BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a power transistor device with electrostatic discharge protection and a low dropout regulator using the power transistor device, wherein an electrostatic discharge path is provided to protect the power transistor device. 
         [0003]    2. Description of Related Art 
         [0004]      FIG. 1  shows a schematic diagram of a prior art low dropout (LDO) regulator. An LDO regulator  100  is a linear regulator for converting an input voltage Vin to an output voltage Vout; its basic structure is as shown in this figure, which includes an error amplifier  10  and a power transistor device  20 . The error amplifier  10  receives an output voltage sampled signal which is obtained from the voltage of a resistor R 2 ; resistors R 1  and R 2  are connected in series between the output voltage Vout and ground. The error amplifier  10  compares the output voltage sampled signal with a reference signal Vref to generate an error amplification signal, and outputs the error amplification signal to the gate of a PMOSFET (P-type metal oxide semiconductor field effect transistor) in the power transistor device  20  to control the conduction between the source and the drain of the PMOSFET, that is, to control the conversion from the input voltage Vin to the output voltage Vout. A cross-section view of the PMOSFET of the power transistor device  20  is shown in  FIG. 2 . As shown by the cross-section view, the PMOSFET is formed on a P-type substrate (P-sub)  21 . A high voltage N-type well (NW)  23  and a high voltage P-type well (PW)  24  which are adjacent to each other are formed below the top surface of the P-sub  21 . Shallow trench isolation (STI) regions  25 , an N+ body  26 , a P+ source  27 , and a P+ drain  29  are formed in the wells  23  and  24 ; and a gate  28  is formed on the top surface. 
         [0005]    Referring to  FIGS. 1 and 2 , the power transistor device  20  has a contact pad  1  which is an output terminal of the power transistor device  20 ; the contact pad  1  may be touched by a human body or may contact various electrical fields in various applications and test environments. Therefore, electrical charges may be accumulated on the contact pad  1  to generate an electrostatic voltage, or the contact pad  1  may contact a high electrostatic voltage directly. When the electrostatic voltage is so high to an extent that the power transistor device  20  can not withstand, the electrostatic voltage will discharge through a discharge path, such as a path indicated by the dash line in  FIG. 2 . This will result in mis-operation of the circuit or serious damages to the device. 
         [0006]    In view of the foregoing, the present invention provides a power transistor device with electrostatic discharge protection and a low dropout regulator using the power transistor device, to overcome drawbacks in the prior art. 
       SUMMARY OF THE INVENTION 
       [0007]    The first objective of the present invention is to provide a power transistor device with electrostatic discharge protection. 
         [0008]    The second objective of the present invention is to provide an LDO regulator employing the power transistor device with electrostatic discharge protection. 
         [0009]    To achieve the objective mentioned above, from one perspective, the present invention provides a power transistor device with electrostatic discharge protection comprising: a PMOSFET having a source and a drain electrically connected to a voltage input terminal and a voltage output terminal, respectively; and an electrostatic discharge protection device electrically connected to the voltage input terminal and the voltage output terminal, for providing an electrostatic discharge path such that the electrostatic voltage of the voltage output terminal discharges through the electrostatic discharge path, to protect the PMOSFET; wherein the voltage output terminal includes a contact pad for providing an electrical connection to a load circuit. 
         [0010]    In a preferred embodiment of the power transistor device, the electrostatic discharge protection device further includes a deep N-well (NW) or an N-type buried layer (NBL). 
         [0011]    In one embodiment, the electrostatic discharge protection device includes an NPN bipolar junction transistor (BJT) having an emitter and a collector electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a base controlled by the voltage output terminal. 
         [0012]    In another embodiment, the electrostatic discharge protection device includes an NMOSFET (N-type metal oxide semiconductor field effect transistor) having a drain and a source electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a gate which is connected to ground or which is controlled by the voltage output terminal. 
         [0013]    In yet another embodiment, the electrostatic discharge protection device includes a silicon controlled rectifier (SCR) having a cathode and an anode electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a gate controlled by the voltage output terminal. 
         [0014]    From another perspective, the present invention provides a LDO regulator with electrostatic discharge protection for converting an input voltage of a voltage input terminal to an output voltage of a voltage output terminal, the LDO regulator with electrostatic discharge protection comprising: an error amplifier circuit generating an error amplified signal according to an output voltage sampled signal and a reference signal, wherein the output voltage sampled signal is sampled from the output voltage; and a power transistor device including: a PMOSFET having a source and a drain electrically connected to the voltage input terminal and the voltage output terminal, and having a gate controlled by the error amplified signal; and an electrostatic discharge protection device electrically connected to the voltage input terminal and the voltage output terminal, for providing an electrostatic discharge path such that the electrostatic voltage of the voltage output terminal discharges through the electrostatic discharge path, to protect the PMOSFET; wherein the voltage output terminal includes a contact pad for providing an electrical connection to a load circuit. 
         [0015]    In the aforementioned LDO regulator, the electrostatic discharge protection device further includes a deep N-well (NW) or an N-type buried layer (NBL). 
         [0016]    In one embodiment of the aforementioned LDO regulator, the electrostatic discharge protection device includes an NPN bipolar junction transistor (BJT) having an emitter and a collector electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a base controlled by the voltage output terminal. 
         [0017]    In another embodiment of the aforementioned LDO regulator, the electrostatic discharge protection device includes an N-type metal oxide semiconductor (NMOS) FET having a drain and a source electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a gate which is connected to ground or which is controlled by the voltage output terminal. 
         [0018]    In yet another embodiment of the aforementioned LDO regulator, the electrostatic discharge protection device includes a silicon controlled rectifier (SCR) having a cathode and an anode electrically connected to the voltage output terminal and the voltage input terminal, respectively, and having a gate controlled by the voltage output terminal. 
         [0019]    The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows a schematic diagram of a prior art LDO regulator. 
           [0021]      FIG. 2  shows a cross-section view of the PMOSFET of the power transistor device  20  in  FIG. 1 . 
           [0022]      FIGS. 3 and 4  show a first embodiment of the present invention. 
           [0023]      FIGS. 5 and 6  show a second embodiment of the present invention. 
           [0024]      FIGS. 7 and 8  show a third embodiment of the present invention. 
           [0025]      FIGS. 9 and 10  show a fourth embodiment of the present invention. 
           [0026]      FIG. 11  shows another embodiment of the power transistor device  30 . 
           [0027]      FIG. 12  shows another embodiment of the power transistor device  40 . 
           [0028]      FIG. 13  shows another embodiment of the power transistor device  50 . 
           [0029]      FIG. 14  shows another embodiment of the power transistor device  60 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The spirit of the present invention is to provide an N-type channel device which forms a discharge path, to solve the problems caused by the electrostatic charges in a P-type power transistor. 
         [0031]      FIGS. 3 and 4  show a first embodiment of the present invention. As shown in  FIG. 3 , an LDO regulator  200  includes an error amplifier circuit  10  and a power transistor device  30 . The present invention is different from the prior art in that, besides a PMOSFET, the power transistor device  30  includes an electrostatic discharge protection device  2 . The source and the drain of the PMOSFET are electrically connected to the voltage input terminal Vin and the voltage output terminal Vout, respectively. In this embodiment, the electrostatic discharge protection device  2  includes an NPN bipolar junction transistor (BJT) which has an emitter  31  and a collector  32  electrically connected to the voltage output terminal Vout and the voltage output terminal Vin, respectively, and has a base controlled by the voltage output terminal Vout. When the contact pad  1  of the voltage output terminal Vout contacts an electrostatic voltage, the electrostatic voltage can be discharged through a path from the collector  32  to the emitter  31  of the electrostatic discharge protection device NPNBJT, to protect the PMOSFET. 
         [0032]      FIG. 4  shows a cross-section view of the power transistor device  30  of the first embodiment. Among the differences between this embodiment and the prior art, one difference is that this embodiment further includes a deep N-type well (deep NW) or an N-type buried layer (NBL)  22 . The deep NW or NBL  22  is formed between a P-type substrate (P-sub)  21  and a high voltage N-type well (NW)  23  and a high voltage P-type well (PW)  24 , for isolating the power transistor device  30  from the P-type substrate  21 . The N+ type emitter  31 , the N+ type collector  32 , and the P+ type base are respectively shown in the figure.  FIG. 4  also shows symbols of the NPNBJT and a resistor to indicate the relationship among the areas in the cross-section view from circuit perspective. The dash arrow line indicates the discharge path when the contact pad contacts an electrostatic voltage. Because the NPNBJT provides an additional electrostatic discharge path, the electrostatic voltage will not impact the function of the PMOSFET and will not damage the structure of the PMOSFET. 
         [0033]      FIGS. 5 and 6  show a second embodiment of the present invention. As shown in  FIG. 5 , an LDO regulator  300  includes an error amplifier circuit  10  and a power transistor device  40 . The power transistor device  40  includes a PMOSFET and an electrostatic discharge protection device  2 . In this embodiment, the electrostatic discharge protection device  2  includes an NMOSFET which has a drain  42  and a source  43  electrically connected to the voltage output terminal Vout and the voltage input terminal Vin, respectively, and has a gate  44  controlled by the voltage output terminal Vout. When the contact pad  1  of the voltage output terminal Vout contacts an electrostatic voltage, the electrostatic voltage can be discharged through a path from the drain  42  to the source  43  provided by a parasitic NPNBJT of the electrostatic discharge protection device NMOSFET, to protect the PMOSFET. 
         [0034]      FIG. 6  shows a cross-section view of the power transistor device  40  of the second embodiment. The deep NW or NBL  22  is formed between the P-type substrate (P-sub)  21  and the high voltage N-type well (NW)  23  and the high voltage P-type well (PW)  24  for isolating the power transistor device  40  from the P-type substrate  21 . The N+ type drain  42 , the N+ type source  43 , and the gate  44  are respectively shown in the figure.  FIG. 6  also shows a symbol of the parasitic NPNBJT of the NMOSFET to indicate the relationship among the areas in the cross-section view from circuit perspective. The dash arrow line indicates the discharge path when the contact pad  1  contacts an electrostatic voltage. Because the parasitic NPNBJT of the NMOSFET provides an additional electrostatic discharge path, the electrostatic voltage will not impact the function of the PMOSFET and will not damage the structure of the PMOSFET. 
         [0035]      FIGS. 7 and 8  show a third embodiment of the present invention. As shown in  FIG. 7 , an LDO regulator  400  includes an error amplifier circuit  10  and a power transistor device  50 . The power transistor device  50  includes a PMOSFET and an electrostatic discharge protection device  2 . In this embodiment, the electrostatic discharge protection device  2  includes an NMOSFET which has a drain  52  and a source  53  electrically connected to the voltage output terminal Vout and the voltage input terminal Vin, respectively, and has a gate  54  electrically connected to ground. When the contact pad  1  of the voltage output terminal Vout contacts an electrostatic voltage, the electrostatic voltage can be discharged through a path from the drain  52  to the source  53  provided by a parasitic NPNBJT of the electrostatic discharge protection device NMOSFET, to protect the PMOSFET. 
