hi, If you want to analyse the mismatch of Band gap, the mismatch parameters from the foundry are needed.
For example, the 3.3V PMOS Vtsat mismatch parameter is 4.9mV*um. If you want to know the Vt mismatch of a PMOS current mirror with size of W=10um, L=5um, the Vt mismatch equals to 4.90mV*um/sqrt(10um*5um)=0.693uV.
Normally the mosfet mismatch expresses as: sigma=slope/sqrt(W*L), the slope has a unit of mV*um.
the resistor mismatch expresses as: sigma=slope/sqrt(Weffe*Leffe), the slope has a unit of %.W: resistor width, Weff = W-ΔW, L: resistor length; Leff = L-ΔL.
With these two equations and the mismatch parameters, you can calculate the vt mismatch and resistor mismatch of you circuitry.