books.google.com/books?isbn=0849302471 If you can find you way to page 35 of this book, it shows the cross section of an epifet. These are handy when you need a device that draws current like a resistor when small voltages are applied, but eventually current limits. Not only does it save power, but it saves space too.
The epifet is usually early in the chain of circuits that determine power on reset, undervoltage lockout, etc.