Abstract:
A modified MOSFET structure comprises an integrated field effect rectifier connected between the source and drain of the MOSFET to shunt current during switching of the MOSFET. The integrated FER provides faster switching of the MOSFET due to the absence of injected carriers during switching while also decreasing the level of EMI relative to discrete solutions. The integrated structure of the MOSFET and FER can be fabricated using N-, multi-epitaxial and supertrench technologies, including 0.25 μm technology. Self-aligned processing can be used.

Description:
RELATED APPLICATIONS 
       [0001]    This application is related to, and claims the benefit of, U.S. patent application Ser. No. 12/238,308, filed Sep. 25, 2008, titled “Adjustable Field Effect Rectifier” and through it U.S. Provisional Patent Application Ser. No. 60/975,467, filed Sep. 26, 2007, as well as U.S. patent application Ser. No. 12/359,094, filed Jan. 23, 2009, entitled “Regenerative Building Block and Diode Bridge Rectifier,” and through it U.S. Provisional Patent Application Ser. No. 61/022,968, filed Jan. 23, 2008, and also provisional U.S. patent application Ser. No. 61/048,336, filed Apr. 28, 2008, entitled “MOSFET with Integrated Field Effect Rectifier,” all of which have the same inventors as the present application and are incorporated herein by reference in full for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to semiconductor transistors, and more specifically to an integration of a field effect rectifier into a MOSFET structure for improving the performance of the MOSFET, and methods therefor. 
       BACKGROUND OF THE INVENTION 
       [0003]    MOSFETs are commonly used for fast switching in electronic circuits. However, where the load is inductive the switching speed is limited due to the intrinsic body diode problem. When the gate voltage is used to switch MOSFET from ON to OFF state (reverse recovery), the intrinsic p-n junction diode has to conduct current, and will inject carriers into the bulk of device. Until the injected carriers are dissipated, the MOSFET will continue to stay in the ON state. This causes slow switching of MOSFET from ON to OFF state and limits the frequency of the MOSFET operation. 
         [0004]    To overcome the slow switching of the body diode an external freewheeling diode is often added between the source and the drain of the MOSFET to prevent the injection of carriers during reverse recovery. However, the addition of the external diode can lead to increased EMI, since the current that was flowing through the MOSFET now has to flow through the external diode and connecting wires. The extra EMI emissions can also limit the switching speed of the MOSFET with freewheeling diode. While the freewheeling diode is typically put as close to MOSFET as possible, the problem of extra EMI remains. 
         [0005]    While integrating the P-N junction diode technology into the MOSFET has an initial appeal, this technology has a limited potential to improve the body diode problem, since the body diode is a P-N junction diode integral to MOSFET structure. Similarly, Schottky diode technology is practically incompatible with MOSFET technologies, since it requires specific metallization not well suited for MOSFET manufacturing. 
       SUMMARY OF THE INVENTION 
       [0006]    To overcome the limitations of the prior art, the present invention integrates a Field Effect Rectifier (FER) into a MOSFET to improve the switching characteristics of the MOSFET when coupled to an inductive load, thus improving switching speed without significant adverse effects on EMI. 
         [0007]    In the new design, the FER does not replace the body diode, but provides a shunt or a bypass for the current flow around the body diode. The FER technology is compatible with MOSFET technology, permitting substantially conventional processing. Depending upon the implementation, the device of the present invention can be configured for either high voltage operation (e.g., a discrete high power device) or low voltage operation (e.g., in an integrated circuit) by adjustment of, for example, the size and resistivity of the epi, gate size, and so on. Further, unlike P-N junction diodes, the low voltage FER is a majority carrier device and prevents the body diode from injecting minority carriers that slow down the MOSFET operation. The high voltage FER will inject fewer carriers due to a particular one sided carrier injection mechanism (Rodov, Ankoudinov, Ghosh, Sol. St. Electronics 51 (2007) 714-718). 
     
    
     
