On Wed, 7 Oct 2020 16:53:26 +0100, Roger wrote:

Hi, I'm hoping that someone out there will have some info on an
ON Semiconductor 5105FA

Not exactly what you were asking:
https://www.onsemi.com/products/discretes-drivers/mosfets/ntr5105p
If you inscribe "5105" in the search box, you'll get a list of other
possible matches.

It would be very helpful if you would describe the package,
application, and provide a photo. I don't think you're reading the
part number or manufacturer correctly.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558