Patent Publication Number: US-7719080-B2

Title: Semiconductor device with a conduction enhancement layer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of semiconductor devices and more particularly to semiconductor devices having a conduction enhancement layer. 
     2. Description of Related Art 
     Semiconductor devices are increasingly required to accommodate high currents and/or high voltages without failing. Many high power applications call for the use of a semiconductor switch which is required to conduct a large current when turned on, and to block a high voltage when off. 
     One device used in such applications is the power metal-oxide-semiconductor field-effect transistor (MOSFET). As discussed in J. Baliga, Power Semiconductor Devices, PWS Publishing Co. (1996) at p. 426, a power MOSFET exhibits excellent fast switching capability and safe-operating-area. When designed to block relatively low voltages (less than 200 volts), the power MOSFET has a low on-resistance. However, on-resistance increases very rapidly when its breakdown voltage is increased. This makes the on-state power losses unacceptable where high DC supply voltages are used. 
     Another approach which has been explored to improve blocking voltage while maintaining low on-resistance has been the fabrication of FETs using silicon carbide (SiC). SiC has a wider bandgap than does silicon (Si), giving it a “critical electric field”—i.e., the peak electric field that a material can withstand without breaking down—that is an order of magnitude higher than that of Si. This allows much higher doping and a much thinner drift layer for a given blocking voltage, resulting in a very low specific on-resistance in SiC-based devices. 
     Unfortunately, many SiC devices developed to date exhibit severe commercialization constraints. One such device is described in “High-voltage Accumulation-Layer UMOSFET&#39;s in 4H-SiC”, IEEE Electron Device Letters, Vol. 19, No. 12 (December 1998), pp. 487-489. This SiC-based device employs a UMOS structure, with an accumulation channel formed on the sidewalls of the trench by epitaxial growth to attain enhancement mode operation. It requires an additional epitaxial layer under the p-base to promote current spreading and achieve low on-resistance. The doping levels and the thicknesses of the sidewall epilayer and the epilayer under the p-base must be tightly controlled to achieve an enhancement mode device with low on-resistance. These demands result in a complex fabrication process which is unsuitable for large-scale manufacturing. 
     Another high power device is the insulated-gate bipolar transistor (IGBT). An IGBT with a trench gate structure is described, for example, in H.-R. Chang and B. Baliga, “500-V n-Channel Insulated-Gate Bipolar Transistor with a Trench Gate Structure”, IEEE Transactions on Electron Devices, Vol. 36, No. 9, September 1989, pp. 1824-1828. In operation, a positive gate voltage forms N-type inversion layers, through which electrons flow to provide the base drive needed to turn on the device&#39;s PNP transistor. 
     However, the IGBT has disadvantages which render it unsuitable for some applications. Because the structure is basically a transistor with gain, there will be some recombination in its N− drift region, causing the device to exhibit a high forward voltage drop. Another drawback to IGBTs is that they can “latch-up”, at which point they are no longer under the control of the gate voltage. When in this mode, conduction through the device can no longer be controlled by the gate voltage. 
     SUMMARY OF THE INVENTION 
     Briefly, and in general terms, the invention is directed to semiconductor devices with a conduction enhancement layer. In one aspect, such a semiconductor device includes a drift layer of a first conductivity type having a first doping concentration and a conduction layer also of the first conductivity type that is on the drift layer and has a second doping concentration greater than the first doping concentration, i.e., the doping concentration of the drift layer. The device also includes a pair of trench structures. Each trench structure includes a trench contact at one end and a region of a second conductivity type that is opposite the first conductivity type, at another end. Each trench structure extends into and terminates within the conduction layer such that the second-conductivity-type region of the trench structure is within the conduction layer. A first contact structure is on the drift layer opposite the conduction layer while a second contact structure is on the conduction layer. During device operation, the conduction layer, with its higher doping concentration, shrinks the depletion region spreading from the second conductivity type region at the end of the trench structures. The shrinking of the depletion region can significantly improve the device on-resistance while maintaining the blocking capability. 
