Patent Publication Number: US-9419130-B2

Title: Semiconductor device and integrated circuit

Description:
BACKGROUND 
     Power transistors commonly employed in automotive and industrial electronics require a low on-state resistance (R on ), while securing a high voltage blocking capability. For example, a MOS (“metal oxide semiconductor”) power transistor should be capable, depending upon application requirements to block drain to source voltages V ds  of some tens to some hundreds or thousands of volts. MOS power transistors typically conduct very large currents which may be up to some hundreds of Amperes at typical gate-source voltages of about 2 to 20 V. 
     Power transistors usually include a body contact region that electrically contacts the body region to the source terminal. Thereby, a parasitic bipolar transistor is widely suppressed or deteriorated. Due to the body contact region, a pn diode is formed that results in a transistor having reverse blocking capabilities in one direction only. 
     Attempts are being made to provide semiconductor devices having further improved characteristics. 
     SUMMARY 
     According to an embodiment, a semiconductor device in a semiconductor substrate comprises a first drain region and a second drain region, a first drift zone and a second drift zone, at least two gate electrodes in the semiconductor substrate, and a channel region between the gate electrodes. The first drift zone is arranged between the channel region and the first drain region, the second drift zone is arranged between the channel region and the second drain region, and the second drain region being disposed on a side of the gate electrode, the side of the gate electrode being remote from the side of the first drain region. 
     According to a further embodiment, a semiconductor device in a semiconductor substrate comprises a first drain region and a second drain region, a first drift zone, at least a first gate electrode and a second gate electrode arranged in the semiconductor substrate, a channel region disposed between the first and the second gate electrodes, and a first gate dielectric between the first gate electrode and the channel region and a second gate dielectric between the second gate electrode and the channel region. The first drift zone is arranged between the channel region and the first drain region, wherein the first gate electrode is electrically connected to a first gate terminal and the second gate electrode is electrically connected to a second gate terminal disconnected from the first gate terminal or wherein the second gate dielectric is different from the first gate dielectric. 
     According to a further embodiment, a semiconductor device comprises a transistor in a semiconductor substrate having a first main surface. The transistor comprises a source region of a first conductivity type, a drain region of the first conductivity type, a channel region of the first conductivity type, a drift zone between the channel region and the drain region, and a gate electrode at the channel region, the gate electrode being arranged in gate trenches formed in the first main surface. 
     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 embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate the main embodiments and together with the description serve to explain the principles. Other embodiments and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts. 
         FIG. 1A  shows a perspective view of a semiconductor device according to an embodiment; 
         FIG. 1B  shows a cross-sectional view of the semiconductor device shown in  FIG. 1A . 
         FIG. 2  shows a modification of the embodiment shown in  FIG. 1 ; 
         FIG. 3A  shows a cross-sectional view of a semiconductor device according to a further embodiment; 
         FIG. 3B  shows an equivalent circuit diagram of the semiconductor device shown in  FIG. 3A ; 
         FIG. 4  shows a cross-sectional view of a semiconductor device according to a further embodiment, taken in a plane parallel to the first main surface of the semiconductor substrate. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as “top”, “bottom”, “front”, “back”, “leading”, “trailing” etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims. 
     The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments. 
     The terms “wafer”, “substrate” or “semiconductor substrate” used in the following description may include any semiconductor-based structure that has a semiconductor surface. Wafer and structure are to be understood to include silicon, silicon-on-insulator (SOI), silicon-on sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could as well be silicon-germanium, germanium, or gallium arsenide. According to other embodiments, silicon carbide (SiC) or gallium nitride (GaN) may form the semiconductor substrate material. 
     The terms “lateral” and “horizontal” as used in this specification intends to describe an orientation parallel to a first surface of a semiconductor substrate or semiconductor body. This can be for instance the surface of a wafer or a die. 
     The term “vertical” as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of the semiconductor substrate or semiconductor body. 
     The Figures and the description illustrate relative doping concentrations by indicating “−” or “+” next to the doping type “n” or “p”. For example, “n − ” means a doping concentration which is lower than the doping concentration of an “n”-doping region while an “n + ”-doping region has a higher doping concentration than an “n”-doping region. Doping regions of the same relative doping concentration do not necessarily have the same absolute doping concentration. For example, two different “n”-doping regions may have the same or different absolute doping concentrations. In the Figures and the description, for the sake of a better comprehension, often the doped portions are designated as being “p” or “n”-doped. As is clearly to be understood, this designation is by no means intended to be limiting. The doping type can be arbitrary as long as the described functionality is achieved. Further, in all embodiments, the doping types can be reversed. 
