Question

Describe and diagram in detail what happens when a pn-junction is formed. Include the band structure in equalibrium using the 'capacitor' approximation and describe and indicate all currents in the junction.

Answer 1

The first type of junction we consider is between two regions in a semiconductor having different doping type. The earliest experiments on pn junctions are attributed to Ohl working at Bell labs in 1940 on the properties of silicon crystals.Following this discovery, the theoretical understanding of such junctions was
largely developed by Shockley.

Thermal Equilibrium and the Built-In Potential We use the fact that the Fermi level must be constant in thermal equilibrium to construct the band edge diagram for junctions between different materials. In the case of a pn junction, before the twomaterials are in contact we have the situation depicted in E0 is the vacuum level and represents the energy of free electrons (in other words, carriers that are no longer bound to the material). The difference between the Fermi level and E0 is called the work function, qΦ, of the material. We also define the difference between the conduction band edge, Ec, and the vacuum level as the electron affinity, qX, which is constant for a given semiconductor. If the two regions now come into contact and combine to form a junction, the
large carrier concentration gradients that exist at the interface will cause a transfer of carriers in order to align the Fermi levels and achieve thermal equilibrium.