Abstract:
There is provided an insulated gate bipolar transistor including: a first semiconductor area of a first conductivity type; a second semiconductor area of a second conductivity type formed on one surface of the first semiconductor area; third semiconductor areas of the first conductivity type continuously formed in a length direction on one surface of the second semiconductor area; a plurality of trenches formed between the third semiconductor areas, extending to an inside of the second semiconductor area, and being continuous in the length direction; a fourth semiconductor area of the second conductivity type formed on one surface of the third semiconductor areas, insulation layers formed inside the trenches; gate electrodes buried inside the insulation layers; and a barrier layer formed in at least one of locations corresponding to the third semiconductor areas inside the second semiconductor area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 10-2012-0132534 filed on Nov. 21, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an insulated gate bipolar transistor having high current density and a reduced on-voltage. 
         [0004]    2. Description of the Related Art 
         [0005]    Low power consumption power conversion apparatuses have recently been promoted. Thus, research into low power consumption by a power semiconductor device that performs a central role in the power conversion apparatus has been actively undertaken. 
         [0006]    In particular, among power semiconductor devices, research into an insulated gate bipolar transistor (IGBT) is actively ongoing, since an IGBT can achieve low on-voltage according to a conductivity modulation effect and can be easily controlled owing to a voltage-driven gate. 
         [0007]    Types of IGBT include a planar IGBT, a trench IGBT, and others. A planar IGBT has a structure in which gate electrodes are formed on a wafer surface. A trench IGBT has a structure in which an oxide film is interposed in trenches formed vertically downward from a wafer surface and gate electrodes are buried therein. 
         [0008]    A trench IGBT includes channels formed in both inner walls of the trenches, which may increase channel density as compared to the case of a planar IGBT. Thus, the trench IGBT can further reduce an on-voltage. 
         [0009]    The structure of a conventional trench IGBT may include an n− type low concentration silicon substrate that is a drift layer, an n type field stop layer formed in one surface of the n− type drift layer, and a relatively high concentration p type thin collector layer formed in one surface of the field stop layer in which an amount of impurities is controlled. 
         [0010]    A plurality of p type base areas are formed in the other surface of the n− type drift layer. Surfaces of the p type base areas have n+ type emitter areas selectively formed thereon. 
         [0011]    The trench is formed from the n+ type emitter area to the n− type drift layer through the p type base area. The gate electrode formed of conductive polycrystalline silicon is formed inside the trench having a gate oxide layer therebetween. 
         [0012]    An interlayer insulation layer coated on an upper portion of the trench insulates an emitter electrode and the gate electrode. 
         [0013]    The emitter electrode formed on an upper portion of the interlayer insulation layer is formed commonly conducting and contacting the n+ type emitter area and the p type base area by using an aperture window installed in the interlayer insulation layer. 
         [0014]    A collector electrode is installed in a rear surface of the p type collector layer. 
         [0015]    To allow the trench IGBT to be on-state, a voltage higher than a threshold voltage needs to be applied to the gate electrode in a state in which a voltage applied to the collector electrode is higher than a voltage applied to the emitter electrode. 
         [0016]    Charges are accumulated in the gate electrodes by the above-described voltage and an n type inverted channel is concurrently formed in a surface of the p type base area side facing the gate oxide layer interposed between the gate electrodes. 
         [0017]    Electrons are injected into the n− type drift layer from the n+ type emitter area through the n channel. The injected electrons make collector bonding forward biased, holes are injected from the p type collector layer, and thus the trench IGBT becomes the on-state. 
         [0018]    A voltage drop value between the collector electrode and the emitter electrode in the on-state is equal to an on-voltage. 
         [0019]    To change the IGBT from the on-state to an off-state, the voltage of the gate electrode needs to be below a threshold value. 
