Diffusion Currents in Semiconductors: Principles and Equations

This lecture discusses the principles of diffusion currents in semiconductors, focusing on the influence of electric fields and doping variations. It begins by reviewing drift currents and Maxwell's equations, then introduces diffusion currents, explaining how they arise from variations in doping concentrations. The instructor presents the diffusion equation, illustrating how the current is proportional to the gradient of the dopant concentrations. The relationship between diffusion constants and thermal speed is established, along with the Einstein relations linking mobility and diffusion. The lecture summarizes the balance of drift and diffusion currents, emphasizing their dependence on electric fields and carrier concentrations. The instructor also explores the equilibrium conditions in semiconductor structures, using an example of an N/N junction to illustrate how diffusion and drift currents interact to achieve balance. The lecture concludes with a discussion of the governing equations for these currents, highlighting their significance in understanding semiconductor behavior under various conditions.