$useconstIg

Since DDCC solver is solved by input bias, therefore it is hard to obtain the exactly desired current density. $useconstIg is a command for the program to get a certain Ig value by tunning the bias itself to reach the required gate current. However, it is very time consuming. This function is used for BJT simulation we Ig is treated as base current to get IC versus V_CE current at a fixed base current density.

$useconstIg
Ig_value V_trial_min V_trial_max Vg_step_min Precision

The Ig_value is the current density of gate ( or Base). The unit is A/cm since it is 2D simulation. The Ig is define as (Jpg-Jng). Be careful about the sign

The simulation program need to set Vg = constant and Vd (VC) in a range. Vs (VE) can be treated as emitter. 
The program will try in a Vg bias ranging from $Vg+V_{trial,min}<Vg< Vg+V_{trial,max}$ in a minimum Vg interval Vg_step_min.
Once the current error is within   Precision

For example 
$Schottky     
1.4
$gatebias     
0.9   0.9   0.01   
$drainbias     
0.5   5.0   0.5   
$sourcebias     
0.0
$useconstIg
1.0e-3 -0.2 0.2 1.0e-4 1.0e-3

The $useconstIg setting in GUI interface is here
After setting up the general structure, press Material Parameter and fill in these fields!