         [0020]    Through the change in states, the charges accumulated in the gate electrodes are discharged to a gate driving circuit through a gate resistor. In this regard, since the n type inverted channel area is converted into a p type area, a path of the electrons disappears, resulting in no electron supply to the n− type drift layer. Accordingly, hole injection from the collector layer is not made, and thus, the electrons and the holes accumulated in the n− type drift layer are discharged to the collector electrode and the emitter electrode, respectively, or are re-coupled to each other. Thus, current is extinguished, and the IGBT becomes the off-state. 
         [0021]    A variety of attempts have been made to increase a current density of the trench IGBT and further reduce the on-voltage. Conventionally, performance enhancement of the IGBT has been promoted by mainly adjusting a trench space of the IGBT. 
         [0022]    However, narrowing of the trench space is restricted due to limited photo processing. Thus, a new method, other than the method of narrowing the trench space needs to be proposed. 
       RELATED ART DOCUMENT 
       [0000]    
       
         (Patent Document 1) Japanese Patent Laid-Open Publication No. 1994-188423. 
       
     
       SUMMARY OF THE INVENTION 
       [0024]    An aspect of the present invention provides an insulated gate bipolar transistor having high current density and reduced on-voltage. 
         [0025]    According to an aspect of the present invention, there is provided an insulated gate bipolar transistor including: a first semiconductor area of a first conductivity type; a second semiconductor area of a second conductivity type formed on one surface of the first semiconductor area; third semiconductor areas of the first conductivity type continuously formed in a length direction on one surface of the second semiconductor area; a plurality of trenches formed between the third semiconductor areas, extending to an inside of the second semiconductor area, and being continuous in the length direction; a fourth semiconductor area of the second conductivity type formed on one surface of the third semiconductor areas, insulation layers formed inside the trenches; gate electrodes buried inside the insulation layers; and a barrier layer formed in at least one of locations corresponding to the third semiconductor areas inside the second semiconductor area. 
         [0026]    The barrier layer may be formed between an area corresponding to 5 μm apart from lower surfaces of the trenches in a thickness direction and the third semiconductor areas. 
         [0027]    A width of the barrier layer may be smaller than a space between the trenches in a width direction. 
         [0028]    The barrier layer may be formed in the at least one of locations corresponding to the third semiconductor areas, except for a location corresponding to the fourth semiconductor area. 
         [0029]    The second semiconductor area may include a buffer layer of the second conductivity type contacting the first semiconductor area, and an impurity concentration of the buffer layer may be higher than that of the second semiconductor area. 
         [0030]    The insulated gate bipolar transistor may further include an emitter electrode commonly ohmic-contacting the third semiconductor areas and the fourth semiconductor area, and a collector electrode ohmic-contacting the other surface of the first semiconductor area. 
         [0031]    The trenches may have a continuous stripe shape in the length direction. 
         [0032]    An impurity concentration of the fourth semiconductor area may be higher than that of the second semiconductor area. 
         [0033]    According to another aspect of the present invention, there is provided an insulated gate bipolar transistor including: a first semiconductor area of a first conductivity type; a second semiconductor area of a second conductivity type formed on one surface of the first semiconductor area; third semiconductor areas of the first conductivity type continuously formed in a length direction on one surface of the second semiconductor area; a plurality of trenches formed between the third semiconductor areas, extending to an inside of the second semiconductor area, and being continuous in the length direction; a fourth semiconductor area of the second conductivity type formed on one surface of the third semiconductor areas, insulation layers formed inside the trenches; gate electrodes buried inside the insulation layers; and a barrier layer formed between the trenches inside the second semiconductor area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0035]      FIG. 1  is a schematic perspective view of an insulated gate bipolar transistor according to an embodiment of the present invention; 
           [0036]      FIG. 2A  is an enlarged view of area v of  FIG. 1 ; 
           [0037]      FIG. 2B  is an enlarged cut perspective view of area v′ of  FIG. 1 ; 
           [0038]      FIG. 3A  is a cross-sectional view taken along line A-A′ of  FIG. 1 ; 
           [0039]      FIG. 3B  is a cross-sectional view taken along line B-B′ of  FIG. 1 ; 
           [0040]      FIG. 4  is a plan view of  FIG. 1 ; and 
           [0041]      FIG. 5  is a schematic cross-sectional view of an insulated gate bipolar transistor according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0042]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0043]    In the drawings, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements. 
