appending claims. The drawings are not scaled and are for illustrative purposes only. Corresponding elements are designated by the same reference sign in the different drawings, respectively, if not stated otherwise. 
     The terms “having”, “containing”, “including”, “comprising” and the like are open, and the terms indicate the presence of stated structures, elements or features but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
     The term “electrically connected” describes a permanent low-ohmic connection between electrically connected elements, for example a direct contact between the concerned elements or a low-ohmic connection via a metal and/or highly doped semiconductor. The term “electrically coupled” includes that one or more intervening element(s) adapted for signal transmission may be provided between the electrically coupled elements, for example resistors or elements that are controllable to temporarily provide a low-ohmic connection in a first state and a high-ohmic electric decoupling in a second state. 
       FIG. 1A  shows a portion of a semiconductor device  500 , for example an MCD such as an MGD (MOS-gated diode) with shorted gate, an RC-IGBT or a device including an MCD or RC-IGBT functionality. Silicon (Si), silicon carbide (SiC), germanium (Ge), a silicon germanium crystal (SiGe), gallium nitride (GaN), gallium arsenide (GaAs) or any other A III B V  semiconductor forms a semiconductor body  100  of the semiconductor device  500 . 
     At a front side the semiconductor body  100  has a first surface  101  which may be approximately planar or which may be spanned by coplanar surface sections. A minimum distance between the first surface  101  and a mainly planar second surface at an opposite rear side and parallel to the first surface  101  defines the voltage blocking capability of the semiconductor device  500 . For example, the semiconductor body  100  of an RC-IGBT specified for a blocking voltage of about 1200 V may have a thickness of 90 μm to 110 μm. Embodiments related to higher blocking capabilities may be based on semiconductor bodies  100  with a thickness of several 100 μm. 
     In a plane perpendicular to the cross-sectional plane, the semiconductor body  100  may have an approximately rectangular shape with an edge length in the range of several millimeters. A normal to the first surface  101  defines a vertical direction and directions orthogonal to the vertical direction are horizontal directions. 
     The semiconductor body  100  includes a drift structure  120  of a first conductivity type. The drift structure  120  forms first pn junctions pn 1  with body zones  115  of the second conductivity type, wherein the body zones  115  are formed between the first surface  101  and the drift structure  120 . 
     Transistor cells TC, first auxiliary cells AC 1  and second auxiliary cells AC 2  are formed along gate structures  150  extending from the first surface  101  down to at least the first pn junctions pn 1 . 
     The gate structures  150  include a conductive gate electrode  155  and a gate dielectric  151  separating the gate electrode  155  from the semiconductor body  100 . The gate electrode  155  may be a homogeneous structure or may have a layered structure including one or more metal containing layers. According to an embodiment, the gate electrode  155  may include or consist of a heavily doped polycrystalline silicon layer. The gate electrode  155  may be electrically connected to a gate connector  330  outside the semiconductor body  100 . The gate connector  330  may form or may be electrically coupled or connected to a gate terminal G. 
     The gate dielectric  151  may have a uniform thickness. According to other embodiments, a bottom portion of the gate dielectric  151  averted from the first surface  101  may be thicker than a top portion oriented to the first surface  101 . The gate dielectric  151  may include or consist of a semiconductor oxide, for example thermally grown or deposited silicon oxide, a semiconductor nitride, for example deposited or thermally grown silicon nitride, or a semiconductor oxynitride, for example silicon oxynitride. 
     The transistor cells TC further include source zones  110  of the first conductivity type forming second pn junctions pn 2  with body zones  115  assigned to the transistor cells TC. The source zones  110  are formed between the first surface  101  and the body zones  115  of the transistor cells TC. 
     The source zones  110  of the transistor cells TC as well as the body zones  115  of the first and second auxiliary cells AC 1 , AC 2  are electrically connected to a first load electrode  310  which may form or which may be electrically coupled or connected to a first load terminal L 1 . The body zones  115  of the transistor cells TC may also be electrically connected to the first load electrode  310 . The drift structure  120  is electrically connected to a second load electrode  320  which may form or which may be electrically coupled or connected to a second load terminal L 2 . 
     At gate voltages above a first threshold voltage Vthn inversion layers are formed in the body regions  115  of the transistor cells TC as well as in the first and second auxiliary cells AC 1 , AC 2  along the gate dielectric  151 . The inversion layers in the transistor cells TC form MOS gated channels for minority charge carriers between the source zones  110  and the drift structure  120 . The inversion layers in the first and second auxiliary cells AC 1 , AC 2  are without connection to the first load electrode  310 . The first and second auxiliary cells AC 1 , AC 2  differ from each other as regards a relationship between a forward voltage across the respective single auxiliary cells AC 1 , AC 2  and the gate voltage when the first pn junctions pn 1  are forward biased. 
     The first pn junctions pn 1  are forward biased in case of a forward biased MCD with a positive voltage applied between the first load terminal L 1  (anode) and the second load terminal L 2  (cathode) or, in case of a reverse biased RC-IGBT with a negative voltage applied between the second load terminal L 2  (collector) and the first load terminal L 1  (emitter). The first and second auxiliary cells AC 1 , AC 2  differ from each other with respect to their behavior concerning charge carrier injection efficiency. 
     The different charge carrier injection characteristics of the first and second auxiliary cells AC 1 , AC 2  may result from different emitter efficiencies, wherein emitter efficiency is the ratio of the hole current density to the total current density. A variation of emitter efficiency may be achieved by different vertical dopant profiles through the first pn junctions pn 1 , by way of example. 
     According to another embodiment, the first and second auxiliary cells AC 1 , AC 2  may have equal or similar emitter efficiencies but differ from each other with respect to a width of the body zones  115  and/or the first pn junctions pn 1  between neighboring gate structures  150 . In the first auxiliary cells AC 1  an injection efficiency given by the integral across the emitter efficiency from one side of the concerned cell to the opposite side may be greater than the injection efficiency in the second auxiliary cells AC 2 . According to a further embodiment, the auxiliary cells AC 1 , AC 2  may have both different emitter efficiencies and horizontal dimensions. 
     An inversion channel formed along a gate structure  150  and connected with the body zone  115  of an auxiliary cell AC 1 , AC 2  increases the injection efficiency of the concerned cell such that injection efficiency at least of the second auxiliary cells AC 2  may be controlled by the gate-to-emitter voltage VGE. 
     During a saturation period the second auxiliary cells AC 2  are effective as saturation injections cells injecting charge carriers into the drift structure  120  at a high rate and establishing a dense charge carrier plasma. In a desaturation period the saturation injection cells AC 2  are considerably less active such that the charge carrier plasma partially dissipates. For example, during desaturation the mean injection efficiency of the saturation injection cells AC 2  for VGE&gt;Vth 2  may be at most 50%, or at most 10%, or at most 1% of the mean injection efficiency of the saturation injection cells AC 2  for VGE&lt;Vth 2 . 
