Knowledge What is the Forward Voltage of an LED?

[Shine]

Forward voltage (VF) is the voltage applied to the semiconductor diode in forward bias. An LED is a p-n junction device formed by oppositely doped semiconductor compound layers. When a sufficient voltage is applied across the p-n junction with a forward bias, the p-n junction can break down causing current to flow from the p-side to the n-side (anode to cathode). Electrons drops down from the conduction band of the n-side to fill the holes from the valence band of the p-side in the active region of the diode. Light is produced when these electrons and holes recombine with each other. LEDs are not designed to be driven with reverse voltage as they may be damaged.

In order for a forward current to flow across the p-n junction, the forward voltage must be high enough. When the forward voltage is below a certain value (called as a threshold value), the current becomes extremely low and little light is emitted. When the voltage exceeds the value, the forward current rises rapidly along with the voltage, and light is emitted. The forward voltage of LEDs varies depending on the semiconductor structure and the band gap energy of the materials forming the multiple quantum well (MQW)

• Infrared (1.6 V): Aluminum gallium arsenide (AIGaAs)
• Red (1.8 V - 2.1 V): Aluminum gallium arsenide (AIGaAs), Gallium arsenide phosphide (GaAsP), Gallium phosphide (GaP)
• Orange (2.2 V): Aluminum gallium indium phosphide (AIGalnP), Gallium arsenide phosphide (GaAsP)
• Yellow (2.4 V): Aluminum gallium indium phosphide (AlGalnP), Gallium arsenide phosphide (GaAsP), Gallium phosphide (GaP)
• Green (2.6 V): Aluminum gallium phosphide (AlGaP), Aluminum gallium indium phosphide (AlGalnP), Gallium nitride (GaN)
• Blue (3.0 V - 3.5 V): Gallium nitride (GaN), Indium gallium nitride (InGaN), Silicon carbide (SiC), Sapphire (AI2O3), Zinc selenide (ZnSe)
• White (3.0 V - 3.5 V): Gallium nitride (GaN [if AIGaN Quantum Barrier present]), Gallium nitride (GaN) based - Indium gallium nitride (InGaN) active layer
• Ultraviolet (3.5 V): Indium gallium nitride (InGaN), Aluminum nitride (AIN), Aluminum gallium nitride (AIGaN)

The forward voltage of an LED is also affected by junction temperature: as junction temperature rises, forward voltage drops. This relationship is a result of the reduction in band gap energy at higher junction temperatures.