The role of diffusion in long-channel field-effect transistors under condition of the velocity saturation

Abstract

An analytical modulation-doped field-effect transistor model including field-dependent mobility and diffusion transport of the electrons in the gradual channel approximation is proposed. The main aim of the model is to clarify the role of diffusion in the condition of the velocity saturation and its influence on the mechanism of the current saturation. Current-voltage characteristics are obtained in the parametric form. An explicit form of the I-V characteristic is deduced for a subthreshold region of the device operation. The influence of the velocity saturation and diffusion on the current flow in different parts of the channel and regions of the I-V characteristic is analyzed. It is shown that diffusion plays an important role in the entire range of the I-V characteristic and cannot be ignored in any realistic model. It follows from the model that in the final stage of current saturation, the latter is governed by the process of diffusion, and change of the current does not depend on the value of the saturated velocity.