     Instead, during the desaturation period, the first auxiliary cells AC 1 , which are effective as desaturation injection cells, still inject sufficient charge carriers to maintain a sufficiently low forward voltage VF across the desaturation injection cells AC 1 . 
     During both the desaturation period and the saturation period, all types of cells AC 1 , AC 2 , TC may inject charge carriers into the drift structure  120 , but in the desaturation period the saturation injection cells AC 2  inject at significantly reduced injection efficiency compared to the saturation period such that in total less charge carriers are injected into the drift structure  120 . 
     By providing two different types of injection cells, one type may be adapted to the requirements of the saturation period and the other type can be adapted to the requirements of the desaturation period. Compared to approaches with only one type of desaturation cells, process constrictions can be relaxed and device parameters for the saturation period and the desaturation period can be tuned independently from each other. 
       FIG. 1B  illustrates the different characteristics of the transistor cells TC, the desaturation injections cells AC 1  and the saturation injection cells AC 2 . The following discussion refers to p-type body zones  115  and n-type source zones  110  as, e.g., in an n-channel RC-IGBT. Corresponding considerations apply to semiconductor devices  500  with p-type source zones  110  and n-type body zones  115  as, e.g., in p-channel RC-IGBTs. 
     According to forward characteristic  701 , when a gate voltage VGL 1  applied between the gate terminal G and the first load terminal L 1  exceeds the first threshold voltage Vthn, inversion layers formed in the body zones  115  of the transistor cells TC along the gate dielectrics  151  form MOS gated channels that connect the source zones  110  with the drift structure  120  and that provide an electron path between the first load electrode  310  and the drift structure  120 . At the same time, due to the absence of source zones or a missing connection between such source zones and the first load electrode  310 , inversion layers formed in the body zones  115  of the desaturation and saturation injection cells AC 1 , AC 2  are without connection to the first load electrode  310 . This holds both when a positive voltage is applied between the first load terminal L 1  and the second load terminal L 2  and when a negative voltage is applied between the first load terminal L 1  and the second load terminal L 2 . The first reverse characteristic  711  illustrates the relationship between a forward voltage VF across single desaturation injection cells AC 1  and the gate voltage VGL 1  in a forward biased mode of the first pn junction pn 1  with a negative voltage applied between the second load terminal L 2  and the first load terminal L 1 . At least for a gate voltage VGL 1  below a further threshold voltage Vth 0 , single desaturation injection cells AC 1  have a low forward voltage VF of at most 0.2%, e.g., at most 0.15%, of the maximum blocking voltage the semiconductor device  500  is specified for. For example, the forward voltage VF is at most 2V for a semiconductor device with a blocking capability of 1200V or at most 5V for a semiconductor device with a blocking capability of 6.5 kV. According to an embodiment, the forward voltage VF is at most 20V at a nominal reverse current. 
     When the gate voltage VGL 1  exceeds the further threshold voltage Vth 0 , the forward voltage VF may increase with increasing VGL 1  or may remain approximately constant up to beyond the first threshold voltage Vthn. According to an embodiment, the injection efficiency of the desaturation injection cells AC 1  sharply decreases and the forward voltage VF across the desaturation injection cells AC 1  increases for VGL 1 &gt;Vth 0  to keep the impact of the desaturation injection cells AC 1  on other device parameters low. Below the further threshold voltage Vth 0  an increase of the forward voltage drop across the first desaturation injection cells AC 1  with increasing gate-to-emitter voltage VGE is less steep than above the further threshold voltage Vth 0 . The second reverse characteristic  721  illustrates the relationship between a forward voltage VF across single saturation injection cells AC 2  and the gate voltage VGL 1  in the forward biased mode of the first pn junction pn 1 . For a gate voltage VGL 1  below a second threshold voltage Vthp which is lower than the further threshold voltage Vth 0 , the forward voltage across a single saturation injection cell AC 2  is at most 5V. For VGL 1 &gt;Vthp the forward voltage drop across single saturation injection cells AC 2  sharply increases with increasing VGL 1  indicating that the charge carrier injection efficiency of the saturation injection cells AC 2  sharply decreases between Vthp and Vth 0 . Below the second threshold voltage Vthp an increase of the forward voltage drop across the saturation injection cells AC 2  with increasing gate-to-emitter voltage VGE is less steep than above the further threshold voltage Vthp. 
       FIG. 1B  further shows that an overall charge carrier injection efficiency ηAC 21  of the saturation injection cells AC 2  for VGL 1 &lt;Vthp is higher than an overall charge carrier injection efficiency ηAC 22  of the saturation injection cells AC 2  for VGL 1 =Vth 0 , wherein the overall charge carrier injection efficiency is the surface integral over the local emitter efficiency for the respective cell type. The desaturation injection cells AC 1  may show a lower overall charge carrier injection efficiency than the saturation injection cells AC 2  for VGL 1 &lt;Vthp but maintain a sufficient charge carrier injection efficiency at least up to VGL 1 =Vth 0  such that despite of the comparatively low overall charge carrier injection efficiency the desaturation injection cells AC 1  can maintain a low forward voltage VF for the desaturation period with a gate voltage VGL 1  between the second threshold voltage Vthp and the further threshold voltage Vth 0 . 
     Typically, three-level approaches for desaturable RC-IGBTs rely on injection cells designed with a large spread between a high injection efficiency at a gate-to-emitter voltage VGE of −15 V on the one hand and a low injection efficiency at VGE=0 V on the other hand. During an injection period of a reverse conducting mode (RC-mode) of the RC-IGBT, in which the first pn junctions pn 1  are forward biased. The injection cells are active and inject charge carriers into the drift structure  120  at high injection efficiency to achieve a dense charge carrier plasma. In a desaturation period preceding commutation, hence before the voltage between the first and second load terminals L 1 , L 2  changes polarity, the injection cells are deactivated and inject charge carriers only at a significantly lower injection efficiency such that the charge carrier plasma density attenuates. 
     The desaturation is the more effective the greater the spread of the injection efficiency is. However, when forming injection cells with high spread between the injection mode and the desaturation mode, it turns out that the forward voltage VF at VGE=0 may strongly depend on process fluctuations. 
     Instead, the present embodiments rely on two types of injection cells. The saturation injection cells AC 2  may be designed with a high spread between the injection efficiency at VGL 1 =−15 V and the injection efficiency at VGL 1 =0 V such that a high desaturation efficiency can be achieved. The desaturation injection cells AC 1  may be designed with no or a low spread between the injection efficiency at VGL 1 =−15 V and the injection efficiency at VGL 1 =0 V but with an injection efficiency that assures a sufficient low forward voltage VF, e.g. less than 3 V, at and close to VGL 1 =0 V, wherein the forward voltage drop VF is less prone to process fluctuations. As a result, the embodiments combine high desaturation efficiency with persistently low forward voltage VF even during the desaturation period. 
