Abstract:
A switching device is described having a semiconductor substrate with a front side and a back side. The switching device includes a first transistor which includes a first region adjacent the front side, a second region within the first region, the semiconductor substrate, and at least one island region adjacent the backside. The switching device also includes a second transistor which includes the first region, the second region, the semiconductor substrate, and a third region coupled to the at least one island region.

Description:
This application is a division of and claims the benefit of U.S. application Ser. No. 08/706,513, filed Sep. 4, 1996, U.S. Pat. No. 5,851,857 the disclosure of which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to power switching devices. More specifically, the present invention relates to a power switching device for high frequency applications which has a relatively low “on resistance.” 
     Metal-oxide semiconductor field effect transistors (MOSFETs) have become the standard power switching device because of their fast switching capabilities. Unfortunately, as the breakdown voltages of power MOSFETs increase, a correlative increase in device “on resistance,” R ON , is encountered. This undesirable increase is largely a result of the high resistivity of the semiconductor layer which makes the increase in breakdown voltage possible. Increased R ON , in turn, translates into conduction losses and increasingly inefficient operation. The relationship between R ON  and the device breakdown voltage, V B , is approximated by the equation: 
     
       
           R   ON   ≈aV   B   2.5   (1)  
       
     
     That is, for every doubling of V B , R ON  is increased by a factor of 5.66. Thus, despite their favorable switching characteristics, at some breakdown voltage, standard power MOSFETs become too inefficient for high power operation. 
     In contrast, insulated gate bipolar transistors (IGBTs) have a lower effective RON than MOSFETs as a result of a four layer structure which facilitates the injection of minority carriers into the high resistivity region. Unfortunately, the injection of these minority carriers results in slower devices which cannot match the switching capabilities of MOSFETs. This is due to the delay required to build up enough minority carriers in the high resistivity region before an IGBT is fully turned on. Similarly, the IGBT experiences a delay turning off because of the time required for the same minority carriers to be removed from this region. 
     In addition, because the four layer structure of an IGBT is similar to that of a thyristor, if the concentration of minority carriers in the high resistivity region exceeds a certain threshold, the IGBT ceases to behave like a transistor and goes into a latching mode. This behavior is described in detail in U.S. Pat. No. 4,199,774, issued on Apr. 22, 1980, the entire specification of which is incorporated herein by reference. Several techniques have been employed to reduce the susceptibility of IGBTs to latching. One of the most effective techniques involves irradiating the device with electrons after completion of standard semiconductor processing. Other techniques include unique device cell layout, source ballasting, and increasing the doping of the body region of the device. For more detailed descriptions of some of these techniques please see Comparison of 300-, 600-, and 1200-V n-Channel Insulated Gate Transistors, Chow et al., IEEE Transactions on Electron Device Letters, Vol. EDL-6, No. 4, April 1985, pp. 161-163, and The Insulated Gate Transistor: A New Three-Terminal MOS-Controlled Bipolar Power Device, Baliga et al., IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984, pp. 821-828, both of which are incorporated herein by reference in their entirety. Unfortunately, while these techniques have had varying measures of success in reducing latching susceptibility, the devices remain slower than MOSFETs operating at similar power levels. 
     A power switching device is therefore desirable which combines the switching speed of a power MOSFET with the low “on resistance” of an IGBT. 
     SUMMARY OF THE INVENTION 
     The present invention provides a power switching device which combines the switching speed of a power MOSFET with the low “on resistance” of an IGBT. Moreover, the switching device of the present invention is less susceptible to the above-described latching phenomenon than standard IGBTs. The operational characteristics of the switching device of the present invention are made possible by its unique structure which provides a power MOSFET and an IGBT in parallel in a single device. The devices share a common source/emitter region and a common gate. The drain of the MOSFET comprises a number of island regions adjacent the back side of the device, at which surface the island regions are surrounded by the collector region of the IGBT. 
     Moreover, by varying the size, shape, number, and alignment of the island drain regions, the operational characteristics of the device of the present invention can be made to be more like either the MOSFET or the IGBT depending upon the application. That is, if in a particular application a fast switching speed is more important than a low “on resistance,” the operational characteristics of the device can be adjusted toward the MOSFET end of the spectrum. This may be accomplished, for example, by increasing the size of the island drain regions. Alternatively, a similar effect may be achieved by aligning the island drain regions more closely with the gate of the device. 
     Thus, according to the invention, a switching device and a method for fabricating the same are provided. The switching device is fabricated in a semiconductor substrate with a front side and a back side. The switching device includes a first transistor which includes a first region adjacent the front side, a second region within the first region, the semiconductor substrate, and at least one island region adjacent the backside. The switching device also includes a second transistor which includes the first region, the second region, the semiconductor substrate, and a third region coupled to the at least one island region. 
     A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a switching device designed according to the present invention; 
     FIG. 2 is a simplified illustration of the backside of a switching device designed according to a specific embodiment of the invention; 
     FIG. 3 is a cross-sectional view of a switching device designed according to a specific embodiment of the invention; 
     FIG. 4 is a cross-sectional view of a switching device designed according to another specific embodiment of the invention; and 
     FIG. 5 is a cross-sectional view of a switching device designed according to yet another specific embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic representation of a switching device  100  designed according to the present invention. Switching device  100  is an integrated device which includes a power MOSFET  102  and an IGBT  104  connected in parallel. The devices share the same MOS gate  106  and the same region as the source of the MOSFET and the emitter of the IGBT, i.e., source/emitter node  108 . Device  100  also includes an intrinsic reversed diode  110  in parallel with both MOSFET  102  and IGBT  104 . Intrinsic diode  110  obviates the need for an external diode to be added across IGBT  104  as would typically be the practice. 
     FIG. 2 is a simplified illustration of the back side  200  of an N-channel switching device designed according to a specific embodiment of the invention. The substrate of the device is an N-type semiconductor crystal into which N+ island regions  202  are formed. At back side  200 , the N+ island regions are surrounded by a P+ doped region. In FIG. 2, island regions  202  are shown as circular. However, according to various embodiments of the invention, these regions may have any of a variety of closed shapes including, for example, ellipses, polygons, triangles, etc. The formation of N+ island regions  202  and the surrounding P+ area  204  may be accomplished sequentially by any of a variety of ion implantation or deposition techniques. 
     The designer may uniquely determine the operational characteristics of the switching device of the present invention for a particular application by controlling the size, shape, and number of island regions  202 . This is because the ratio of the island region area to the total back side area determines whether the device operates more like a MOSFET or an IGBT. This ratio R may be represented: 
     
