Abstract:
An SiC Schottky diode die or a Si Schottky diode die is mounted with its epitaxial anode surface connected to the best heat sink surface in the device package. This produces a substantial increase in the surge current capability of the device.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit and priority of U.S. Provisional Application No. 60/696,634, filed Jul. 5, 2005 the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to semiconductor devices and more particularly relates to a structure to improve the surge capability of a Schottky diode.  
       BACKGROUND OF THE INVENTION  
       [0003]     Silicon Carbide (SiC) Schottky diodes are well known and have reduced switching losses, increased breakdown voltage and reduced volume and weight as compared to their silicon (Si) counterparts. Such devices are therefore replacing Si Schottky devices in numerous applications such as converter/inverters, motor drives, and the like.  
         [0004]     However, higher voltage SiC Schottky diodes, such as those rated at 600 volts, for example, have a reduced surge capability than the equivalent Si device. Thus, in an application such as an AC/DC power factor correction circuit, where surge ruggedness is important, the surge capability of the conventional SiC Schottky diode was reduced by a factor of 4, compared to the equivalent Si Schottky diode.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     In accordance with the present invention, a SiC Schottky die or even a silicon (Si) Schottky die is mounted in a package which is arranged to more effectively remove heat from its epitaxial anode side, which is the hottest side of the die thereby to reduce the effect of “self heating”, which we have recognized is the source of the reduced surge capability of the SiC Schottky diode and the equivalent Si Schottky die.  
         [0006]     This is accomplished by mounting the die with its anode side well coupled to a conductive heat sink surface. Thus, a SiC die or a Si die may be inverted from its usual orientation and the guard ring surrounding the active area is well insulated so that the active anode area can be soldered or secured with a conductive adhesive to the heat sink surface without shorting the guard ring. The support surface may be a conventional lead frame as used for a TO-220 type package, or the like, or may be the interior surface of the conductive “can” of a DirectFET® type housing. Such DirectFET® type housings or packages are shown in U.S. Pat. No. 6,624,522 (IR-1830) the disclosure of which is incorporated herein in its entirety.  
         [0007]     To ensure good electrical and/or thermal connection of the anode to the heat sink surface, a solderable top metal of the type shown in copending application Ser. No. 11/255,021, filed Oct. 20, 2005 (IR-2769), the entirety of which is incorporated herein by reference, is formed on the anode surface of the die, particularly a SiC die. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a SiC Schottky diode forward voltage drop and forward current at a plurality of different temperatures.  
         [0009]      FIG. 2  shows a measured forward voltage drop as a function of time for different values of 0.5 m sec. pulses of forward current at 25° C. in the prior art package of  FIG. 4 .  
         [0010]      FIG. 3  is like  FIG. 2  but shows a reduced forward voltage drop when the Schottky die is mounted in accordance with the invention as shown in  FIG. 5 .  
         [0011]      FIG. 4  is a cross-section of a SiC Schottky diode of the prior art in which the anode layer, or epitaxially formed layer faces away from the main package heat sink.  
         [0012]      FIG. 5  shows the structure of  FIG. 4  where the die is flipped over, and the hotter epi surface side of the die faces and is thermally coupled to the main heat sink surface of the device package or assembly. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     We performed a thermal and electrical analysis of SiC Schottky diodes and learned that the reduction in their surge capability, as compared to equivalent Si devices is related to the “self heating” of the die under high current and relatively long pulse conditions when the die is unable to effectively dissipate the heat produced. This limitation on device performance during forward conduction since, at high current, the positive temperature coefficient forces a thermally reduced voltage drop which increases until device destruction.  
         [0014]     This is due to the characteristic of SiC (of any of the various polytypes such as 4H, 3C, 6H and others) and is strongly dependent on temperature particularly with lightly doped material as is normally found in the top epitaxially grown layer of a typical SiC devices.  
         [0015]     Thus, as shown in  FIG. 1 , we have recognized from calculation and simulation the strong effect of temperature on the forward voltage drop and forward current due to self heating (R th =2.5 K/W). In  FIG. 1 , current saturation is apparent.  