         [0044]      FIG. 1  is a schematic perspective view of an insulated gate bipolar transistor according to an embodiment of the present invention. 
         [0045]      FIG. 2A  is an enlarged view of area v of  FIG. 1 . 
         [0046]      FIG. 2B  is an enlarged cut perspective view of area v′ of  FIG. 1 . 
         [0047]    To clearly explain the present invention, directions of the insulated gate bipolar transistor are defined such that y, x, and z of  FIG. 1  respectively indicate a length direction, a width direction, and a thickness direction. In this regard, the thickness direction may be used to have the same meaning as a stacking direction of semiconductor areas. 
         [0048]    Referring to  FIGS. 1 ,  2 A, and  2 B, an n− type drift layer  200  may be formed on a p type collector area  100  used as a collector area. 
         [0049]    A p type well area  300  may be formed on an upper surface of the n− type drift layer  200 . 
         [0050]    The p type well area  300  may be continuously formed on one surface of the n− type drift layer  200  in a length direction. An n+ type source area  400  may be formed on a portion of an upper surface of the p type well area  300 . 
         [0051]    In a case in which the n+ type source area  400  is plural, the n+ type source areas  400  may be formed on the upper surface of the p type well area  300  to be spaced apart from each other by a predetermined interval in the length direction. 
         [0052]    A plurality of trenches  500  may be formed to pass through the p type well area  300  in a thickness direction to extend to the inside of the n− type drift layer  200 . 
         [0053]    That is, the plurality of trenches  500  may be formed between the plurality of p type well areas  300  to extend to the inside of the n− type drift layer  200  and have a continuous stripe shape in the length direction. 
         [0054]    A thin gate insulation layer  600  may be formed on inner walls of the trenches  500 . A gate electrode  700  may be formed to be buried in the gate insulation layer  600 . 
         [0055]    A barrier layer  800  may be formed inside the n− type drift layer  200 . 
         [0056]    Meanwhile, an emitter electrode that commonly ohmic-contacts the p type well area  300 , the p type well area  300  in the surface of the n+ type source areas  400 , and the n+ type source area  400  may be formed. 
         [0057]    A collector electrode that ohmic-contacts one surface of the p type collector area  100  may be formed. 
         [0058]    Meanwhile, an upper plane of the insulated gate bipolar transistor of  FIG. 1  is defined as an IGBT upper plane. 
         [0059]    Meanwhile, the collector area  100 , the drift layer  200 , the well area  300 , and the source area  400  used herein may be defined as a first semiconductor area, a second semiconductor area, a third semiconductor area, and a fourth semiconductor area, respectively. 
         [0060]    Also, the p type and the n type used herein may be defined as a first conductivity type and a second conductivity type, respectively. 
         [0061]    Further, in general, “+” means a state doped at a high concentration, and “−” means a state doped at a low concentration. 
         [0062]    With respect to the above-described insulated gate bipolar transistor (IGBT), a channel area is formed in side wall portions of the trench  500  inside the p type well area  300 . That is, if a voltage higher than a threshold voltage is applied to the gate electrode  700 , a conductivity type of the side wall portions of the trench  500  inside the p type well area  300  is inverted so that a channel is formed, and an electron current flows from the n+ type source area  400  to the n− type drift layer  200  through the channel. 
         [0063]    The electron current functions as a base current of a transistor formed by the p type well area  300 , the n− type drift layer  200 , and the p type collector area  100 . A hole current corresponding to the base current flows from the p type collector area  100  through the n− type drift layer  200  and the p type well area  300  to the emitter electrode. 