       FIG. 1C  schematically shows the mode of operation of an RC-IGBT based on the semiconductor device  500  of  FIGS. 1A and 1B . 
     During a saturation period Sat of the RC-mode of the RC-IGBT, a gate-to-emitter voltage VGE is below Vthp and the saturation injection cells AC 2  inject charge carriers at high efficiency into the drift structure  120  resulting in a high storage charge QF. The collector-to-emitter voltage VCE is given by the forward voltage VF 1  of the reverse diode during the saturation period Sat, wherein the characteristics of the reverse diode are governed by the saturation injection cells AC 2 . The forward voltage VF 1  as well as the forward resistance Rfwd of the reverse diode is low. 
     At t=t 1 , VGE rises to Vthp&lt;VGE&lt;Vth 0  and a desaturation period Desat starts. The saturation injection cells AC 2  switch to a low injection mode. The charge carrier plasma density and the storage charge sharply decrease whereas forward voltage rises and forward resistance Rfwd increases. But the desaturation injection cells AC 1  are still active and ensure an ongoing comparatively low forward voltage VF 2  of the reverse diode. 
     After some time a process may be triggered that may change the voltage bias across the RC-IGBT. For example, a semiconductor switch in the other part of a half bridge is turned-on while the RC-IGBT remains turned off. During turn-off the voltage bias of the RC-IGBT may repeatedly change from reverse biased to forward biased and vice versa. Finally, the voltage bias may change to forward biased whereby the RC-IGBT commutates and may directly change into a blocking state. 
     Since the charge carrier plasma density has been reduced, a smaller number of charge carriers have to be drained off from the semiconductor body  100  than without desaturation period Desat and switching losses are reduced. Since desaturation gets along without formation of any MOS gated channel, the RC-IGBT can immediately sustain the full blocking voltage in a blocking phase Blk of the forward biased state. When the commutation charge carrier flow ends, the gate voltage may be again lowered to below the second threshold voltage in order to improve robustness against strong current filaments in the wake of the commutation charge carrier flow and to avoid a dynamic increase of the gate potential above Vthn. 
     Later, e.g., at t=t 3 , VGE may rise to above Vthn, the MOS gated channels in the transistor cells TC turn on and the RC-IGBT changes to a conducting phase Cnd of the forward biased state. 
     The first, second and further threshold voltages Vthn, Vthp, Vth 0  are selected to meet worst case conditions specified for the gate voltage levels. For example, Vth 0  may be selected such that the forward voltage of the desaturation injection cells AC 1  is below 15V for a 1200V device over the whole admissible gate voltage range for the desaturation mode. The second threshold voltage Vthp may be selected such that the second auxiliary cells AC 2  contribute to overall hole injection only to a low degree, e.g., at most 30%, or at most 15%, or at most 5% over the complete admissible gate voltage range for the desaturation mode. 
     For example, datasheets of depletable three-level RC-IGBTs may specify gate voltage levels of +10 to +25 V for switching on the MOS-gated channels, −3 V to +3 V for the desaturation mode and −15 to −25 V for the saturation mode. For an RC-IGBT with the above specifications, the first threshold voltage Vthn may be about half way between +1 V and +10 V, e.g. close to 5.5 V. The further threshold voltage Vth 0  may be between the maximum admissible voltage level for the desaturation mode and the first threshold voltage Vthn, hence in a range between +1.0 V and +5.0 V, for example between 2 V and 3 V. The second threshold voltage Vthp may be half way between −15 V and the minimum admissible voltage level for the desaturation mode, hence in a range between −10 V and −1 V, for example at −5.5 V. 
       FIG. 2A  illustrates an RC-IGBT or a semiconductor device including an RC-IGBT  501  according to an embodiment. At a front side of a silicon semiconductor body  100  including a drift structure  120  as described above a transistor module  601  includes controllable cells Ce, which may be transistor cells TC, desaturation injection cells AC 1  or saturation injection cells AC 2  as described above. Control electrodes of the controllable cells CE are electrically connected to a gate conductor  330 , which may form or which may be electrically connected or coupled to a gate terminal G. Source and body zones of the controllable cells CE are electrically connected to a first load electrode  310 , which may form or which may be electrically connected or coupled to an emitter terminal E. 
     The drift structure  120  includes a drift zone  121  and may directly adjoin to the body zones in the controllable cells Ce. According to other embodiments, a heavier doped barrier layer may be sandwiched between the body zones  115  and the drift zone  121 . In the drift zone  121 , a dopant concentration may gradually or in steps increase or decrease with increasing distance to the first surface  101  at least in portions of its vertical extension. According to other embodiments, the dopant concentration may be approximately uniform in the complete drift zone  121 . A mean dopant concentration in the drift zone  121  may be between 1E12 cm −3  and 1E15 cm −3 , for example in a range from 5E12 cm −3  to 5E13 cm −3 . 
     A pedestal layer  130  formed along a second surface  102  at an opposite rear side directly adjoins a second load electrode  320  which may form or which may be electrically connected to a collector terminal C. The pedestal layer  130  includes first zones  131  of the conductivity type of the body zones  115  and second zones  132  of the conductivity type of the drift structure  120 . The first and second zones  131 ,  132  extend from the drift structure  120  to the second load electrode  320 , respectively. The first zones  131  are effective as rear side emitter zones injecting minority charge carriers into the drift structure  120  in a conducting phase of the forward biased state. The second zones  132  are effective as collector shorts bypassing the rear side emitter zones in the RC-mode. The first zones  131  may alternate with the second zones  132  in a bimodal region  620  of the RC-IGBT  501 . In addition to the bimodal region  620 , the RC-IGBT  501  may include a pilot region  610  with a pilot zone  133 , wherein a horizontal extension of the pilot zone  133  is at least twice, e.g. at least ten times as large as a horizontal extension of the first zones  131  in the bimodal region  620 . The pilot zone  133  supports ignition of the conducting phase of the forward biased state. 
     The dopant concentrations in the first and second zones  131 ,  132  and, if applicable, in the pilot zone  133 , are sufficiently high to ensure a low ohmic contact to the second load electrode  320 . For example, a maximum dopant concentration along the second surface  102  in p doped first, second or pilot zones  131 ,  132 ,  133  may be at least 1E16 cm −3 , for example at least 5E17 cm −3 . A maximum dopant concentration in n-doped first, second or pilot zones  131 ,  132 ,  133  may be at least 1E18 cm −3 . 