       
           R=nA   i   /AT   (2)  
       
     
     where n is the number is island regions, A i  is the area of each individual island region, and A T  is the total area of the back side of the device. According to various embodiments of the invention, the designer may vary the ratio R between 0 and 1. For R=0, the device is a standard IGBT. For R=1, the device is a standard MOSFET (if no P+ back side doping is used). The alignment of island regions  202  also affects the operational characteristics of the device and will be discussed below with reference to FIGS. 3 and 4. 
     FIG. 3 is a cross-sectional view of a switching device  300  designed according to a specific embodiment of the invention. Device  300  is formed in an N− semiconductor crystal  302  with P wells  304  formed in the front side thereof. N+ regions  306  are formed within P wells  304 . In the back side of crystal  302  N+ island regions  308  are separated by a continuous P+ region  310 . A collector/drain electrode  312  on the back side of crystal  302  electrically connects island regions  308  with region  310 . Gate electrode  314  and emitter/source electrode  316  are formed on the front side of crystal  302  with oxide layer  318  separating gate electrode  314  from the surface of crystal  302 . Similar device features in subsequent drawings will employ the same reference numbers. 
     The device features of FIG. 3 combine to form the device of FIG. 1 in the following manner. N+ region  306 A, P well  304 A, N− crystal  302 , and N+ island regions  308  combine to form MOSFET  102 . N+ region  306 A, P well  304 A, N− crystal  302 , and P+ region  310  combine to form IGBT  104 . Intrinsic diode  110  corresponds to P well  304 A, N− crystal  302 , and N+ island regions  308 . Edge of die termination for each device can be a continuous P+ region in the back side of the device and the corresponding N+ scribe line configuration in a standard MOSFET or IGBT device. 
     Switching devices made according to the present invention such as, for example, device  300  of FIG. 3, have advantages over conventional MOSFETs and IGBTS. A conventional high voltage MOSFET has a high “on resistance” R ON . A typical 1000V MOSFET with an area of 64 mmsq has an R ON  of 1 ohm. A 1600V MOSFET with the same die area has an R ON  of 3.24 ohms at 25° C. However, as the device heats up, R ON  increases. At a junction temperature of 150° C., R ON  typically doubles from its rating at 25° C. resulting in an R ON  greater than 6 ohms; an unacceptably high value for many applications. 
     By contrast, and much like the back side P+ region of an IGBT, back side P+ region  310  of switching device  300  injects minority carriers, i.e., “holes,” into the N− region of crystal  302 , thereby reducing the resistance of the switch. The injection of minority carriers into the high resistivity region represented by the N− region is sufficient to reduce R ON  of device  300  without slowing it down overly much. Of course, as discussed above, the designer may manipulate the size, shape, number, and alignment of N+ island regions  308  to increase or decrease the effects of minority carrier injection to suit a particular application. An example of such manipulation is described with reference to FIG.  4 . 
     FIG. 4 is a cross-sectional view of a switching device  400  designed according to another specific embodiment of the invention. Switching device  400  is similar to device  300  except that P+ region  310  is aligned with P wells  304  rather than gate electrode  314 . This configuration results in greater minority carrier injection, and thus device  400  acts more like an IGBT than device  300 . Conversely, the configuration of device  300  in which P+ region  310  is aligned with gate electrode  314  results in lesser minority carrier injection, and thus device  300  acts more like a MOSFET. 
     In yet another alternate embodiment, the spacing of regions  308  and  310  differs from those of P wells  304  such that no particular alignment occurs. Where precise control over the operational characteristics of the device is desired, the coincident spacing of device  300  or  400  is preferable to a device in which no such alignment may be accomplished. However, it is easier (and therefore cheaper) to manufacture a device without such alignment requirements. Thus, where a wider range of operational characteristics is acceptable, it is not necessary to require alignment of these device features. 
