If you know the device area then you can manually calculate in origin by exporting .csv file from tonyplot log file of I-V curve. The other way is that you can probe total current density at a specific point in dc sweep.  

probe name=current_density x=12 y=0.015 j.total

solve vfgate=-30 vgate=-30 vsource=0 vdrain=30 vstep=5 vfinal=30 name=gate

good luck.