     The drift structure  120  may include a field stop layer  128  of the conductivity type of the drift zone  121 , wherein the field stop layer  128  separates the drift zone  121  from the pedestal layer  130 . A mean net dopant concentration in the field stop layer  128  is at least twice as high as a maximum net dopant concentration in the drift zone  121  and at most half as high as the mean dopant concentration in the second zones  132  of the pedestal layer  130 . The drift structure  120  may include further doped zones, for example zones forming a compensation structure, barrier zones for locally increasing a charge carrier plasma density and/or buffer zones locally shaping the electric field. 
       FIG. 2B  refers to an MCD  502  or a semiconductor device comprising an MCD  502 . The source and body zones of the controllable cells Ce may be electrically connected to the first load electrode  310 , which may form or which may be electrically connected to an anode terminal A. The gate electrodes of the controllable cells Ce may be electrically connected a gate terminal or to the first load electrode  310 . The pedestal layer  130  is a heavily doped layer of the conductivity type of the drift zone  121 , wherein a maximum net dopant concentration in the pedestal layer  130  along the second surface  102  ensures a low ohmic contact to the second load electrode  320 , which forms or which is electrically connected to a cathode terminal K. For further details reference is made to the description of the RC-IGBT  501  of  FIG. 2A . 
     The total area ratio of desaturation injection cells AC 1  to saturation injection cells AC 2  may range from 1:1 to 1:10000, e.g., from 1:20 to 1:500. The total area ratio of transistor cells TC to injection cells AC 1 , AC 2  may be in a range from 200:1 to 1:50, e.g., from 10:1 to 1:10. Placement of the saturation and desaturation injection cells AC 2 , AC 1  may be unrelated to the position of the first, second and pilot zones  131 ,  132 ,  133  at a rear side. 
     In  FIG. 3A , the RC-IGBT  501  includes a bipolar region  620  and comparatively small first auxiliary cells AC 1  are evenly distributed within the complete bipolar region  620 . Evenly distributed desaturation injection cells AC 1  avoid high local current densities during commutation. An avalanche induced portion of a reverse recovery charge Qrr can be kept small. According to other embodiments, the saturation and desaturation injection cells AC 2 , AC 1  at the front side are aligned to the pattern of the first, second and pilot zones  131 ,  132 ,  133  at the rear side. 
     In  FIG. 3B , the desaturation injection cells AC 1  are placed in a bimodal region  620  surrounding a pilot region  610 . In a portion of the semiconductor body  100  oriented to the rear side, the reverse current mainly flows in the bimodal region  620  during the saturation period of the RC-mode. By placing the desaturation injection cells AC 1  only in the bimodal region  620 , the reverse current nearly exclusively flows in the bimodal region  620  over the complete vertical extension of the semiconductor body  100  shortly before commutation. Injecting, during the desaturation period, charge carriers exclusively in the bimodal region  620  results in that the charge carrier plasma is concentrated in the bimodal region  620 . As a consequence, a commutation charge carrier flow has no or only a weak horizontal component and does not generate a sufficient horizontal voltage drop to ignite charge carrier injection from the pilot region  610 . The pilot zone  133  is prevented from unwanted ignition. 
     As illustrated in  FIG. 3B  a contiguous desaturation injection cell AC 1  may surround the pilot region  610 , wherein the desaturation injection cell AC 1  may be centered to the bimodal region  620  that surrounds the pilot region  610 . A resulting reverse current gain of the pilot region  610  is low. According to other embodiments, a plurality of isolated desaturation injection cells AC 1  may be arranged in a stripe centered to the bimodal region  620  and surrounding the pilot region  610 . 
       FIGS. 3C to 3D  show RC-IGBTs  501  with the desaturation injection cells AC 1  formed in the pilot region  610  in the vertical projection of the pilot zone  133 . The desaturation injection cells AC 1  locally increase the reverse current gain of the pilot region  610  and in this way may improve switching softness of the RC-IGBT  501 . Minority charge carriers that have been previously injected through the desaturation injection cells AC 1 , e.g., holes in case of an n-channel RC-IGBT  501 , may ignite the pilot zone  133  during commutation such that a bipolar transistor formed by the body zones  115 , the drift structure  120  and the pilot zone  133  turns on. Ignition of the pilot zone  133  may improve softness of the switching behavior and robustness against oscillations at costs of increased switching losses. 
     In the RC-IGBT  501  of  FIG. 3C  one or a small number of desaturation injection cells AC 1  is/are placed in the center of the pilot zone  133  to increase the horizontal path lengths of charge carriers previously injected by the desaturation injection cells AC 1 . 
     In  FIG. 3D , the first desaturation injection cells AC 1  are arranged symmetrically in the edges of the pilot region  610  and close to the bimodal region  620  to achieve a tradeoff between low switching losses and high switching softness. 
       FIG. 4  shows possible positions of desaturation injection cells AC 11 , AC 12 , AC 13  in an RC-IGBT  501  with expanded second zones  132  providing collector shorts for the RC-mode. According to an embodiment with the desaturation injection cells AC 11  placed in the vertical projection of the second zones  132  and at a large horizontal distance z 1  to the adjoining first zone  131 , a reverse current through the semiconductor body  100  during desaturation mainly flows in a vertical direction between the second zones  132  and the desaturation injection cells AC 11 . Charge carrier plasma density remains low in the vertical projection of the first zones  131 . When the RC-IGBT commutates, the resulting commutation charge carrier flow has no or only a weak horizontal component, generates no or only a low horizontal voltage drop along the pn junctions between the first zones  131  and the drift structure  120  and does not ignite the bipolar transistor formed by the body zones  115  of the controllable cells CE, the drift structure  120  and the first zones  131 . 
     By placing the desaturation injection cells AC 12 , AC 13  at a small horizontal distance z 2  to the first zones  131  in the vertical projection of the second zones  132  and/or at a horizontal distance z 3  to the second zones  132  in the vertical projection of the first zones  131  results in an increased charge carrier plasma density in the vertical projection of the first zones  131 , such that a charge carrier flow during commutation has a horizontal component. The resulting horizontal voltage drop may trigger ignition of the bipolar transistor such that switching softness can be improved, if applicable at costs of increased switching losses. Placement of the desaturation injection cells AC 11 , AC 12 , AC 13  with respect to the second zones  132  forming the collector shorts, as well as number and lateral extension of the desaturation injection cells AC 1  determines the conditions at which the commutation charge carrier flow triggers charge carrier injection from the first zones  131 . 
     The desaturation injection cells AC 1  differ from the saturation injection cells AC 2  in that the desaturation injection cells AC 1  have a higher threshold voltage up to which they inject charge carriers at high efficiency. This effect can be achieved by a variation of geometrical dimensions and/or dopant gradients in the injection cells AC 1 , AC 2 , by way of example. 
