characterization, modeling, and design of esd protection circuits

66 Chapter 3. Simulation: Methods and Applications
V terminal
Device
...
other contacts
Load
(a)
V external
terminal current
V terminal
V external
terminal voltage
Fig. 3.24 (a) Schematic of a general device with external load and
voltage; (b) adapting the load line (solid lines) along the I-V
curve allows for optimal convergence at each operating point.
load line perpendicular to this new point (Fig. 3.25b). Projection and recalibration are then
repeated until the trace is complete. The scheme must keep track of turning points in a
curve to ensure that the external voltage is always projected in the right direction. For
example, in a trace of a MOSFET snapback, the load resistance and external voltage steps
are positive before the trigger point, but when the curve’s slope becomes negative at the
onset of snapback, the perpendicular load resistance must also become negative and the
external voltage must be stepped negatively. Turning points are more fully discussed in
[28], as are issues concerning how to keep the curve trace smooth, how projection step
sizes are determined, and the necessity of a scaling scheme.
With this method, a simulator can automatically generate any arbitrarily shaped I-V curve
given only a user-specified starting point, ending point (maximum voltage or current), and
initial step size. The scheme has been implemented as a C program, “Tracer,” a virtual
instrument which functions as a wrapper around any device simulator which supplies
voltage, current, and tangent information, i.e., no modifications need to be made to the
simulation code. Tracer communicates with a device simulator by modifying the simulator
(b)