     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated 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. 
     As employed in this specification, the terms “coupled” and/or “electrically coupled” are not meant to mean that the elements must be directly coupled together—intervening elements may be provided between the “coupled” or “electrically coupled” elements. The term “electrically connected” intends to describe a low-ohmic electric connection between the elements electrically connected together. 
     The present specification refers to a “first” and a “second” conductivity type of dopants, semiconductor portions are doped with. The first conductivity type may be p type and the second conductivity type may be n type or vice versa. As is generally known, depending on the doping type or the polarity of the source and drain regions, MOSFETs may be n-channel or p-channel MOSFETs. For example, in an n-channel MOSFET, the source and the drain region are doped with n-type dopants, and the current direction is from the drain region to the source region. In a p-channel MOSFET, the source and the drain region are doped with p-type dopants, and the current direction is from the source region to the drain region. As is to be clearly understood, within the context of the present specification, the doping types may be reversed. If a specific current path is described using directional language, this description is to be merely understood to indicate the path and not the polarity of the current flow, i.e. whether the transistor is a p-channel or an re-channel transistor. The Figures may include polarity-sensitive components, e.g. diodes. As is to be clearly understood, the specific arrangement of these polarity-sensitive components is given as an example and may be inverted in order to achieve the described functionality, depending whether the first conductivity type means n-type or p-type. 
       FIG. 1A  shows a perspective view of a semiconductor device  200  according to an embodiment. The semiconductor device  200  comprises a first drain region  210  and a second drain region  211 , a first drift zone  220  and a second drift zone  221  and at least two gate electrodes  230 ,  231  arranged in the semiconductor substrate. The semiconductor device further comprises a channel region  260  disposed between the gate electrodes  230 ,  231 . The semiconductor device may further comprise a first gate dielectric layer  235  that insulates the first gate electrode  230  from the adjacent substrate material. Likewise, the semiconductor device  200  may comprise a second gate dielectric layer  236  that insulates the second gate electrode  231  from the adjacent substrate material. 
     The first drift zone  220  is arranged between the channel region  260  and the first drain region  210 , and the second drift zone  221  is arranged between the channel region  260  and the second drain region  211 . The second drain region  211  is disposed on a side of the gate electrode  230 , the side being remote from the side of the first drain region  210 . Accordingly, the first drain region  210 , the first drift zone  220 , the channel region  260 , the second drift zone  221  and the second drain region  211  are arranged in this order along a first direction which may be parallel to the first main surface  110  of the semiconductor substrate  100 . According to this embodiment, the semiconductor device implements a power transistor including two drift zones. As will be explained in the following, as a result, the blocking characteristics of the power transistor may be improved when the transistor is operated in different polarities. 
     The first and the second drain region may be of a first conductivity type, for example, n-type. Further, the channel region  260  may be of the first conductivity type, having a lower doping concentration than the drain regions. Moreover, the drift regions may as well be of the first conductivity type, at a doping concentration which may be between the doping concentration of the channel region and the drain regions. According to a further implementation, the doping concentration of the drift region may be lower than doping concentration of the channel region and the drain region. The transistor implements a transistor having a channel region of the first conductivity type which does not include a body region of the second conductivity type. Accordingly, this transistor implements a depletion transistor or a normally-on transistor. Such a normally on-transistor usually is in a conducting state when no gate voltage or a gate voltage corresponding to 0 V is applied. 
     As is shown in  FIG. 1 , the gate electrodes include a first gate electrode  230  and a second gate electrode  231 . According to an embodiment, the first gate electrode  230  is held at a potential different from the potential of the second gate electrode  231 . For example, this may be accomplished by electrically connecting the first gate electrode to a first gate terminal different from a second gate terminal electrically connected to the second gate electrode. According to a further embodiment, this may be accomplished by providing the second gate dielectric adjacent to the second gate electrode with negative charges that shift the threshold voltage of the gate electrode. 
     For example, this may be accomplished by forming an initial dielectric layer that may comprise silicon oxide, silicon nitride or a combination of these materials. Thereafter, an aluminum oxide layer may be formed, the aluminum oxide layer having a thickness of e.g. less than 1 nm. Then, an annealing process is performed in an oxidizing atmosphere to form a dielectric layer having a large amount of fixed charge that is due to the incorporation of Al 2 O 3 . This may result in a significant shift of the threshold voltage of the transistor. For example, the concentration of the negative charges may be 10 11  to 10 13  cm −2 . According to a further method, an oxide layer may be deposited on the aluminum oxide layer followed by an annealing process. 