         [0016]     The effect is strongly dependent on lightly doped material, (i.e. the epitaxial layer carrying the anode contact of the Schottky. Thus, mobility in this layer decreases with temperature according to the following formula:  
         μ   ⁡     (   T   )       =         μ   o     ⁡     [     T   300     ]         -   2.5           
 
 where μ o =400. 
 
         [0017]     From the above, it can be seen that the high mobility at high junction temperatures T j  will lead to high resistivity high forward voltage drop V f  and poor surge capability. It should be noted that the same analysis applies to the Si Schottky die as well as the SiC Schottky die and the benefits of the invention apply equally.  
         [0018]     In accordance with the invention, and with the above understanding, it is critically necessary to improve the cooling of the epitaxial silicon side of the die (the anode) since that is the hottest side of the die. Thus, the epitaxial side of the die must contact the best heat dissipation surface available in the package for the die. Thus, in a plastic package, this would be the lead frame supporting the die, or the interior top surface of the can in a DirectFET® type package.  
         [0019]     To this end, the SiC or other die must be flipped with the epitaxial layer in the position of the cathode in the standard package. The top metal on the epitaxial surface is preferably solderable, for example, using the solderable top metal disclosed in application Ser. No. 11/255,021, filed Oct. 20, 2005 (IR-2769). The device back metal, now on the cathode side of the die may be any suitable bondable metal.  
         [0020]     When flipped die is used, special protection is needed to prevent the device termination region from contacting the lead frame. As will be shown, a suitable epoxy passivation mask, or the like can be used.  
         [0021]     Referring next to  FIG. 4 , there is shown a prior art SiC Schottky diode device  20  and at least a portion of the package for the device. The Schottky die is shown as die  21 , having a substrate  22  and a top epi layer  23 . The resistivity and thickness of the SiC is based on the blocking voltage required, for example, 600 volts. A barrier metal interface  24  is a top epi layer  23  and receives a suitable anode contact  25 , which may be A1 or any bondable metal. The active area of the device is terminated by a diffused termination guard ring  26  which is passivated by a suitable insolation layer  27 , which could be an oxide. A similar structure is present in the Si Schottky die.  
         [0022]     The cathode side of substrate  22  receives a cathode electrode  28  which can, for example, be a tri-layer of CrNiAg or any suitable solderable metal.  
         [0023]     The package for die  22  will include a heat sinking surface such as the metal lead frame  30  in  FIG. 4 . Any other metal layer of the package will serve as a good heat sink for die  22 , and in  FIG. 4 , the die  22  is soldered or secured by a conductive cement or epoxy to lead frame  30  so that a good thermal connection is obtained. Frequently, the heat sink  30  will also serve as a cathode contact for the package.  
         [0024]     The package is then completed in any desired manner to fully house the die  22 .  
         [0025]     As pointed out previously, this structure has produced unexpectedly poor surge capability.  
         [0026]     In accordance with the invention, and as shone in  FIG. 4 , the die  22  of  FIG. 4  is flipped so that the epi side  23  of the die makes contact with the best heat sink surface of the package.  
         [0027]     In  FIG. 5 , components identical to those of  FIG. 4  have the same identifying numeral. However, an epoxy passivation mass  40  is added around the edge of contact  25  and under termination passivation  27  to prevent the accidental contact of guard ring  26  to metal body  30 . A solder paste  41  is also employed to thermally and electrically connect anode contact  25  to heat sink  30 .  
         [0028]      FIG. 2  shows the forward voltage drop for the device of  FIG. 4  as a function of time for different current values of 0.5 m sec. current pulses at 25° C. The plural curves shown are for pulses of 15 amperes (the bottom-most line) to 40 amperes (the top most line), with intermediate pulse currents of 17, 20, 22, 25, 27, 30, 32 and 37 Amperes. Note the dramatic increase in forward voltage drop at the 37 and 40 ampere levels.  
         [0029]      FIG. 3  shows curves like those of  FIG. 2  for the die of  FIG. 5 , containing the novel invention. Note the substantially reduced forward voltage drop and thus the reduced heating of the die at the higher current pulse values.  
         [0030]     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein.