zhiling0229,
If all you need to do is turn on the power to your regulator, A PNP power transistor might be a better choice, Connect the emitter to your raw supply, the colllector to your load. Connect a resistor from the raw supply to the PNP base. Use an NPN transistor or N channel Fet to turn on the PNP transistor. The NPN base or FET gate would be driven by the microcontroller. Connect the NPN collector or FET drain to the PNP transistor base through a resistor. The value of the resistor would be chosen to provide enough base current to the PNP to ensure that it is saturated when turned on.
Regards,
Kral