     In another aspect, the invention relates to an N type field-effect transistor (FET) switch that includes an N− drift layer having a doping concentration and an N type conduction layer on the N− drift layer that has a doping concentration greater than the doping concentration of the N-drift layer. The FET also includes a pair of gate structures, each including a gate contact at one end and a P region at another end. Each gate structure extends into and terminates within the conduction layer such that the P region is within the conduction layer. A drain contact structure including an N+ layer is on the N− drift layer opposite the conduction layer while a source contact structure including an N+ layer is directly on the conduction layer. With this arrangement, the doping concentration of the entire portion of the device between the N− drift layer and the source contact structure is greater than the doping concentration of the N− drift layer. 
     In another aspect, the invention relates to a P type FET switch that includes a P− drift layer having a doping concentration and a P type conduction layer on the P− drift layer that has a doping concentration greater than the doping concentration of the P− drift layer. The switch also includes a pair of gate structures, each including a gate contact at one end and an N region at another end. Each gate structure extends into and terminates within the conduction layer such that the N region is within the conduction layer. The switch also includes a drain contact structure including a P+ layer on the P− drift layer opposite the conduction layer and a source contact structure including a P+ layer directly on the conduction layer. With this arrangement, the doping concentration of the entire portion of the device between the P− drift layer and the source contact structure is greater than the doping concentration of the P− drift layer. 
     These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of one embodiment of a semiconductor device, i.e., a FET switch, configured in accordance with the present invention. 
         FIG. 2  is a sectional view of the switch of  FIG. 1 , illustrating its operation when off. 
         FIG. 3  is a sectional view of the switch of  FIG. 1 , illustrating its operation when on. 
         FIG. 4  is a graph plotting drain current vs. drain bias at several different gate-source voltages for a FET switch configured in accordance with the present invention. 
         FIG. 5  is a sectional view of an opposite polarity version of the device of  FIG. 1 . 
         FIG. 6  is a graph plotting drain current vs. drain bias at several different gate-source voltages for a conventional FET switch and a FET switch configured in accordance with the present invention. 
         FIG. 7  is a cross-sectional view of a multiple-cell implementation of a switch, cut along section lines  7 - 7  in  FIGS. 8 ,  9  and  10 . 
         FIG. 8  is a plan view of one embodiment of a multiple-cell implementation of a switch. 
         FIG. 9  is a plan view of another embodiment of a multiple-cell implementation of a switch. 
         FIG. 10  is a plan view of yet another possible embodiment of a multiple-cell implementation of a switch. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary embodiment of a semiconductor device  10 , e.g., a FET switch, configured in accordance with the present invention is shown in  FIG. 1 . An N− drift layer  12  having a doping concentration is on a first N+ layer  14 . A N type conduction layer  16  having a doping concentration greater than that of the N− drift region  12  is on the N− drift layer. 
     Recessed into the conduction layer  16  opposite the drift layer  12  are a pair of trench structures  18 ,  20 , which are separated by a mesa region  22  that is comprised of the portion of the conduction layer  16  between the trenches. The trenches  18 ,  20  are recessed to a predetermined depth, with the depth defining each trench&#39;s “bottom”. Each trench has oxide side-walls  24 ,  26 ,  28 ,  30  and is filled with a conductive material  32 . At the bottom of each trench is a shallow P region  34 ,  36 , which extends across the bottom and around the corners of each trench&#39;s side-walls. The depth of the trench structures  18 ,  20  is such that the P regions  34 ,  36  are entirely within the conduction layer  16 . A second N+ layer  38  is on the conduction layer  16  within the mesa region  22 . 
     The first N+ layer  14  provides an ohmic contact to the drift layer  12 , and a first layer of metal  40  on the N+ layer  14  provides a drain connection for the FET switch. The first N+ layer  14  and the first metal layer  40  define all or a portion of a drain contact structure. The second N+ layer  38  provides an ohmic contact to the mesa region  22 , and a second layer of metal  42  on the N+ layer  38  provides a source connection for the switch. The second N+ layer  38  and the second metal layer  42  define all or a portion of a source contact structure. A third layer of metal  44  contacts the conductive material  32  in each of the trenches  18 ,  20 , and provides a gate connection, or contact, for the FET switch. 
     The structure of the switch when “off” is illustrated in  FIG. 2 . The switch is turned “off” by lowering gate voltage Vg toward a negative voltage sufficient to reverse-bias the junction between the shallow P regions  34 ,  36  and the conduction layer  16 . This causes depletion regions  46  to form around each P region  34 ,  36  and adjacent to the oxide sidewalls  24 ,  26 ,  28 ,  30 . The depletion regions  46  merge in mesa region  22  to form a potential barrier for electrons. This barrier prevents the flow of current between the drain  40  and the source  42 . The Vg value needed to turn the device off is dependent on several factors, the most significant of which is the width of the mesa region  22 : the wider the mesa region  22 , the greater the turn-off value of Vg. 
