How to change gamma while NF simulation in SpectreRF

With decreasing L, γ in channel thermal noise formula increased to about 2~3. However, we don't know the γ value in SpecteRF simulation. If simulation tool uses BSIM3 model, the thermal noise model based on inversion channel charge is used, and no parameter like γ can be found.

So the problem is if I want to increase the γ manually, How to ?