The Fermi Level is the energy level at which there is a 50% probability of finding an electron in a material. In a pn junction, there is a diffusion of electrons from the n-type region to the p-type region and vice versa. This diffusion creates an electric field which balances the potential difference between the two regions, resulting in a constant Fermi Level.
The constant Fermi Level creates a built-in potential barrier at the junction, preventing further diffusion of electrons. This is what allows a pn junction to act as a diode, allowing current to flow in only one direction.
The Fermi Level can change if an external voltage is applied to the pn junction. This changes the potential difference between the two regions and can result in the Fermi Level shifting towards one side or the other.
No, the Fermi Level is not constant in all types of junctions. It is only constant in equilibrium conditions, where there is no external voltage applied. In non-equilibrium conditions, such as when a voltage is applied, the Fermi Level can change.
The constant Fermi Level allows for precise control of the behavior of a pn junction, making it a crucial component in many semiconductor devices. It also plays a role in determining the energy levels of electrons in different materials, which affects their conductivity and other properties.