Definition
The small voltage that naturally exists across the junction of a semiconductor device, such as a diode or transistor, caused by the diffusion of charge carriers across the boundary between P-type and N-type material. This voltage must be overcome by an external applied voltage before the device will conduct in the forward direction. For a silicon junction the barrier voltage is approximately 0.7 volts, and for a germanium junction it is approximately 0.3 volts.
Plain English
A semiconductor like a diode has a tiny built-in voltage at the spot where its two halves meet. Until the voltage you apply from outside is bigger than this built-in voltage, no current flows through the device.
Context Anchor
Seen in aircraft electrical and electronics study, especially when learning how diodes, transistors, rectifiers, and solid-state circuits work.
Derivation
Barrier comes from the Old French barriere, meaning a fence or obstacle. The word fits because this voltage acts like a small electrical fence at the junction. Current cannot pass until the applied voltage is high enough to climb over it.
Why Pilots Care
Pilots maintaining or troubleshooting aircraft electronics need to know that semiconductor devices do not conduct the moment any voltage is applied. Understanding barrier voltage explains why a slightly weak power source may fail to drive a circuit, and why silicon and germanium components behave differently in the same role.
Analogy
Think of a light spring door. A tiny push will not open it, but once the push is strong enough, the door moves and lets you through. Barrier voltage is that minimum push for current inside the electronic part.
Intuition Check
Barrier voltage is not a separate battery or an added resistor. It is a built-in electrical threshold inside the semiconductor junction.
Example Sentence 1
Because the silicon diode has a barrier voltage of about 0.7 volts, the circuit will not conduct until the applied voltage rises above that level.
Example Sentence 2
A lower than expected barrier voltage reading showed the transistor junction had failed during the electrical system check.