P-N Junctions are types of semiconductors with a unique band structure and are used in variety of applications from solar cells to LEDs to transistors. This Demonstration shows the electronic band structure, above, of a P-N junction as well as the physical junction, below, for a generic semiconductor. The left side, in blue, contains p-type dopant, which provides excess holes, brown circles, as the charge carriers. The right side, in yellow, contains a n-type dopant, which provides excess electrons, green circles, as the charge carriers. The dotted red line represents the Fermi level
The performance of a P-N junction depends on the dopant concentration of the two regions. The key features of the band structure that determine performance are the depletion regions of the two sides, shown by the red lines, and the built-in voltage. At zero applied voltage, there are no excess charge carries in the depletion region, only the ion cores. This creates the built-in voltage, the magnitude of which is given by the green line. Changing the dopant concentration will change both the thickness of the depletion region and the magnitude of built in voltage. Applying a bias voltage lowers the built-in voltage, causing current to flow and decreasing the depletion width.
A P-N junction is formed when a p-type semiconductor and an n-type semiconductor are joined with a metallurgical junction. Because of the difference in concentration of dopants on each side, there is a driving force for diffusion across the interface; excess electrons from the n side and excess holes from the p side diffuse until the chemical potential (Fermi level) is equal across both sides. This creates the bent band structure and a depletion region within the junction, an area where all the free charge carriers are depleted. Without free carriers, the ions left behind generate an electric field and a built-in voltage, creating a barrier for diffusion. Applying a forward bias voltage lowers the built-in voltage, reducing the barrier and allowing current to flow.
Based on material from Electrical, Optical, and Magnetic Properties of Materials, MIT, spring 2012, taught by Professor Polina Anikeeva.