W6-03 Diffusion versus drift #SemiconductorPhysics #HCVerma

This lecture from the Physics Lectures channel, part of a series on semiconductor physics, explains the concepts of diffusion and drift currents in a pn junction. The instructor begins by reviewing the energy band diagram of a pn junction, including the Fermi level, density of states, and carrier distributions. He then discusses how concentration gradients lead to diffusion of majority carriers, but only those with sufficient energy can overcome the potential barrier. This diffusion creates a current from p to n side. However, in thermal equilibrium, there must be a balancing current due to minority carriers drifting under the built-in electric field, which flows from n to p side. The drift current arises from minority carriers generated thermally, which are swept by the electric field. In equilibrium, diffusion and drift currents cancel, resulting in zero net current. The lecture then introduces forward biasing, where an external voltage lowers the energy barrier, exponentially increasing the diffusion current while leaving the drift current largely unchanged. This leads to the derivation of the ideal diode equation: I = I0 (e^(eV/kT) - 1). The instructor notes assumptions and limitations, such as small applied voltage compared to built-in potential.