     The intrinsic diode formed in each of the above devices (i.e., diode  110  of FIG. 1) can be made a fast switching diode by one or a combination of several techniques. For example, the device may be irradiated with a high energy, i.e., greater than 2 mega electron volts (MEV), electron beam. “Deep traps” may also be incorporated in the device crystal by implantation, deposition, or evaporation followed by diffusion of heavy metal atoms such as, for example, gold and platinum. The diffusion step may employ conventional techniques or a rapid thermal process (RTP). Additionally, high energy implantation of ions such as, for example, He+ ions, or “alpha particles.” Any irradiation damage caused by any of these techniques may require a controlled anneal step for correction of the damage. 
     Even with the use of the above-described techniques to speed up the intrinsic diode, there are some applications for which it is too slow. Therefore, it may be desirable to block it off from operation. This may be accomplished by the formation of a P− layer  502  as shown in FIG.  5 . Switching device  500  is substantially identical to switching device  300  of FIG. 3 except for the introduction of P− layer  502  which blocks the intrinsic diode by isolating N+ island regions  308  from the N− region of crystal  302 . The effectiveness of the blocking is dependent, at least in part, on the net width of layer  502 . In addition, the doping concentration of layer  502  must be carefully controlled to ensure that sufficient hole injection by P+ region  310  will still occur, and that there is effective “emitter shorting” in the back side of the device by N+ island regions  308 . One method of forming layer  502  which facilitates achieving these goals employs an ion implantation and diffusion process. Following implantation in the back side of the N− crystal, the diffusion step brings P− layer  502  relatively deep within crystal  302  as compared to the diffusion depths of the front side regions  304  and  306 . If the doping concentration is relatively high and the net width of layer  502  is greater than about 5 microns, device  500  will tend to behave somewhat like a typical IGBT with a low V SAT  (e.g., 2V) and a relatively slow turn-off time (500 ns). For relatively low doping concentrations and net widths less than 5 microns, device  500  will have a greater V SAT  (e.g., 2.3-6V) and a much faster turn-off time (e.g., 50 ns), behaving more like device  300  of FIG.  3 . 
     One method for forming the back side structure of FIG. 5 includes the following steps: 
     1. Backside implantation of a low dose of boron for creation of P− layer. 
     2. High temperature (T&gt;1100° C.) diffusion for longer than 12 hours to drive the boron to a depth of greater than 6 microns. 
     3. Masking and etching of back side to define openings for introduction of impurities for formation of N+ island regions. 
     4. Introduction of a high dose of N+ impurities (e.g., &gt;1E15) either by ion implantation or by standard diffusion pre-deposition process to form N+ island regions. 
     5. Etching of the remaining back side oxide. 
     6. Deposition of boron either by ion implantation or diffusion pre-deposition. The relative concentrations of the boron and the N+ doping in the island regions are such that the N+ regions are not converted to P regions. 
     7. Diffusion of boron at T&lt;1150° C. for less than 9 hours, the specific temperature and diffusion time of course being dependent upon the desired device characteristics. The shallower the diffusion, the higher the V SAT  and the faster the device. For example, for depths of less than 5 microns for both the N+ island regions and the P+ backside region, V SAT ≈3-4V and the device turn-off time is approximately 200 ns. It will be understood that the order of formation of the N+ island regions and the P+ back side region may be reversed. 
     While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit or scope of the invention. For example, the present invention has been described primarily with regard to an N-channel device. However, it will be understood that the invention may just as easily be implemented as a P-channel device with complementary doped regions and substrate. The scope of the invention should therefore be determined by reference to the appended claims.