characterization, modeling, and design of esd protection circuits

3.3. Mixed Mode Simulation 69
V c
C c
L s
Fig. 3.26 Mixed-mode circuit model for an NMOS transistor subjected to the
human-body model. The voltage source and all circuit elements are
defined with SPICE models, except for the transistor, which is defined by
the 2D device simulator.
a mixed-mode circuit (up to 10 in TMA-MEDICI), two or more MOSFETS could be
placed in parallel to simulate a multiple-finger ESD structure (Fig. 3.27). Slight layout
variations between the structures can be introduced to model random variations in processing
which result in nonuniform turn-on. The circuit can then be modified to ensure
turn-on of all fingers, perhaps by incorporating a ballast resistor on each drain.
V c
+
-
+
-
C c
L s
C s
R c
C t
2D device model
R gate
Fig. 3.27 Mixed-mode circuit model for a multiple-finger ESD NMOS structure
subjected to the human-body model. Ballast resistors are placed between
the output of the HBM circuit and each drain to facilitate uniform turn-on
of transistors M1 and M2.
C s
R c
C t
R ballast
M1
R ballast
M2
R gate
R gate