       THE FIGURES 
         [0008]      FIGS. 1A-1B  show a MOSFET with integrated Field Effect Rectifier in accordance with the invention, where  FIG. 1A  shows a DMOS structure and  FIG. 1B  shows a UMOS structure. The current flow between source and drain is controlled by the gate electrode. The current will flow through the FER during switching, once the Gate voltage does not allow the current flow through the MOSFET. The Adjustment area (optional) provides a control of the leakage current. 
           [0009]      FIG. 2  shows in graphical form forward voltage drop vs. current for the body diode of a regular MOSFET (red) and a MOSFETR (green) in accordance with the invention. With VG=+5V the RDS,ON per MOSFET occupied area is about the same. Dark blue curve is for MOSFETR and the light blue is for MOSFET. 
           [0010]      FIG. 3  shows leakage current vs. reverse voltage for a conventional MOSFET (red), and MOSFETR with (green) and without (blue) the adjustment area in accordance with the invention. Scale (1A=2.5e-7) 
           [0011]      FIG. 4  illustrates transients for a 10A 20V MOSFETR according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The present invention comprises a new MOSFET structure that has integrated therein a field effect rectifier (hereinafter sometimes referred to as a “MOSFETR”). The field effect rectifier provides the alternative path for the current flow when the gate voltage switches OFF the current flow through MOSFET. The injection of the carriers from P-N junction can be reduced or completely eliminated, leading to faster MOSFET switching without significant EMI. 
         [0013]    Referring first to  FIGS. 1A , the DMOS MOSFETR structure indicated generally at  100  comprises a MOSFET  100 A on the left and an FER  100 B on the right. In some embodiments, the FER device can be adjustable as described in U.S. Provisional Patent Application Ser. No. 60/975,467, filed Sep. 26, 2007, although an FER without adjustable area also can be used in other embodiments. As shown in  FIG. 1 , the MOSFETR  100  has three electrodes: source  105 , gate  110  and drain  115 . The main current flows between the source and drain electrodes through the epitaxial N-layer  125 . The P-well  130  is provided to create a depletion layer when reverse bias is applied. The N++ regions  135  provide ohmic contact for the current flow. The adjustment area comprises the window  140  inside the FER gate and the P++ implantation  145 . The adjustment area permits control of the current flow, and thus can be desirable in some embodiments, depending upon the implementation. The gate oxide thickness and the doping levels control the height of the potential barrier under the FER gate  165 , and therefore the gate oxide under FER gate  150  can, in some embodiments, have a different thickness than the gate oxide under the MOSFET gate  155 . The gate voltage controls the conductivity of the narrow channel  160  under MOS gate  110  and switches the MOSFET between OFF and ON states. The transition from ON to OFF happens at the threshold voltage, which can be adjusted either by using a doping profile under the gate or by changing the thickness of the gate oxide  155 . The gate oxide thickness for the sides of MOSFET  155  and FER gate  150  can be varied independently of one another to ensure proper operation of both components. 
         [0014]    When the MOSFETR  100  is in the ON state (e.g., VGS=+5V), current flows through MOSFET channel  160 . A forward characteristic of an embodiment of MOSFETR in accordance with the invention is shown in  FIG. 2 , where the device is capable of operating at 10A at 20V. It can be appreciated by those skilled in the art that the MOSFETR has RDS,ON equal to 3.6 milliohm. If the right portion of the device is also a MOSFET, then simulated RDS,On is 2.0 milliohm. Notice that, for the characteristics shown in  FIG. 2 , the area of the MOSFET is on the order of 50% of the total area of the MOSFETR. Thus RDS,On of MOSFETR per unit area is about 10% smaller. This results because part of the epitaxial layer under the diode is used for conduction during MOSFET operation. It will be appreciated that this effect will, for some embodiments, increase for higher voltage devices. The increase of RDS,ON is typically smaller for high voltage devices, since the epitaxial layer becomes thicker to withstand higher reverse voltage. For some embodiments the area covered by MOSFET can be increased to reduce RDS,On, while the reduced FER area is still effective for the stored charge reduction. 
         [0015]      FIG. 1B  shows a UMOS MOSFETR structure which operates in a manner substantially identical with that shown in  FIG. 1A , and like elements are shown with like reference numbers, but with the most significant digit incremented by one. 
         [0016]    When the MOSFET is turned OFF (VGS=−5V), the current will flow through the body diode of MOSFET with VF=0.76V or through the FER of the MOSFETR with VF=0.58. In at least some embodiments, VF will preferably be kept below the knee voltage (˜0.7V) of the body diode, where P-N junction starts to inject carriers. Thus, a conventional MOSFET will inject carriers during switching while a MOSFETR according to the present invention substantially eliminates this undesirable behavior. 
         [0017]    In the OFF state, the small leakage current will flow through the AFER channel  165 . In at least some embodiments, this leakage is controlled by the potential barrier height and how fast the pinch-off effect takes place.  FIG. 3  demonstrates that the leakage of a MOSFETR is about 500 μA at 20V, which is similar to the leakage of a MOSFET. The adjustment area of the MOSFETR plays a role to keep leakage under control, and the leakage of a MOSFETR that does not have adjustment area is on the order of twice as much, or 1 μA. At higher voltages, the effect of the adjustment area can decrease. 
         [0018]      FIG. 3  illustrates the absence of injected carriers during switching with inductive load. For the example of  FIG. 3 , VGS is set to −5V, which turns OFF the current through MOSFET channel. The electron density distribution in MOSFETR for a 10A forward current is substantially the same as the electron density distribution with no current, and thus confirms that no carrier density modulation occurs. The electron concentration in the middle of the epitaxial N-layer can be seen to be on the order of 2.9e16. In contrast, operation of a conventional MOSFET shows significant injection at 10A current: the electron concentration becomes 5.1e16, or almost double. These injected carriers significantly slow down the operation of a conventional MOSFET with inductive loads. 
         [0019]      FIG. 4  shows the simulated transient behavior of MOSFET with and without the integrated diode. The transient time and stored charge are significantly smaller for MOSFETR. The low stored charge and small dl/dt of MOSFETR demonstrate that the device of the present invention is highly suitable for fast switching applications. 
         [0020]    In summary, the static characteristics of MOSFETR are very similar to those of the regular MOSFET, while exhibiting faster switching due to the absence of injected carriers during switching. The integrated FER diode structure is preferred over the discrete solution since it will decrease the level of EMI and allow the faster switching with low EMI. 
         [0021]    While the embodiment of the invention described is based on an N-epitaxial layer, it will be appreciated by those skilled in the art that equivalent structures can be formed using multi-epitaxial or supertrench methods, and it is specifically intended that the present invention encompass such alternatives. Likewise, it will be appreciated that the present structure is typically integrated into the larger circuits, and can be fabricated using standard methods including, for example, 0.25 μm technology with a mask alignment accuracy of approximately 20 nm, as well as others. 
         [0022]    The present invention has been described in detail, including numerous alternatives and equivalents. It is therefore to be understood that the invention is not to be limited by the embodiments specifically described herein, but only by the appended claims.