Which device emits light when forward biased?

• A. Transistor
• B. LED
• C. Photodiode
• D. Zener diode

Answer: (B)

Light emission from a forward-biased junction comes from electroluminescence, which LEDs are built to produce. In an LED, the semiconductor material is chosen so that when current flows from the n-type side to the p-type side, electrons and holes recombine across the junction and release energy as photons. The material’s direct bandgap energy determines the color of the light, so the device efficiently radiates visible light instead of wasting energy as heat.

The other devices aren’t designed to emit light. A transistor mainly amplifies or switches signals and may glow only under unusual conditions. A photodiode is made to detect light and convert it into an electrical current. A Zener diode is used for voltage regulation in reverse bias and doesn’t purposefully emit light during normal operation.