Before revision: For example, when a bias is applied to two ends of a chunk of semiconductor, or a light is shining in one place (see right figure), electron will diffuse from high density regions (center) to low density regions (two ends), forming a gradient of electron density.
After revision: For example, when a bias is applied to two ends of a chunk of semiconductor, or a light is shining in one place (see right figure), electrons will diffuse from high density regions (center) to low density regions (two ends), forming a gradient of electron density.