       FIG. 3  illustrates how the switch of  FIG. 1  is turned “on”. A positive Vg voltage, sufficient to overcome the built-in potentials of the p-n junctions present at the interfaces between the P regions  34 ,  36  and the conduction layer  16 , is applied to the gate connection  44 . This turns the switch on, such that current  48  is allowed to flow from the drain  40  to the source  42  via the mesa region  22 . As described below with reference to  FIG. 4 , the turn-on voltage Vg for the device is typically less than 2V. Due to this voltage level there is little or no hole injection from the P regions  34 ,  36  into the conduction layer  16 . Because the conduction layer  16  is more heavily doped than the N− drift layer  12 , it reduces the out-diffusion of P material from the P regions  34 ,  36 ; shrinks the depletion region  46  spreading from the P regions  34 ,  36 ; and thereby provides a switch having a lower on-resistance than would be provided by a switch whose P regions are within the less heavily doped N− drift layer  12 . 
     The degree of conductivity through the conduction layer  16  is regulated with the voltage applied to the gate contact  44 . Experimental plots of the on-state characteristics of the switch are shown in  FIG. 4 , which plots drain current versus drain bias voltage for gate-to-source voltages Vg of 0, 0.5, 1.0, 1.5 and 2.0 volts. From the plots it is noted that the device provides a high forward current, i.e., drain current density, with a low on-resistance and that the value of Vg necessary to provide a high forward current is low. This low Vg value ensures a negligible gate current, i.e., the current from the gate to the source. 
     The device is preferably arranged such that a negative gate voltage is required to form the potential barrier needed to turn the device off. That is, with no voltage applied to the gate connection  44 , the mesa region  22  should not be completely depleted by the potentials created by the work function difference between the conductive material  32  and the conduction layer  16 . If the device was arranged such that the mesa region  22  was completely depleted with Vg=0, the device&#39;s on-resistance may be unacceptably high. 
     The trenches  18 ,  20  are preferably recessed vertically into the conduction layer  16 ; i.e., with their side-walls approximately perpendicular to the top surface of the conduction layer. However, the device is not limited to vertically-recessed trenches: each trench may be wider at the top than it is at the bottom, or vice versa. Trenches that are wider at the top than at the bottom make the mesa region wider between the trench bottoms, which tends to lower the switch&#39;s on-resistance but may degrade its blocking voltage. Trenches that are wider at the bottom constrict the mesa region, which may increase on-resistance but improve blocking voltage. Vertically-recessed trenches provide a good balance between on-resistance and blocking voltage, and are preferred. 
     Referring back to  FIG. 1 : the conductive material  32  in the trenches  18 ,  20  is preferably polysilicon which has been heavily-doped with acceptors. Polysilicon is preferred because it easily fills the trenches, but other materials that can fill the trenches and provide good conductivity could also be used. 
     When the switch is required to have a high blocking voltage, i.e., greater than about 300 volts, its conduction layer  16 , N+ drift layers  14 ,  38 , N− drift layer  12 , and shallow P regions  34 ,  36 , are preferably made from a semiconductor material having a bandgap voltage that is higher than that of silicon (Si), such as silicon carbide (SiC), gallium nitride (GaN), or diamond. The peak electric field that a material can withstand without breaking down, i.e., its “critical field”, is proportional to its bandgap voltage. Thus, an SiC layer, for example, is able to sustain a peak field that is about ten times greater than that supportable by an Si layer of comparable thickness. Furthermore, the doping concentration a material is capable of attaining is proportional to its critical field. Thus, SiC&#39;s higher critical field enables the switch&#39;s material layers to have a doping concentration that is an order of magnitude higher than is possible with Si (˜5×10 15  vs. ˜8×10 13  carriers/cm 3 ). 