       FIGS. 5A to 5C  refer to RC-IGBTs  501  or other semiconductor devices including an RC-IGBT  501  with desaturation injection cells AC 1  formed by a local variation of a barrier structure  125  between the body zones  115  of the injection cells AC 1 , AC 2  and the drift zone  121 . The RC-IGBT  501  is based on a semiconductor body  100  as described in detail with regard to  FIGS. 1A to 1C , wherein the semiconductor body  100  includes a drift structure  120  of a first conductivity type, a body zone  115  of a second, opposite conductivity type between the first surface  101  and the drift structure  120  as well as a pedestal layer  130  sandwiched between the drift structure  120  and the second surface  102 . 
     For the illustrated n-channel RC-IGBT  501 , the first conductivity type is n-type and the second conductivity type is p-type. Similar considerations as outlined below apply to p-channel RC-IGBTs with the first conductivity type being p-type and the second conductivity type being n-type. 
     The drift structure  120  includes a drift zone  121  with a dopant concentration that may gradually or in steps increase or decrease with increasing distance to the first surface  101  at least in portions of its vertical extension. According to other embodiments the dopant concentration in the drift zone  121  may be approximately uniform. For an RC-IGBT  501  based on silicon, a mean dopant concentration in the drift zone  121  may be between 1E12 cm −3  and 1E15 cm −3 , for example in a range from 5E12 cm −3  to 1E14 cm −3 . In case of an RC-IGBT  501  based on SiC, a mean dopant concentration in the drift zone  121  may be between 5E14 cm −3  and 1E17 cm −3 , for example in a range from 1E15 cm −3  to 1E16 cm 3 . 
     The pedestal layer  130  includes first zones  131  of the conductivity type of the body zones  115  and second zones  132  of the conductivity type of the drift zone  121 . The first zones  131  are effective as rear side emitter zones injecting minority charge carriers into the drift zone  121  in the conducting phase. The second zones  132  form collector shorts bypassing the first zones  131  in the RC-mode. Impurity concentrations in the first and second zones  131 ,  132  are sufficiently high for forming an ohmic contact with a metal directly adjoining the second surface  102 . A mean dopant concentration for p-type zones may be at least 1E16 cm −3 , for example 5E17 cm −3 , and a mean dopant concentration for n-type zones may be at least 1E18 cm −3 , for example at least 5E19 cm −3 . 
     The drift structure  120  may include a field stop layer  128  of the conductivity type of the drift zone  121 . The field stop layer  128  separates the pedestal layer  130  from the drift zone  121 , wherein a mean dopant concentration in the field stop layer  128  may be lower than the mean dopant concentration in the second zones  132  of the pedestal layer  130  by at least 50%, e.g., by at least one order of magnitude and may be higher than in the drift zone  121  by at least 100%, e.g. by at least one order of magnitude. 
     The first and second zones  131 ,  132  of the pedestal layer  130  extend from the second surface  102  to the field stop layer  128  or, in absence of a field stop layer, to the drift zone  121 , respectively. The first zones  131  may be dots horizontally embedded by second zones  132  forming a grid or vice versa. According to other embodiments, the first and second zones  131 ,  132  may be stripes running parallel to a first horizontal direction or may form nested rectangular frames, by way of example. Control structures  150  of transistor cells TC, saturation injection cells AC 2  and desaturation injection cells AC 1  extend from the first surface  101  into the drift zone  121 . Portions of the semiconductor body  100  between neighboring control structures  150  form cell mesas  170 . 
     The control structures  150  may be stripes extending along an extension direction of the cell mesas  170 . According to an embodiment the extension direction may be exclusively parallel to a first horizontal direction such that the cell mesas  170  and the control structures  150  are straight stripe structures. According to another embodiment, the extension direction alters with respect to the first horizontal direction such that the cell mesas  170  and the control structures  150  form staggered stripes. 
     The cell mesas  170  may be regularly arranged at a uniform center-to-center distance of, for example 400 nm to 20 μm, for example 800 nm to 2 μm. A distance between the first surface  101  and the bottom of the control structures  150  may range from 1 μm to 30 μm, e.g., from 2 μm to 6 μm. A lateral width of the cell mesas  170  may range from 0.05 μm to 10 μm, e.g., from 0.1 μm to 1 μm. 
     The control structures  150  include a gate electrode  155  and a gate dielectric  151  separating the gate electrode  155  from the semiconductor body  100 . The gate electrode  155  may be a homogenous structure or may have a layered structure including one or more conductive layers. According to an embodiment the gate electrode  155  may include or consist of heavily doped polycrystalline silicon. The gate electrodes  155  may be electrically connected to a gate terminal G. 
     The gate dielectric  151  may include or consist of a semiconductor oxide, for example thermally grown or deposited silicon oxide, a semiconductor nitride, for example deposited or thermally grown silicon nitride, or a semiconductor oxynitride, for example silicon oxynitride. 
     Transistor cells TC, saturation injections cells AC 2  and desaturation injections cells AC 1  may be directly concatenated to each other along a horizontal direction. 
       FIG. 5A  shows transistor cells TC, saturation injection cells AC 2  and desaturation injection cells AC 1  directly concatenated along a first horizontal direction defined by the longitudinal axes of the control structures  150 . Transistor cells TC, saturation injection cells AC 2  and desaturation injection cells AC 1  may directly adjoin to each other, wherein transitions between the different cell types may be gradual or abrupt. According to other embodiments, the transistor cells TC and the desaturation injection cells AC 1  are formed along different control structures  150  running parallel to each other. 
     The body zones  115  are formed in first sections of the cell mesas  170  oriented to the first surface  101  and may directly adjoin the first surface  101  in the saturation and desaturation injections cells AC 2 , AC 1 . A mean net impurity concentration in the body zones  115  may be in the range from 1E16 cm −3  to 5E18 cm −3 , for example between 1E17 cm −3  and 5E17 cm −3 . Each body zone  115  may form a first pn junction pn 1  with the drift structure  120 . 
     Portions of the cell mesas  170  assigned to the transistor cells TC include source zones  110  forming second pn junctions pn 2  with the body zones  115  of the transistor cells TC. Portions of the cell mesas  170  assigned to the saturation and desaturation injections cells AC 2 , AC 1  may be devoid of any source zone or include source zones without connection to the first load electrode  310 . 
     The source zones  110  may be formed as wells extending from the first surface  101  into the body zones  115  and define the transistor cells TC which are arranged along a longitudinal horizontal axis of the respective cell mesa  170 . Shadowed regions without source zones  110  separate neighboring transistor cells TC assigned to the same cell mesa  170 , wherein in the shadowed regions the body zones  115  of the saturation and desaturation injections cells AC 2 , AC 1  directly adjoin the first surface  101 . Transistor cells TC and shadowed regions alternate along the longitudinal axis of the respective cell mesa  170 . 
     A distance between neighboring source zones  110  arranged along the longitudinal axis may be in a range from 1 μm to 200 μm, for example in a range from 3 μm to 100 μm. 
