In this work we have extended the analytical model on noise, by taking into account the effect of the depletion region that extends into the gate to drain spacing. The model utilizes the charge-control model based on analytical functions that relate 2-D electron gas concentration and the Fermi level. The effects of this high field extension region on the noise performance of the device have been investigated. Using the proposed model, the noise characteristics of two HEMTs are analytically calculated and compared with the measured data. It is observed that the contribution of the extended depletion region to the overall device noise is rather significant and it should not be ignored. The theoretical predictions based on the model are found to be in good agreement with the measured noise data.