     The higher doping concentration achievable with a wide-bandgap material also lowers the device&#39;s on-resistance when compared with an Si implementation. Use of a wide-bandgap material also reduces reverse leakage current. SiC&#39;s wide bandgap enables a device&#39;s reverse leakage current to be several orders of magnitude less than an Si-based device of comparable thickness. This factor also serves to increase the temperature at which the switch can be operated. Because reverse leakage current increases exponentially with temperature, conventional devices must be operated at lower temperatures to achieve leakage currents as low as that provided by an SiC implementation. Conversely, a switch fabricated from a wide-bandgap material such as SiC can be operated at higher temperatures while still meeting a given reverse leakage current specification. 
     The device&#39;s reverse blocking capability is determined by a number of factors, including the width of the mesa region  22  and the doping and thickness of the conduction layer  16  and the N− drift layer  12 . A lower doping concentration of the N type conduction layer  16  requires a wider mesa region  22 , which tends to lower the device&#39;s on-resistance, but also lowers its blocking voltage. Conversely, a higher doping concentration of the N type conduction layer  16  allows for a narrow mesa region  22 , which improves the device&#39;s blocking capability, but also increases on-resistance. The doping of the N type conduction layer  16  needs to be properly designed so as to maintain the blocking capability determined by the properties of the N type layer  12 . 
     Reverse blocking voltage increases with increasing thickness and decreasing doping level of the N− drift region  12 . A thick and lightly-doped drift region tends to have a high resistance. However, the more heavily doped N type conduction layer acts to significantly lower on-resistance, thereby enabling the fabrication of a device having lightly doped N− drift region  12  to provide a high reverse blocking voltage, while still providing a low on-resistance via the heavily doped N type conduction layer. 
     The shallow P regions  34 ,  36  protect the trench oxide from high electric fields. Thus, the doping of the shallow P regions  34 ,  36  should be sufficient to prevent them from becoming completely depleted when subjected to the switch&#39;s rated breakdown voltage. The depletion region from the shallow P regions  34 ,  36  extend around the trench corners, to protect the corners from premature breakdown caused by high electric fields. 
     The P regions  34 ,  36  are preferably made shallow to limit lateral diffusion. To further limit lateral diffusion, it is preferred that the P regions  34 ,  36  comprise a slow-diffusing material. For an SiC implementation of the switch, the preferred material for the shallow P regions is aluminum. 
     Careful consideration must be given to the width of the mesa region  22 , and the widths and depths of the trenches  18 ,  20 . For example, if a mesa is too narrow, the depletion regions from the P regions  34 ,  36  may act to pinch off the conductive path and block current flow. If too wide, the reverse blocking voltage may be adversely affected. 
     The present FET switch is not limited to the structure shown in  FIG. 1 . An opposite polarity embodiment of a FET switch  110  is shown in  FIG. 5 , in which each of the materials has been swapped with its opposite polarity counterpart. Here, a P− drift layer  112  having a doping concentration is on a first P+ layer  114  and a P type conduction layer  116  having a doping concentration greater than that of the P− drift layer is on the P− drift layer. 
     A pair of trenches  118 ,  120  are recessed into the conduction layer  116  opposite the drift layer  112  and P+ layer  114 , and are separated by a mesa region  122 . Each of the trenches  118 ,  120  has oxide side-walls  124 ,  126 ,  128 ,  130  and is filled with a conductive material  132 . A shallow N region  134 ,  136  is at the bottom of each trench. The depth of the trench structures  118 ,  120  is such that the N regions  134 ,  136  are entirely within the conduction layer  116 . 
     A second P+ layer  142  is on the P− drift layer  138  within mesa region  122 . The first P+ layer  114  provides an ohmic contact to the drift layer  112 , and a first layer of metal  140  on the P+ layer  114  provides a drain connection for the FET switch. The second P+ layer  138  provides an ohmic contact to the mesa region  122 , and the second layer of metal  142  on the P+ layer  138  provides a source connection for the switch. A third layer of metal  144  contacts conductive material  132  in each of the trenches, providing a gate connection for the switch. The conductive material is preferably polysilicon which has been heavily-doped with donors, or acceptors. 
     The switch functions as before, except that the switch is turned on by applying a negative gate voltage to gate connection  144  sufficient to overcome the built-in potentials of the p-n junctions present at the interfaces between N regions  134 ,  136  and the conduction layer  116 . This allows current to flow from source  142  to drain  140 . A positive gate voltage sufficient to reverse-bias the junction between the shallow N regions  134 ,  136  and the conduction layer  116  turns the device off. 