         [0064]    A potential barrier experienced by holes injected from the p type collector area  100  is formed by the barrier layer  800 , and a conductivity modulation effect increases by increasing an accumulation of holes around the barrier layer  800 . 
         [0065]      FIGS. 3A and 3B  are cross-sectional views taken along line A-A′ and line B-B′ of  FIG. 1 , respectively.  FIG. 4  is a plan view of  FIG. 1 . 
         [0066]    As shown in  FIGS. 3A ,  3 B, and  4 , the barrier layer  800  may not be continuously formed in the length direction and may be formed under a portion of the p type well area  300  on which the n+ type source area  400  is not formed. That is, since the n+ type source area  400  is a channel area for transferring an electron current, the barrier layer  800  is not formed in a location corresponding to the upper portion of the p type well area  300  having the n+ type source area  400  formed thereon, in the length direction, in order to allow the barrier layer  800  not to interrupt a transfer path of the electron current. 
         [0067]    In other words, the barrier layer  800  does not exist in a location in which the n+ type source area  400  is formed on the upper plane. 
         [0068]    The barrier layer  800  is formed in a location corresponding to the p type well area  300  inside the n− type drift layer  200  in the width direction. That is, the barrier layer  800  is disposed between the plurality of trenches  500  that are adjacent in the width direction. 
         [0069]    Also, in a case in which the holes formed in a lower portion of the barrier layer  800  fail to escape to the emitter electrode by the barrier layer  800 , the IGBT may be damaged by a latch-up operation due to a parasitic pnpn structure of the IGBT. Thus, a width of the barrier layer  800  may be smaller than a space between the trenches  500  in the width direction so that some holes can escape from a hole accumulation area formed in the lower portion of the barrier layer  800  to the emitter electrode. 
         [0070]    Furthermore, the channel is formed in the side wall portions of the trenches  500 , and the electron current flows from the n+ type source area  400  to the n− type drift layer  200  through the channel, and thus the width of the barrier layer  800  may be smaller than the space between the trenches  500  in the width direction in order not to interrupt an electron current flow. 
         [0071]    Also, the barrier layer  800  may be disposed between an area corresponding to 5 μm apart from lower surfaces of the trenches  500  in the thickness direction and the p type well area  300 . In other words, the barrier layer  800  may be disposed within a thickness range H2 from a boundary of the p type well area  300  and the n− type drift layer  200  to H1 that is within 5 μm apart from the lower surfaces of the trenches  500 . 
         [0072]    In a case in which the barrier layer  800  is disposed too far from the trenches  500 , spreading of the electron current that starts being generated from the lower portions of the trenches  500  may be interrupted. That is, in a case in which the barrier layer  800  is disposed more than 5 μm apart from the lower surfaces of the trenches  500 , this is analyzed as an additional resistance component in the electron current spreading phenomenon, resulting in lowering a voltage of a device. 
         [0073]    The barrier layer  800  may be formed by injecting oxygen ions to the n− type drift layer  200  and oxidizing the n− type drift layer  200 . 
         [0074]    Also, an n type buffer layer may be further formed between the n− type drift layer  200  and the p type collector area  100 . 
         [0075]    The buffer layer may provide a field stop function. Thus, as compared with an IGBT having no buffer layer, the IGBT according to the present embodiment may allow the n− type drift layer  200  to be thin in the same internal pressure condition. 
         [0076]    According to the embodiment of the present invention, the barrier layer  800  may be formed between the adjacent trenches  500  as shown in  FIG. 5 . 
         [0077]    The insulated gate bipolar transistor according to the embodiment of the present invention may increase a conductivity modulation phenomenon owing to the barrier layer  800  inside the n− type drift layer  200 , thereby enhancing a current density of the IGBT and further reducing an on-voltage. 
         [0078]    As set forth above, according to embodiments of the invention, an insulated gate bipolar transistor can increase current density and reduce an on-voltage. 
         [0079]    While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.