     By shifting the threshold voltage or applying a different voltage to the second gate electrode, the current voltage characteristics of the power transistor may be shifted so that at a gate voltage corresponding to 0 V, the transistor is in a non-conducting state. 
     For example, the material of the first and the second gate electrode may be doped polysilicon, for example, n doped polysilicon. By way of example, the polysilicon may be doped with phosphorous. In another embodiment the first and the second gate electrodes may be p doped polysilicon, e.g. boron doped polysilicon. The work function of p-doped polysilicon may be beneficial for increasing the threshold voltage for an n-channel transistor. 
     The respective components are formed in a portion of a semiconductor substrate  100 . For example, the semiconductor substrate  100  may be an SOI (“silicon-on-insulator”) substrate including a buried oxide layer  150  and a semiconductor base layer  160 . The first and second gate electrodes  230 ,  231  may be formed so as to extend to the buried oxide layer  150 . Accordingly, no conductive portion is present between the gate electrode and the buried oxide layer  150  and the gate electrodes accomplish a lateral isolation. The drift zones  220 ,  221  may be formed so as to have the same blocking voltage characteristics. For example, the first and second drift zones  220 ,  221  may have the same doping concentrations and the same lengths measured along the direction between drain region and gate electrode. According to a further embodiment, the first and second drift zones  220 ,  221  may be formed so as to have different blocking voltage characteristics. For example, they may have different doping concentrations at a certain distance measured from the gate electrode. According to an embodiment, the first and second drift zones  220 ,  221  may have a different length. According to the embodiment shown in  FIG. 1A , the gate electrodes  230 ,  231  are disposed in gate trenches  238  that are formed in the first main surface  110  of the semiconductor substrate  100 . The first and second drain regions  210 ,  211  may be disposed adjacent to the first main surface  110 . According to a further embodiment, the first and second drain regions  210 ,  211  may be arranged on opposing surfaces of the semiconductor substrate  100 . In this case, the gate electrodes  230 ,  231  may be formed as buried gate electrodes. According to this embodiment, the further components of the power transistor may be manufactured by epitaxially growing a further semiconductor layer and forming the respective components in the epitaxially grown semiconductor layer. The semiconductor device shown in  FIG. 1A  further comprises contact regions  225 , i.e. portions of the second conductivity type. The contact regions  225  are disposed at the side of the first and second drain regions and are electrically connected to the first or second drain terminal, respectively. The contact regions  225  extract minority carriers from the drift zones  220 ,  221 . For example, in case of a drift zone having n-type conductivity, holes are extracted by the contact regions  225 . Alternatively, the contact regions  225  may also comprise Schottky contacts in contact with regions of the first conductivity type, the regions of the first conductivity type having a doping concentration lower compared to the drain regions. The contact regions  225  may also comprise Schottky contacts in contact with the drift zones  220 ,  221 . 
     Field plates may be disposed at the first and/or second drift zone  220 ,  221  or embedded in the first and/or second drift zone. For example, the field plates may be formed in field plate trenches, the field plate trenches being disposed in the first main surface  110  of the semiconductor substrate  100 . As is common, the field plates may be insulated from the first and/or second drift zone by means of a field plate dielectric layer. 
     When a suitable voltage is applied to the gate electrode  230 , an inversion layer is formed at the boundary between the channel regions  260  and the insulating gate dielectric material  235 . Accordingly, the transistor is in a conducting state from the first drain region  210  to the second drain region  211  or vice-versa. The polarity of the “suitable voltage” may, for example, depend on the polarity of the dopants of the channel region. The conductivity of the channel that is formed in the channel region  260  is controlled by the gate electrode. The second gate electrode  231  controls the threshold voltage of the transistor. To be more precise, the first gate electrode controls the conductivity of the channel, and the threshold voltage is set by applying a different, typically time independent voltage to the second gate electrode  231  or by setting the same gate voltage to the second gate electrode, wherein the second gate dielectric  236  is modified in comparison with the first gate dielectric layer  235 . By controlling the conductivity of the channel formed in the channel region, the current flow from the first drain region  210  to the second drain region  211  via the channel formed in the channel region  260  may be controlled. 
     When the transistor is switched off, no conductive channel is formed at the boundary between the channel region  260  and the first gate dielectric material  235 , so that only a subthreshold current flows. Due to the presence of the first and/or second drift zones  220 ,  221 , the blocking or breakdown voltage characteristics may be further improved. 
