MOS Junctions: Surface Potential and Band Bending

This lecture discusses the operation modes of a MOS junction on an n-type substrate, focusing on the surface potential and band bending. The instructor begins by describing the flatband condition, where no charge exists due to the absence of band curvature. As a negative voltage is applied to the gate, the bands bend downwards, attracting holes and leading to accumulation. The lecture then transitions to the effects of positive voltage, which pushes the energy levels down and creates a depletion region. The instructor explains the transition to weak inversion as the energy level approaches the conduction band. The discussion continues with the behavior of the junction under an n-type substrate, detailing how positive voltage attracts electrons, leading to accumulation, while negative voltage creates a depletion region. The concepts of flatband, accumulation, depletion, and weak inversion are thoroughly examined, providing a comprehensive understanding of the electrical characteristics of MOS structures.