# Calculation time DC sweep vs. fixed voltage (DEVICE, electrical contact)

#1

Hi

When I run two exact same simulations with a 3D electric field monitor, it takes 7 times as long to calculate results when I use a fixed voltage at 2.5V (electrical contact boundary condition) compared to sweeping the voltage from 0 to 5V with 5 interval points (thus also including the 2.5V voltage point). I seem to get exactly the same results with both.

Anybody has a clue why this could be happening?

#2

The CHARGE solver always starts with an initial guess (the initialization stage) before it starts the iterations at each bias point. If this initial guess is a good one then the solver is able to reach a solution quickly. However, if the guess is a poor one then the solver will diverge or take a long time to reach the solution.

The solver always starts from the equilibrium case (zero bias everywhere). So when you are running a single simulation for a bias of 2.5 volt, the system is far from equilibrium and the initial guess becomes a poor guess. In the other case where you are sweeping from 0 to 5 V, the solver easily finds the solution for 0 V since it is in equilibrium. In the next step the solver uses the result from the previous bias point as the guess. Since 1 V is much closer to 0 V, the guess is a good one and the solver reaches the solution quickly. For 2V it uses the result of 1 V as the initial guess and so on. Since at all bias points the solver starts with a goo guess, the simulation in the second case is much faster even though you are sweeping through more points.

Hope this clarifies the situation.

pn_junction_Forward bias
#3

Thank you. That clarifies things.