The review of modern physics has given us a description of nature. Waves are described with a wave equation. Particles are described with particle equations. Experiments indicated that particle phenomena could be explained with a wave theory. Experiments indicated that wave phenomena could be described with particle theory. Schrodinger equation evolved whose solutions yielded a probability density of finding a state. Fermi function determined whether the state was filled.

Materials could now be described. Materials were understood in terms of an energy band diagram. Conduction band – valence band – Fermi energy. Electrons being promoted from the valence band into the conduction band left a vacancy (“hole”) in the valence band. “n type” semiconductor and “p type” semiconductor.

Basic model pn Homogeneous doping model

Homogeneous doping model p n +- Electrons diffuse into the p region and holes diffuse into the n region. This creates an electric field.

p n

Forces on electrons and holes p n diffusion force on electrons diffusion force on holes E field force on holes E field force on electrons

Basic model – thermal equilibrium p n +-

electrons and holes face a barrier p n

Electron density in the conduction band of the n type material p n +-

Hole density in the p type semiconductor material p n +-

Barrier potential can now be defined

Potential distribution in the depletion region p n +-

V = 0

Voltage is continuous

Reverse biased PN junction p n p n

Reverse biased PN junction energy diagram

Voltage-dependent capacitor

Simple three-dimensional unit cell