Semiconductor p-n and Metal-Semiconductor Junctions

Until now, our discussion was based solely on homogeneous semiconductors whose properties are uniform in space. Although a few devices can be made from such semiconductors, the majority of devices and the most important ones utilize non-homogeneous semiconductor structures. Most of them involve semiconductor p-n junctions, in which a p-type doped region and an n-type doped region are brought into contact. Such a junction actually forms an electrical diode. This is why it is usual to talk about a p-n junction as a diode. Another important structure involves a semiconductor in intimate contact with a metal, leading to what is called a metalsemiconductor junction. Under certain circumstances, this configuration can also lead to an electrical diode.

The objective of this Chapter will first be to establish an accurate model for the p-n junction which can be at the same time mathematically described. This model will be the ideal p-n junction diode. The basic properties of this ideal p-n junction at equilibrium will be described in detail. The non-equilibrium properties of this p-n junction will then be discussed by deriving the diode equation which relates the current and voltage across the diode. Deviations from the ideal diode case will also be described. Finally, this Chapter will also discuss the properties of metalsemiconductor junctions and compare them with those of p-n junctions.