The resistor model consists of process-related device data that allow the resistance to be calculated from geometric information and to be corrected for temperature. The parameters available are:

The sheet resistance is used with the narrowing parameter and l and w from the resistor device to determine the nominal resistance by the formula:

[\begin{array}{ll} {R_{nom} = {\lbrack font\ rm\ \lbrack char\ r\ mathalpha\rbrack\lbrack char\ s\ mathalpha\rbrack\lbrack char\ h\ mathalpha\rbrack\rbrack}\frac{l - {\lbrack font\ rm\ \lbrack char\ S\ mathalpha\rbrack\lbrack char\ H\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ R\ mathalpha\rbrack\lbrack char\ T\ mathalpha\rbrack\rbrack}}{w - {\lbrack font\ rm\ \lbrack char\ N\ mathalpha\rbrack\lbrack char\ A\ mathalpha\rbrack\lbrack char\ R\ mathalpha\rbrack\lbrack char\ R\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ W\ mathalpha\rbrack\rbrack}}} & \ \end{array}]

DEFW is used to supply a default value for w if one is not specified for the device. If either rsh or l is not specified, then the standard default resistance value of 1 mOhm is used. TNOM is used to override the circuit-wide value given on the .options control line where the parameters of this model have been measured at a different temperature. After the nominal resistance is calculated, it is adjusted for temperature by the formula:

[\begin{array}{ll} {R\left( T \right) = R\left( {\lbrack font\ rm\ \lbrack char\ T\ mathalpha\rbrack\lbrack char\ N\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ M\ mathalpha\rbrack\rbrack} \right)\left( 1 + TC_{1}\left( {T - {\lbrack font\ rm\ \lbrack char\ T\ mathalpha\rbrack\lbrack char\ N\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ M\ mathalpha\rbrack\rbrack}} \right) + TC_{2}\left( T - {\lbrack font\ rm\ \lbrack char\ T\ mathalpha\rbrack\lbrack char\ N\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ M\ mathalpha\rbrack\rbrack})^{2} \right) \right.} & \ \end{array}]

where (R\left( {\lbrack font\ rm\ \lbrack char\ T\ mathalpha\rbrack\lbrack char\ N\ mathalpha\rbrack\lbrack char\ O\ mathalpha\rbrack\lbrack char\ M\ mathalpha\rbrack\rbrack} \right) = R_{nom}|R_{acnom}). In the above formula, `(T)' represents the instance temperature, which can be explicitly set using the temp keyword or calculated using the circuit temperature and dtemp, if present. If both temp and dtemp are specified, the latter is ignored. Ngspice improves SPICE's resistors noise model, adding flicker noise ((\frac{1}{f})) to it and the noisy (or noise) keyword to simulate noiseless resistors. The thermal noise in resistors is modeled according to the equation:

[\begin{array}{ll} {\bar{i_{R}^{2}} = \frac{4kT}{R}\Delta f} & \ \end{array}]

where `(k)' is the Boltzmann's constant, and `(T)' the instance temperature.

Flicker noise model is:

[\begin{array}{ll} {\bar{i_{Rfn}^{2}} = \frac{{\lbrack font\ rm\ \lbrack char\ K\ mathalpha\rbrack\lbrack char\ F\ mathalpha\rbrack\rbrack}I_{R}^{\lbrack font\ rm\ \lbrack char\ A\ mathalpha\rbrack\lbrack char\ F\ mathalpha\rbrack\rbrack}}{W^{WF}L^{LF}f^{EF}}\Delta f} & \ \end{array}]

A small list of sheet resistances (in (\frac{\Omega}{\square})) for conductors is shown below. The table represents typical values for MOS processes in the 0.5 - 1 um

range. The table is taken from: N. Weste, K. Eshraghian - Principles of CMOS VLSI Design 2nd Edition, Addison Wesley.