     As is indicated in the perspective view shown in  FIG. 1A , the channel region  260  has the shape of a ridge. This is further illustrated in the cross-sectional view shown in  FIG. 1B , which is taken between I and I′. The cross-sectional view is taken so as to intersect a plurality of first and second gate electrodes  230 ,  231  and channel regions  260 . The ridge may have a topside  260   c , and side walls  260   b.    
     According to an embodiment, the width d 1  of the channel region  260  fulfills the following relationship: d 1 ≦l d , wherein l d  denotes a length of a depletion zone which is formed at the interface between the gate dielectric layer  235  and the channel region  260 . For example, the width of the depletion zone may be determined as: 
     
       
         
           
             
               
                 
                   
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     wherein ∈ s  denotes the permittivity of the semiconductor material (11.9×∈ 0  for silicon, ∈ 0 =8.85×10 −14  F/cm), k denotes the Boltzmann constant (1.38066×10 −23  J/K), T denotes the temperature, ln the denotes the natural logarithm, N A  denotes the impurity concentration of the semiconductor body, n i  denotes the intrinsic carrier concentration (1.45×10 10  cm −3  for silicon at 27° C.), and q denotes the elementary charge (1.6×10 −19  C). 
     Generally, the length of the depletion zone varies depending from the gate voltage. It is assumed that in a transistor the length of the depletion zone at a gate voltage corresponding to the threshold voltage corresponds to the maximum width of the depletion zone. For example, the width of the first ridges may be approximately 10 to 200 nm, for example, 20 to 60 nm along the first main surface  110  of the semiconductor substrate  100 . 
     According to the embodiment in which the width d 1 ≦l d , the transistor  200  is a so-called “fully-depleted” transistor in which the channel region  260  is fully depleted when the first gate electrode  230  is set to an on-voltage. In such a transistor, an optimal sub-threshold voltage may be achieved and short channel effects may be efficiently suppressed, resulting in improved device characteristics. 
       FIG. 2  shows a perspective view of a semiconductor device according to a further embodiment. The semiconductor device shown in  FIG. 2  basically corresponds to the semiconductor device illustrated in  FIG. 1 . In addition,  FIG. 2  shows field plates  240  that are arranged in field plate trenches formed in the first main surface  110  of the semiconductor substrate. The field plates  240  are insulated from the adjacent semiconductor material by means of a field plate dielectric layer  241 . 
     Further, the semiconductor device comprises a source region  250  that is disposed between adjacent gate trenches  238 . The source region  250  may be of the first conductivity type, for example n-type. The source region  250  provides a reference for applying the gate voltage. The source region  250  may be connected to a source terminal and may be used for charging and discharging the channel region when charging and discharging the gate electrodes  230 ,  231 . The source region  250  may be interrupted. Further, the source region  250  may extend to a smaller depth than is indicated in  FIG. 2 . The first drain region  210 , the first drift zone  220 , one of the gate electrodes  230 ,  231 , the source region  250 , another corresponding one of the gate electrodes  230 ,  231 , the second drift zone  221 , and the second drain region  211  may be arranged along the first direction. In the arrangement of  FIG. 2 , the current flows from the first drain regions  210  to the second drain regions  211  and vice versa. 
       FIG. 3A  shows a cross-sectional view of a semiconductor device of still a further embodiment. According to the embodiment of  FIG. 3A , the first drain region  310  is disposed adjacent to the first main surface  110  of the semiconductor substrate  100 . Further, the second drain region  311  is disposed adjacent to a second main surface  120  of the semiconductor substrate  100 . The first and the second gate electrodes  330 ,  331  are implemented as buried gate electrodes and a further semiconductor material is epitaxially grown over the gate electrodes  330 ,  331 . The first drift zone  320  and the first drain region  310  may be formed in this overgrown semiconductor material. In a similar manner as has been discussed above with reference to  FIG. 1A , the first and second drain regions  310 ,  311  are of the first conductivity type, for example, with n-type. The drift zones  320 ,  321  are of the first conductivity type, having a lower impurity concentration than the drain regions. The first and second drift zones  320 ,  321  may be doped at the same or at different doping levels at a given distance measured from one of the gate electrodes  330 ,  331 . Further, the length of the first and second drift zones  320 ,  321  may be equal to each other or different. The gate electrodes include first gate electrodes  330  and second gate electrodes  331  which may be implemented in the manner as has been discussed above. Further, a channel region  360  is disposed between adjacent gate electrodes. The channel region  360  may be doped with dopants of a first or a second conductivity type. The semiconductor device further may comprise a source region  350  for providing a reference of a voltage applied to the gate electrode. The source electrode  350  may be electrically connected via a source contact  355  to a source terminal. The source contact  355  is insulated from adjacent semiconductor material by means of a dielectric material  356 . The gate dielectric layer  335  may comprise silicon oxide, silicon nitride, or any other suitable dielectric material. Likewise, the second gate dielectric layer  336  may comprise any of these materials. Optionally, the second gate dielectric layer  336  may be doped so as to shift the threshold voltage as has been discussed above. For example the gate dielectric layer that is adjacent to the channel region may be thinner than the gate dielectric material that is disposed between the gate electrode and the adjacent drift zone  320 ,  321 . For example, as is indicated in  FIG. 3A , the portion  336   a  of the gate dielectric layer may be thinner than the portion  336   b  of the gate dielectric material. Thereby, the gate-drain capacitance may be decreased, resulting in reduced switching losses. 
