Hi,

I am designing a LDO with about 20mA output current.

Would you please introduce any books or tutorials on the layout for power transistor?

Thanks
Yawei

MOS or Bipolar?

The MOS devices are straightforward, just be very fussy about minimizing all your metal and contac resistance paths.

A bipolar one is a lot more picky, bipolar power transistors require ballast in the form of emitter resistance to make sure the currents all balance across the Vbe junctons of the parallel devices. Otherwise one junction runs hot, and all the current goes through there until that junction fries. (Warning - Not seen in simulation!)

On the LDO, be careful with the feedback stability, and the ESR of the compensation network.

Jerry