A semiconductor's bandgap is principally determined by the materials used as substrates and epitaxial layers. Since the colors (i.e., wavelengths) emitted from LEDs vary with bandgap, manufacturers have commercialized LEDs using various materials to create various LED colors, ranging from long to short wavelengths: e.g., red, yellow, green, and blue (Figure 4).
The luminous efficiency, which determines the intensity of an LED, depends on the structure of the pn junction in its epitaxial layers. Accordingly, manufacturers have performed R&D into the materials used as substrates and epitaxial layers along with their coupling structures and production methods. The results have led to a substantial increase in the intensity of LEDs over the early low-intensity red and green LEDs, and have recently enabled the use of LEDs in outdoor environments and the practical use of blue LEDs.