     A dielectric structure  200  may separate the first load electrode  310  from the first surface  101 . The dielectric structure  200  may include one or more dielectric layers from silicon oxide, silicon nitride, silicon oxynitride, doped or undoped silicon glass, for example BSG (boron silicate glass), PSG (phosphorus silicate glass), or BPSG (boron phosphorus silicate glass), by way of example. 
     The first load electrode  310  may form an emitter terminal E or may be electrically coupled or connected to an emitter terminal E of the RC-IGBT  501 . 
     Contact structures  315  extend from the first load electrode  310  through the dielectric structure  200  into the semiconductor body  100 . The contact structures  315  electrically connect the first load electrode  310  with the source zones  110  and the body zones  115 . A plurality of spatially separated contact structures  315  may directly adjoin the respective cell mesa  170 , wherein at least some of the contact structures  315  may be assigned to the source zones  110 . Other embodiments may provide stripe-shaped contact structures  315  that extend along the whole longitudinal extension of the respective cell mesa  170  and that directly adjoin the body zones  115  in the shadowed regions. 
     A second load electrode  320  directly adjoins the second surface  102  and the pedestal layer  130 . The second load electrode  320  may form or may be electrically connected to a collector terminal C. 
     Each of the first and second load electrodes  310 ,  320  may consist of or contain as main constituent(s) aluminum (Al), copper (Cu), or alloys of aluminum or copper, for example AlSi, AlCu or AlSiCu. According to other embodiments, at least one of the first and second load electrodes  310 ,  320  may contain as main constituent(s) nickel (Ni), titanium (Ti), tungsten (W), tantalum (Ta), silver (Ag), gold (Au), platinum (Pt), and/or palladium (Pd). For example, at least one of the first and second load electrodes  310 ,  320  may include two or more sub-layers, wherein each sub-layer contains one or more of Ni, Ti, Ag, Au, Pt, W, and Pd as main constituent(s), e.g., a silicide, a nitride and/or an alloy. 
     The cell mesas  170  further include a barrier structure  125  which may be sandwiched between the body zones  115  and the drift zone  121  such that the barrier structure  125  forms the first pn junctions pn 1  with the body zones  115  and unipolar homojunctions with the drift zone  121 . The barrier structure  125  has the same conductivity type as the drift zone  121 . A mean dopant concentration in the barrier structure  125  is at least ten times as high as the mean dopant concentration in the drift zone  121 . According to an embodiment, the mean dopant concentration in the barrier structure  125  may range from 1E16 cm −3  to 1E18 cm −3 , for example from 1E17 cm −3  to 5E17 cm −3 . The impurities in the barrier structure  125  may be phosphorous (P), arsenic (As), selenium (Se) and/or sulfur (S) atoms/ions in case of an n-channel IGBT  501 . 
     According to other embodiments, the barrier structure  125  may be embedded within the body zones  115  such that portions of the body zones  115  separate the barrier structure  125  from the drift zone  121 . According to further embodiments, the barrier structure  125  is formed within the drift zone  121  at a distance to the first pn junctions pn 1 . 
     In the conducting phase of the forward biased state, the barrier structure  125  forms a barrier for charge carriers to escape from the charge carrier plasma and increases charge carrier plasma density. Further, in the more heavily doped barrier structure  125  minority charge carriers recombine at a higher rate such that the barrier structure  125  reduces minority charge carrier emitter efficiency with respect to the drift zone  121 . The barrier structure  125  is formed at least in the saturation injection cells AC 2  and may also be formed in the transistor cells TC. 
       FIG. 5C  shows gaps  125   a  in the barrier structure  125 . The gaps  125   a  locally increase emitter efficiency with respect to the drift zone  121  and define the desaturation injection cells AC 1 . 
     The n-channel RC-IGBT  501  of  FIG. 6A  shows local attenuated portions  125   b  of the barrier structure  125  defining the desaturation injection cells AC 1 . In the desaturation injection cells AC 1 , the mean dopant concentration/dose in the attenuated portions  125   b  is at most 50%, for example at most 10% of the dopant concentration/dose in portions of the barrier structure  125  outside the attenuated portions  125   b  in the desaturation injection cells AC 1 . 
     In  FIG. 6B  an RC-IGBT  501  includes saturation injections cells AC 2  in narrow portions of cell mesas  170 , wherein the narrow portions have a narrow mesa width y 1 , and desaturation injection cells AC 1  in wide portions of cell mesas  170 , wherein the wide portions have a wide mesa width y 2 . In the desaturation injection cells AC 1  the local injection efficiency per cell length unit is higher than in the saturation injection cells AC 2 . But since the total area assigned to saturation injection cells AC 2  is greater than the total area assigned to desaturation injection cells. AC 1 , for VGE&lt;Vthp the total injection through the saturation injection cells AC 2  may exceed the total injection through the desaturation injection cells AC 1 . 
     In  FIG. 6C  an RC-IGBT  501  includes saturation injections cells AC 2  in narrow cell mesas  170   x  with a narrow mesa width y 1  and desaturation injection cells AC 1  in wide cell mesas  170   y  with a wide mesa width y 2 . According to an embodiment, the wide mesa width y 2  may be in a range from 100 nm to 20 μm, e.g., in a range from 300 nm to 1000 nm or from 400 nm to 800 nm, whereas the narrow mesa width y 1  may be in a range from 10 to 400 nm, e.g., in a range from 50 to 200 nm and wherein the wide mesa width y 2  is at least 90 nm greater than the narrow mesa width y 1 . 
     The transistor cells TC may be formed in the narrow cell mesas  170   x , in the wide cell mesas  170   y , or in both of them. Though in the desaturation injection cells AC 1  the local injection efficiency per cell length unit may be higher than in the saturation injection cells AC 2 , for VGE&lt;Vthp the total injection through the saturation injection cells AC 2  may exceed the total injection through the desaturation injection cells AC 1  if the total area assigned to saturation injection cells AC 2  is sufficiently great with respect to the total area assigned to the desaturation injection cells AC 1 . 
     The embodiments of  FIGS. 6A to 6C  may be combined with each other. For example, the RC-IGBT  501  of  FIG. 6C  may include a patterned barrier structure  125  as illustrated in  FIG. 6A  or a non-patterned barrier structure  125  or does not include any barrier structure. 
       FIGS. 7A to 7C  refer to n-channel RC-IGBTs  501  with bottle-shaped control structures  150 . The control structures  150  include bulged sections  150   a  and narrow sections  150   b  between the bulged sections  150   a  and the first surface  101 , wherein the narrow sections  150   b  extend from the first surface  101  down to at least the first pn junction pn 1 . The narrow sections  150   b  have a width wc 1 . In the bulged sections  150   a  the control structures  150  have a maximum width wc 2  which is greater than the first width wc 1 . The maximum width wc 2  is at least 50 nm, for example at least 100 nm greater than the width wc 1  of the narrow sections  150   b.    
