Depltion mode MOSFET- Vth simulation

Hello everybody, I am running simulation for a depletion mode NMOSFET. I am using ISE-TCAD, I want to ramp the gate and drain voltage from -5V to +5V , but I am not able to do so as there is error. At the begging of the code in the electrode section I defined the gate and drain voltage level to be around -5V and in the solve section I have set the goal to 5V. The error message that I am getting is that it cannot converge at t=0. Can anyone help me with the code. this is the first time I am using this forum, if I done any mistake in the way of asking the question kindly forgive me. Below is the ISE code which I am using.
File {
Grid = "@grid@"
Doping = "@doping@"
Current = "@plot@"
Plot = "@dat@"
*Parameter = "VDMOS"
}

Electrode {
{ Name="Drain" Voltage=-5.0 }
{ Name="Source" Voltage=0.0 }
{ Name="Gate" Voltage=-5.0 }
}

Physics {

EffectiveIntrinsicDensity( OldSlotboom )

Mobility(
DopingDependence
HighFieldSaturation
Enormal
)

Recombination(SRH(DopingDep)
)
}

Physics (MaterialInterface= "Oxide/Silicon"){
Charge(SurfConc=@QF@)
}

Plot {
*--Density and Currents, etc
eDensity hDensity
TotalCurrent/Vector eCurrent/Vector hCurrent/Vector
eMobility hMobility
eVelocity hVelocity
eQuasiFermi hQuasiFermi

*--Temperature
* eTemperature
* hTemperature Temperature

*--Fields and charges
ElectricField/Vector Potential SpaceCharge

*--Doping Profiles
Doping DonorConcentration AcceptorConcentration

*--Generation/Recombination
SRH Auger
AvalancheGeneration eAvalancheGeneration hAvalancheGeneration

*--Driving forces
eGradQuasiFermi/Vector hGradQuasiFermi/Vector
eEparallel hEparalllel

*--Band structure/Composition
BandGap
BandGapNarrowing
Affinity
ConductionBand ValenceBand
}

Math {
Extrapolate
Digits=5
Notdamped=50
Iterations=20
RelerrControl
Derivatives
NewDiscretization
ErRef(hole)=1e10
ErRef(electron)=1e10
}

Solve {
Poisson

NewCurrentFile=""

Quasistationary(
InitialStep=1e-2 Increment=1.3
MinStep=1e-5 MaxStep=0.2
Goal { Name="Gate" Voltage=-5 }
Goal { Name="Drain" Voltage=-5 }
){
Coupled { Poisson Electron }
}
Quasistationary(
InitialStep=1e-2 Increment=1.3
MinStep=1e-5 MaxStep=0.2
Goal { Name="Gate" Voltage=5 }
Goal { Name="Drain" Voltage=5 }
){
Coupled { Poisson Electron }
}
}

I think my problem is basically in the initialstep, minstep, maxstep and increment. Can anyone suggest me how to modify this.
Thanks in advance