       FIG. 3B  illustrates an equivalent circuit diagram of the semiconductor device illustrated in  FIG. 3A . As is illustrated, a current flow is accomplished between the first and the second drain regions  310 ,  311 . Further, the gate voltage is applied to the gate electrode  330  with reference to the potential of the source electrode  350 . According to the embodiment of  FIGS. 3A and 3B , the source electrode  350  is optional. 
       FIG. 4  shows a plan view or, differently stated, a cross-sectional view of a semiconductor device  400  according to a further embodiment. In  FIG. 4 , the same components are designated by references which are incremented by 100 with respect to  FIG. 3A , or incremented by 200 with respect to  FIG. 2 . The semiconductor device illustrated in  FIG. 4  includes a first and a second drain region  410 ,  411 . Further, the semiconductor device includes first gate electrodes  430  and second gate electrodes  431 . A channel region  460  is disposed between adjacent gate electrodes  430 ,  431 . A first drift zone  420  and a second drift zone  421  are disposed on either sides of the gate electrodes  430  between the gate electrode and the first or second drain region, respectively. The semiconductor device  400  further comprises a termination region  470  inclosing the transistor cell array. The second gate electrodes  431  are disposed adjacent to the termination region  470 . Thereby, an appropriate setting of the threshold voltage is ensured in all of the transistor cells illustrated in  FIG. 4 . 
     As has been described above, the semiconductor device according to the embodiments may eliminate a pn junction so as to eliminate the reverse recovery losses for fast switching applications. Further, due to the presence of the drift zones on either side of the gate electrode, the blocking voltage characteristics of the device is further improved. In particular, the blocking characteristic of the device may be exhibited in each direction, for example, without dependence from a polarity of the applied voltage. Accordingly, the semiconductor device may implement a bidirectional switch. 
     According to an embodiment, a bidirectional switch in a semiconductor substrate may comprise a first drain region and a second drain region, a first drift zone and a second drift zone, and at least two gate electrodes arranged in the semiconductor substrate. The bidirectional switch may further comprise a channel region between the gate electrodes. The first drift zone may be arranged between the channel region and the first drain region, the second drift zone may be arranged between the channel region and the second drain region. The second drain region may be disposed on a side of the gate electrode, the side being remote from the side of the first drain region. 
     According to a further embodiment, a bidirectional switch in a semiconductor substrate may comprise a first drain region and a second drain region, a first drift zone, and at least a first gate electrode and a second gate electrode arranged in the semiconductor substrate. The bidirectional switch further comprises a channel region disposed between the first and the second gate electrodes, a first gate dielectric between the first gate electrode and the channel region and a second gate dielectric between the second gate electrode and the channel region. The first drift zone is arranged between the channel region and the first drain region. The first gate electrode may be electrically connected to a first gate terminal, and the second gate electrode may be electrically connected to a second gate terminal disconnected from the first gate terminal. Alternatively, the second gate dielectric may be different from the first gate dielectric. 
     According to a further embodiment, a bidirectional switch comprises a transistor in a semiconductor substrate having a first main surface. The transistor comprises a source region of a first conductivity type, a drain region of the first conductivity type, a channel region of the first conductivity type, a drift zone between the channel region and the drain region, and a gate electrode at the channel region. The gate electrode is arranged in gate trenches formed in the first main surface. 
     While embodiments of the invention have been described above, it is obvious that further embodiments may be implemented. For example, further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above. Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.