     Accordingly, the cell mesas  170  have wide sections  170   b  sandwiched between neighboring narrow sections  150   b  of the control structures  150  and bottleneck sections  170   a  sandwiched between neighboring bulged sections  150   a  of the control structures  150 . The wide sections  170   b  include at least the body zones  115  and the first pn junctions pn 1  as well as portions of the drift zone  121 . A mesa width wm 1   a  of wide sections  170   b  in portions of the cell mesas  170  assigned to transistor cells TC and saturation injection cells AC 2  is in a range from 100 nm to 900 nm, for example in a range from 300 nm to 800 nm. A minimum mesa width wm 2   a  of bottleneck sections  170   a  in portions of cell mesas  170  assigned to transistor cells TC and saturation injection cells AC 2  may be in a range from 10 nm to 400 nm, for example in a range from 50 nm to 200 nm. The bottleneck sections  170   a  include portions of the drift zone  121  and may include portions of a barrier structure, respectively. 
     Mesa widths of the cell mesas  170  in the saturation injection cells AC 2  may be the same as in the transistor cells TC. Portions of the cell mesas  170  assigned to the desaturation injection cells AC 1  may be wider than portions of cell mesas  170  assigned to the saturation injection cells AC 2 . For example, a width wm 2   b  of narrow portions of the bottleneck sections  170   a  of portions of cell mesas  170  in the desaturation injection cells AC 1  is at least 10% greater, for example at least 30% greater than a width wm 2   a  of narrow portions of the bottleneck sections  170   a  of portions of cell mesas  170  in the saturation injection cells AC 2 . For example, the width wm 2   b  may be at least 50 nm, e.g., at least 150 nm greater than the width wm 2   a.    
     In the following, reference is made to the definition of threshold voltages in  FIG. 1B . When a gate voltage VGE lower than the second threshold voltage Vthp is applied to the gate terminal G, p-type inversion layers are formed in the drift zone  121  around the control structures  150 . The inversion layers are connected to the body zones  115  which in turn are connected to the first load electrode  310  such that the inversion layers in the drift zone  121  are effective as charge carrier emitters. The injected charge carriers increase charge carrier plasma density in the drift zone  121 . A high charge carrier plasma density results in low forward resistance and low forward voltage of the reverse diode in the RC-mode of the RC-IGBT  501  during a saturation period. Both the bulgy form of the gate structures  150  and a barrier structure as illustrated in  FIGS. 5A to 5C  contribute to increasing the spread between the injection efficiencies at VGE&lt;Vthp and at VGE&gt;Vthp. The higher the spread is the better is the desaturation efficiency in the desaturation period. 
     During a desaturation period of the RC-mode preceding a commutation of the RC-IGBT  501 , the gate voltage VGE is raised to a voltage greater than the second threshold voltage Vthp but lower than the further threshold voltage Vth 0 . The inversion channels dissipate. In the saturation injection cells AC 2  the bulged sections  150   a  of the control structures  150  shield the body zones  115  against a contiguous portion of the drift structure  120  between the control structures  150  and the pedestal layer  130 . The remaining charge carrier injection efficiency of the body zones  115  in the saturation injection cells AC 2  is low. 
     Instead, in the desaturation injection cells AC 1 , the body zones  115  preserve a comparatively high injection rate through the wider bottleneck portions  170   a  of the cell mesas  170  despite the absence of p-type inversion layers along the control structures  150 . The overall injection efficiency of the desaturation injection cells AC 1  remains sufficiently high to ensure a sufficient charge carrier plasma density in the drift zone  121  during the desaturation period and a sufficiently low forward voltage drop across the reverse diode even in the desaturation period of the RC-mode. Due to the less critical mesa dimensions in the desaturation injection cells AC 2 , the forward voltage drop governed by the desaturation injection cells AC 2  is less susceptible to dimensional variations and process fluctuations. 
     The RC-IGBT  501  combines a high spread of the overall injection efficiency and, as a consequence high desaturation efficiency, with a sufficient minimum injection efficiency in the desaturation period, and, as a consequence a stable forward voltage behavior in the RC-mode. 
       FIG. 7D  refers to a layout with the desaturation injection cells AC 1  formed in wide cell mesas  170   y  that have a mesa width wm 1   b  greater than a mesa width wm 1   a  of narrow cell mesas  170   x  including the saturation injection cells AC 2  or the saturation injection cells AC 2  and the transistor cells TC. The cross-section along line B-B may correspond to the cross-section along line B-B in  FIG. 7A  with the mesa widths wm 1   a  and wm 1   b  referring to the widths of the wide mesa sections of bottleneck mesas. The wide cell mesas  170   y  may include source zones  110  or may be devoid of source zones  110 . Transistor cells TC and saturation injection cells AC 1  may be formed in the same narrow cell mesas  170   x  or in different narrow cell mesas  170   y.    
     According to another embodiment, the drift structure  120  includes barrier structures as illustrated in  FIGS. 5A to 5C  and the sidewalls of the wide and narrow cell mesas  170   y ,  170   x  may be approximately vertical. 
       FIGS. 8A to 8C  illustrate the correlation of the forward voltage VF of the reverse diode in the RC-mode, the storage charge QF in the drift zone  121  and the vertical extension of a narrow portion in the bottleneck section of the cell mesas  170 . 
       FIG. 8A  shows injection cells AC which control structures  150  have a total vertical extension of about 5 μm. The control structures  150  are bottle-shaped with a minimum width wc 1  in a narrow section  150   b  close to the first surface  101  and a maximum width wc 2  in a bulged section  150   a  in a distance to the first surface  101 . The minimum width wc 1  is about 1 μm and the maximum width wc 2  is about 1.2 μm. 
     The cell mesas  170  include bottleneck sections  170   a  with a width wm 2  of about 200 nm in a narrow portion of approximately constant width and wide sections  170   b  between the first surface  101  and the bottleneck sections  170   a  with a width wm 1  of about 400 nm. The wide sections  170   b  include the body zones  115 . A width of the first pn junctions pn 1  approximates the width wm 1  of the wide sections  170   b . The vertical extension of the narrow portions of the bottleneck sections  170   a  may be in a range from 300 nm to 4 μm. 
     In  FIG. 8B  the contiguous lines  801 - 804  show the collector-to-emitter voltage VCE in the RC-mode of an RC-IGBT including the injection cells AC of  FIG. 8A  as a function of the gate voltage VGE at vertical extensions of the narrow portions of the bottleneck sections  170   a  of 2.5 μm ( 801 ), 2.1 μm ( 802 ), 1.8 μm ( 803 ) and 1.5 μm ( 804 ). The dotted lines  811 ,  812 ,  813 ,  814  show the storage charge QF, which is proportional to the injection efficiency, at a vertical extension of the narrow portions of the bottleneck sections of the cell mesas  170  of 2.5 μm ( 811 ), 2.1 μm ( 812 ), 1.8 μm ( 813 ) and 1.5 μm ( 814 ). The amount of current that flows through the structure does not depend on the gate voltage VGE and is in the range of the nominal value. 
