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I am using LTSpice IV. I would like to know the default parameter values of nmos and pmos . We can pick a mosfet model in which other parameters are present(eg: IRLML2346 etc) but the valu of W/L is not given. Can anybody suggest me a way to know this.

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If you read the help file, all is revealed.

First, read the section on the M circuit element. For the level 1 through 3 MOSFET models, the default L and W values are given by the parameters defl and defw, respectively.

These parameters are defined in a .OPTIONS card. The help file page for .OPTIONS tells you the defaults for defl and defw are both 100 μm.

So if you do nothing, your MOSFETS will be assumed to be 100 x 100 μm.

However, if high level parameters VTO, KP, LAMBDA, PHI, and GAMMA, (particularly KP if I recall correctly) are specified, the low-level physical structure parameters are ignored, so these default dimensions won't affect the simulation.

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EDIT: The Photon's answer is correct and I need to read documentation better.

AFAIK LTSPICE IV provides off the shelf MOSFET models with characterized behavior for an internal design by the manufacturer based on the part number of the MOSFET.

Specifying W/L ratios for MOSFETs is more in the realm of Cadence's Virtuoso Analog Design Environment, where certain technologies (e.g. 180nm vs 90nm) must be specified and whose models are created by foundries.

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