Base/Collector Source Circuit (Excitation menu)

There are two identical Thevinin-equivalent source circuits; one associated with the base contact and one associated with the collector contact. Both circuits contain a voltage source and a series resistance. Different values of voltage and resistance can be specified for steady state versus transient conditions. Changing the voltage between its steady-state and initial transient value causes a step change in voltage at t=0. Setting the final transient value different from the initial transient value causes the voltage to sweep linearly from the initial to the final value during the course of the transient solution.

The source series resistance affects how much current flows in the device for a given source voltage, but the loss associated with these elements is not reflected in the device performance as revealed in plots of collector or base current versus voltage. The value of series resistance can be specified either in ohms, or in ohm-cm. In the latter case, the series resistance is adjusted for each solution based on the area of the device being simulated.

A constant-voltage condition at a contact is achieved by setting the corresponding source series resistance to zero. A constant-current condition is maintained by setting both the source voltage and series resistance to large values, so that their ratio gives the desired current. The open- circuit voltage at either the collector or base contact can be obtained by setting the source voltage to a small (can be zero) value, and the corresponding resistance to a large value (1 megohm is usually plenty).

When a simple linear voltage ramp is not adequate, time-dependent source-voltage values can be supplied from an external ASCII data file having a filename with suffix VLT. Each line in the file should contain three numerical values, separated by one or spaces or by a tab. The first value on each line should be the time (in seconds), the second value is the source voltage (in volts), and the third value should be the series resistance (in ohms).The time values must be monotonically increasing. The maximum number of lines read from the file is 200. Note that the time values in this file do not affect the time steps used in the solution, which are determined by the Mode command. Rather, voltage values for each time step are interpolated from this file.