       FIG. 8C  shows a portion of the diagram of  FIG. 8B  around VGE=0 in more detail. 
     Both the collector-to-emitter voltage VCE and the storage charge QF strongly depend on the gate voltage VGE and strongly vary at and around VG=0 V. The collector-to-emitter voltage gradient  804  assigned to a vertical extension of the narrow portion of 1.5 μm ensures a low forward voltage drop for the respective injection cell at VG=0, which is the typical gate voltage level for the desaturation mode of a three-level desaturable RC-IGBT. On the other hand, process fluctuations may result in that the vertical extension of the narrow portions of the bottleneck sections is smaller than 1.8 μm, resulting in the collector-to-emitter voltage gradient  802  and a forward voltage VF of the concerned injection cells AC of more than 100 V. 
     The embodiments allow for combining injection cells AC with the charge storage gradient  811  ensuring a high spread of the charge storage between VGE=−15V and VGE=0V with injection cells having a collector-to-emitter voltage gradient similar to the collector-to-emitter voltage gradient  804  ensuring a low voltage drop even at a desaturation gate voltage VG=0V. 
       FIGS. 9A to 9B  refer to embodiments including, in addition to the first and second auxiliary cells AC 1 , AC 2  third auxiliary cells AC 3  (meta cells) for maintaining a sufficient degree of charge carrier injection even at a gate voltage VGL 1  that exceeds the first threshold voltage Vthn, which is the threshold voltage for the MOS gated channels through the body zones  115  in the transistor cells TC. 
     The meta cells AC 3  may be evenly distributed among the transistor cells TC, the saturation injection cells AC 2  and the desaturation injection cells AC 1 . The meta cells AC 3  are designed such that they have a sufficiently high charge carrier injection efficiency even at gate voltage levels above the first threshold voltage Vthn. In an n-channel RC-IGBT, the meta cells AC 3  are effective as hole emitters even if a positive gate voltage VGE induces inversion channels through the body zones  115  of the transistor cells TC. 
     In a typical three-level operation mode for desaturable n-channel RC-IGBTs, a negative gate voltage VGE is used to increase the hole injection efficiency in injection cells. A gate voltage VGE of about 0 V is applied for a desaturation period in which injection efficiency of the injection cells is reduced. 
     On the other hand, typical applications of RC-IGBTs include driver units controlling the gate voltage of the RC-IGBT and sensing a current, wherein the driver unit may turn the RC-IGBT on when the sensed current is below a certain threshold, which may be about 10% of a nominal collector current I C,nom  the RC-IGBT is specified for. Since for low currents the driver units typically do not always reliably detect the actual current direction, the driver unit may turn on the RC-IGBT even if the RC-IGBT is reversed biased. As a result, the driver unit may apply a gate voltage VGE of +15 V to the gate terminal of the RC-IGBT even in the RC-mode. For this case, the meta cells AC 3  may ensure that sufficient holes are injected into the drift structure  120  to maintain a sufficiently dense charge carrier plasma and to avoid a runaway of the voltage drop across the reverse diode in the RC-mode. The meta cells AC 3  inject sufficient charge carriers for maintaining a bipolar current at gate voltages above the first threshold voltage Vthn. 
     The three types of auxiliary cells including saturation injection cells AC 2 , desaturation injection cells AC 1  and meta cells AC 3  allow for adapting the modes of operation below Vthp, between Vthp and Vth 0  and above Vth 0  in the RC-mode independently from each other. The meta cells AC 3  may be arranged exclusively in the bipolar regions, e.g., exclusively in the vertical projection of collector channels  132 . Meta cells AC 3  may have a reduced anode efficiency compared to the injection cells AC 1 . The meta cells AC 3  may be defined by local dimension variations of the body zones  115  or by a variation of the vertical dopant profiles in the cell mesas  170 . 
     According to an embodiment, the body zones  115  in the meta cells AC 3  may have a lower dopant dose/concentration than the body zones  115  of the saturation and desaturation injection cells AC 2 , AC 1 . According to another embodiment, a dopant concentration in the drift zone  121  along the first pn junctions pn 1  is increased and may form a local barrier structure  125  or a locally enhanced portion  125   c  of a barrier structure  125  extending along the first pn junctions pn 1 . According to other embodiments, an increased concentration of recombination centers may reduce the effective dopant concentration in the body zones  115 . Another embodiment may increase the width of the cell mesas  170  in the meta cells AC 3 . 
     Transitions between the meta cells AC 3  and adjoining transistor cells TC, saturation injection cells AC 2  or desaturation injection cells AC 1  may be smooth or steep. The different cell types may alternate along the same cell mesa  170  or may be formed in different cell mesas  170 . Since the function of the meta cells AC 3  includes to inject charge carriers even when the MOS gated channels in the transistor cells are turned on, the meta cells AC 3  are formed in a minimum distance of at least 5 μm, e.g., at least 20 μm to the transistor cells TC and at least one of a desaturation injection cell AC 1  and a saturation cell AC 2  extends from the transistor cell TC to the meta cell AC 3 . 
       FIGS. 10A to 10C  refer to an embodiment with meta cells AC 3  directly adjoining the transistor cells TC. In the meta cells AC 3 , enhanced sections  125   x  of the barrier zones  125  may locally reduce the injection efficiency of the body zones  115 . 
     Along the contact structures  315  the body zones  115  of the transistor cells TC may include heavily doped contact zones  115   a  that improve the ohmic contact and overcurrent switching ruggedness in the transistor cells TC. By contrast, the body zones  115  of the saturation and desaturation injection cells AC 1 , AC 2  may be devoid of heavily doped contact zones  115 . 
     In addition, outside the contact zones  115   a  the body zones  115  of the transistor cells TC may have a higher dopant concentration than the body zones  115  of the desaturation injection cells AC 1 , the saturation injection cells AC 2 , or both such that emitter efficiency of the body zones in the concerned injections cells is lower than in the transistor cells TC. 
     The RC-IGBT  501  illustrated in  FIG. 11  combines a barrier structure  125  with cell mesas  170  with bottleneck sections  170   a  as described with reference to  FIGS. 7A to 7C . In the illustrated embodiment the barrier structure  125  is uniform. According to other embodiments the barrier structure  125  may be patterned and may include gaps or attenuated portions in the desaturation injection cells AC 1  as described with reference to  FIGS. 5A to 6A . Further embodiments may include meta cells AC 3  as described with reference to  FIGS. 9A to 10C . 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.