$sourcebias

sourcebias is to determine the bias added to the source contact. It is different from the $gatebias and the $drainbias it is written with only one number. For example

$sourcebias
0.1 

This means that the bias of the source contact is at 0.1V.

The $sourcebias setting in GUI interface is here
Press the Contact Setting and set up the bias of the source contact!