     With reference to  FIG. 6 , a comparison of current-voltage family curves between a conventional JFET (i.e., one without a conduction layer  16 ,  116 ) and a novel JFET configured in accordance with the present invention is shown. Both JFETs were fabricated on an SiC wafer with a doping concentration of 100 μm, ˜5×10 14  cm −3  for their respective drift layers and a doping concentration of 4 μm, ˜1×10 16  cm −3  for the conduction layer in the novel JFET. As shown in  FIG. 6 , the novel JFET can conduct more than five times higher forward current than the conventional JFET at a significantly lower gate voltage Vg−2V verses 7V. It has been observed that the conventional JFET exhibited a gate leakage current in the mA range for a Vg of 7V, while the novel JFET exhibited a gate leakage current only in the μA range for a Vg of 2V. 
     To provide a high power switch, the structures of  FIGS. 1 ,  5  (each of which depict an individual device “cell”), are repeated across a die having an area sufficient to provide the necessary current carrying capacity. This is illustrated in the cross-sectional view shown in  FIG. 7  of an exemplary high power FET switch. A die  200  has an N− drift layer  202  on an N+ layer  204 , with a metal layer  206  on the N+ layer providing a drain connection. The die also has a N conduction layer  205  on the N− drift layer  202 . Each of these layers runs approximately the full length and width of the die. 
     A number of trench structures  208  are spaced periodically across the die, each of which has the same structure as the trenches shown in  FIG. 1 ; i.e., filled with a conductive material  210  and having oxide side-walls  212 , with a shallow P region  216  at its bottom that lies within the conduction layer  205 . A layer of metal  218  contacts the conductive material in each trench to provide a gate connection. Mesa regions  220  are located between each pair of trenches; an N+ layer  222  is on the N conduction material in each of the mesa regions; and a layer of metal  224  connects all of the N+ layers together to provide a source connection. 
     The switch operates as described above: when a low-level positive gate voltage, e.g., approximately 2V or less, is applied, current flows between drain and source via the mesa regions. 
     The trench structures  208  may be arrayed across the die in a wide variety of ways. One arrangement is illustrated in  FIG. 8 , which is a plan view that corresponds with the cross-sectional view of  FIG. 7  (metal layers  206 ,  218 , and  224  are not shown for clarity). The trench structures  208  form channels that run the length of the die  200  and are spaced periodically across its width. 
     Another possible trench structure arrangement is shown in  FIG. 9 , which also corresponds with the cross-sectional view of  FIG. 7 . Here, the N+ layers  222  and the mesa regions  220  below them are cylindrical in shape and the oxide side-walls  212  surrounding the layers and regions are spaced periodically within the die  200 . The trench structures&#39; conductive material  210  and the buried P regions  216  occupy the area between the cylindrical mesa regions. Note that an alternative arrangement to that shown in  FIG. 9  is also possible, in which the mesa regions  220  and the trench structures  208  are reversed. The trench structures could be circular in shape and surrounded by cylindrical oxide side-walls, and spaced periodically within the die  200 , with the trench structures  208  recessed in the area between the mesa regions  220 . 
     The perimeter of each of the mesa regions or trenches may also describe a polygon. One particularly efficient example of this is shown in  FIG. 10 , which also corresponds with the cross-sectional view of  FIG. 7 . Here, the N+ layers  222 , the mesa regions  220  below them, and the oxide side-walls  212  surrounding them are hexagonal in shape, and are spaced periodically within the die  200 . 
     The trench structure arrays shown in  FIGS. 7-10  are merely exemplary; many other possible trench and mesa geometries and arrangements are possible which will result in switches that adhere to the principles of the invention. 
     A “termination” typically surrounds an array of cells as described above, to protect the cells on the outer edges of the array. For the present switch, it is preferable that the termination depth extend well into the drift layer. This serves to better protect the outlying trench structures, and reduces the sharpness of the termination&#39;s corners, which enhances the termination&#39;s ability to protect the trenches from high electric fields. 
     When the conduction layer is N type material, the termination is typically P type. Thus, the termination may be formed with the same P type material as is used for the buried P regions below each trench. 
     It will be apparent from the foregoing that while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. For example, while the above detailed description has focused on embodiments of the invention related to FETs, the invention may be used in other semiconductor devices such as insulated gate bipolar transistors (IGBTs). Accordingly, it is not intended that the invention be limited, except as by the appended claims.