         [0036]      FIG. 8  shows a cross-section view of the power transistor device  50  of the third embodiment. The deep NW or NBL  22  is formed between the P-type substrate (P-sub)  21  and the high voltage N-type well (NW)  23  and the high voltage P-type well (PW)  24  for isolating the power transistor device  50  from the P-type substrate  21 . The N+ type drain  52 , the N+ type source  53 , and the gate  54  are respectively shown in the figure.  FIG. 8  also shows a symbol of the parasitic NPNBJT of the NMOSFET to indicate the relationship among the areas in the cross-section view from circuit perspective. The dash arrow line indicates the discharge path when the contact pad  1  contacts an electrostatic voltage. Because the parasitic NPNBJT of the NMOSFET provides an additional electrostatic discharge path, the electrostatic voltage will not impact the function of the PMOSFET and will not damage the structure of the PMOSFET. 
         [0037]      FIGS. 9 and 10  show a fourth embodiment of the present invention. As shown in  FIG. 9 , an LDO regulator  500  includes an error amplifier circuit  10  and a power transistor device  60 . The power transistor device  60  includes a PMOSFET and an electrostatic discharge protection device  2 . In this embodiment, the electrostatic discharge protection device  2  includes a silicon controlled rectifier (SCR) which has a cathode  62  and an anode  63  electrically connected to the voltage output terminal Vout and the voltage input terminal Vin, respectively, and has a gate electrically connected to the voltage output terminal Vout through a resistor connected in series. When the contact pad  1  of the voltage output terminal Vout contacts an electrostatic voltage, the electrostatic voltage can be discharged through a path from the cathode  62  to the anode  63  provided by the electrostatic discharge protection device SCR, to protect the PMOSFET. 
         [0038]      FIG. 10  shows a cross-section view of the power transistor device  60  of the fourth embodiment. The deep NW or NBL  22  is formed between the P-type substrate (P-sub)  21  and the high voltage N-type well (NW)  23  and the high voltage P-type well (PW)  24  for isolating the power transistor device  60  and the P-type substrate  21 . The cathode  62 , the anode  63 , and the gate of the SCR are respectively shown in the figure.  FIG. 10  also shows symbols of the BJTs which form the SCR to indicate the relationship among the areas in the cross-section view from circuit perspective. The dash arrow line indicates the discharge path when the contact pad  1  contacts an electrostatic voltage. Because the cathode  62  to the anode  63  of the SCR provides an additional electrostatic discharge path, the electrostatic voltage will not impact the function of the PMOSFET and will not damage the structure of the PMOSFET. 
         [0039]      FIG. 11  shows another embodiment of the power transistor device  30 . This embodiment is different from  FIG. 4  in that, an STI  25 , and a high voltage N-type well  23  and a high voltage P-type well  24  are provided to isolate the PMOSFET and the NPNBJT from each other. 
         [0040]      FIG. 12  shows another embodiment of the power transistor device  40 . This embodiment is different from  FIG. 6  in that, an STI  25 , and a high voltage N-type well  23  and a high voltage P-type well  24  are provided to isolate the PMOSFET and the NMOSFET from each other. 
         [0041]    As shown in  FIG. 13 , the power transistor device  50  shown in  FIG. 8  may also be modified to a semiconductor structure similar to the one shown in  FIG. 12 . 
         [0042]      FIG. 14  shows another embodiment of the power transistor device  60 . This embodiment is different from  FIG. 10  in that, an STI  25 , and a high voltage N-type well  23  and a high voltage P-type well  24  are provided to isolate the PMOSFET and the SCR from each other. 
         [0043]    The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, a device which does not substantially influence the primary function of a signal can be inserted between any two devices in the shown embodiments, such as a switch or the like. As another example, the shallow trench isolation region can be replaced by a LOCOS (local oxidation of silicon) region, etc. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.
The present invention discloses a power transistor device and a low dropout regulator (LDO) with electrostatic discharge protection. The power transistor device includes: a P-type metal oxide semiconductor (PMOS) field effect transistor (FET), having a source and a drain electrically connected to a voltage input terminal and a voltage output terminal respectively; and an electrostatic discharge protection device, electrically connected to the voltage input terminal and the voltage output terminal, for providing an electrostatic discharge path to protect the PMOSFET.