We show that the local thermal noise sources for the impedance field method (IFM) can be calculated accurately using the Monte Carlo method (MC). Using the results from MC method, we investigate the accuracy of the conventional thermal noise sources used to calculate the noise with the IFM. The results for a 0.1 μm N+-N-N+ device show that the conventional local thermal noise sources can be wrong for submicron devices. We also present the method of terminal noise calculation using an impedance field which is suitable for the Monte Carlo method. This method is compared with the conventional method which uses the autocorrelation function in Monte Carlo method. The results show that the method using the impedance field is very stable and fast numerically.