Driving a LED Matrix with a N-channel MOSFET (2N3819) using an AVR
The 2N3819 is an RF 'N' channel depletion mode JFET and is not really suitable for LED switching. Depletion mode means that the gate needs to be driven abut 7 volts more negative than the source to turn it off, and driven to the source voltage but no higher to turn it on. The maximum drain current is probably not high enough for your needs either.

Look for an N channel enhacement mode MOSFET instead.