In the early days, the "Level 1" SPICE model used a simple square-law equation derived from long-channel physics. However, as transistors shrank, these equations failed to capture "short-channel effects."
[Level 1: Schichman-Hodges] -> [Level 3: Semi-Empirical] -> [BSIM3/BSIM4: Sub-micron] -> [BSIM-CMG: FinFET] Best for basic educational circuit analysis.
: To optimize designs in a virtual environment, reducing the need for costly manufacturing iterations.