Optical Absorption: Understanding Semiconductor Behavior

This lecture discusses the principles of optical absorption in gases and semiconductors. It begins with an explanation of energy levels in gases, detailing how photons interact with these states to cause absorption. The instructor illustrates the concept using the hydrogen spectrum, highlighting the relationship between emission and absorption lines. The lecture then transitions to semiconductors, explaining the significance of the valence and conduction bands. It emphasizes that for absorption to occur, the energy of incoming photons must exceed the bandgap energy. The instructor provides examples of materials like silicon and gallium arsenide, demonstrating how their absorption characteristics change with photon energy. The lecture concludes with a discussion on measuring absorption coefficients, emphasizing the importance of sample thickness in these measurements. Overall, the lecture provides a comprehensive overview of how optical absorption operates in different materials, linking theoretical concepts to practical applications in semiconductor technology.