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When simulating a MOSFET circuit, the user needs to specify both the values of the
model parameters and the dimensions of each MOSFET in the circuit being simulated. At
least, the channel length L and width W must be specified. The areas AD and AS and the
perimeters PD and PS need to be specified for SPICE to model the body-junction capacitances
(otherwise, zero capacitances would be assumed). The exact values of these geometry
parameters depend on the actual layout of the device (Appendix A). However, to estimate
these dimensions, we will assume that a metal contact is to be made to each of the source
and drain regions of the MOSFET. For this purpose, typically, these diffusion regions must
be extended past the end of the channel (i.e., in the L-direction in Fig. 4.1) by at least
2 . 7 5 ^ m i n - Thus, the minimum area and perimeter of a drain/source diffusion region with a
contact are, respectively,
AD = AS = 2.75LminV7 eq(4.168)
and
PD = PS = 2 x 2 . 7 5 Lmin + W eq.(4.169)
Unless otherwise specified, we will use Eqs. (4.168) and (4.169) to estimate the dimensions
of the drain/source regions in our examples.
Finally, we note that SPICE computes the values for the parameters of the MOSFET
small-signal model based on the dc operating point (bias point). These are then used by
SPICE to perform the small-signal analysis (ac analysis).
model parameters and the dimensions of each MOSFET in the circuit being simulated. At
least, the channel length L and width W must be specified. The areas AD and AS and the
perimeters PD and PS need to be specified for SPICE to model the body-junction capacitances
(otherwise, zero capacitances would be assumed). The exact values of these geometry
parameters depend on the actual layout of the device (Appendix A). However, to estimate
these dimensions, we will assume that a metal contact is to be made to each of the source
and drain regions of the MOSFET. For this purpose, typically, these diffusion regions must
be extended past the end of the channel (i.e., in the L-direction in Fig. 4.1) by at least
2 . 7 5 ^ m i n - Thus, the minimum area and perimeter of a drain/source diffusion region with a
contact are, respectively,
AD = AS = 2.75LminV7 eq(4.168)
and
PD = PS = 2 x 2 . 7 5 Lmin + W eq.(4.169)
Unless otherwise specified, we will use Eqs. (4.168) and (4.169) to estimate the dimensions
of the drain/source regions in our examples.
Finally, we note that SPICE computes the values for the parameters of the MOSFET
small-signal model based on the dc operating point (bias point). These are then used by
SPICE to perform the small-signal analysis (ac analysis).
