Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


The pn Junction

As an alternative to the Schottky Barrier contact described in the section Metal–Semiconductor Junction - Rectifying Contact, a junction between an n-type semiconductor and a p-type semiconductor can be used as a rectifying contact. To see why, browse through the animation below. The various parts of the animation are discussed in detail later in this section, so do not be concerned if you do not understand every stage. You can return to this animation as you read more about the p-n junction.

It should be noted in the above animation that the relative quantity of electrons in the p-type material and the relative quantity of holes in the n-type semiconductor before they are joined together has been greatly exaggerated for the purposes of illustration. Both of these are minority carriers in their respective environments – remember that electrons are the majority carriers in n-type semiconductors and that holes are the majority carriers in p-type semiconductors.

When the two semiconductors are initially joined together, electrons will flow from the n-type semiconductor into the p-type semiconductor, and holes will flow from the p-type semiconductor into the n-type semiconductor. The chemical potentials of the two semiconductors will come to equilibrium, and the band structures will be deformed accordingly. A depletion layer is formed at the interface between the two types of doped semiconductor, in which numbers of electrons in the conduction band and holes in the valence band are both significantly reduced.

In equilibrium, there is a potential barrier for electrons to diffuse from the n-type semiconductor into the p-type semiconductor, and also for holes to move from the p-type semiconductor into the n-type semiconductor. These are the majority carriers. In addition, there will be currents from minority carriers, i.e., holes on the n-type side and electrons on the p-type side. For example, holes generated as a result of thermal excitation of electrons in the n-type semiconductor finding themselves in the depletion layer between the n- and p-type semiconductors will be swept over to the p-type side of the junction by the strong electric field within the depletion layer – since the electric field deters electrons from diffusing from the n-type side, it necessarily helps holes entering the depletion layer from the n-type side.

At equilibrium, the total current across the junction has be the same in both directions, so that the overall net current is zero. Any imbalance in current would mean that the system was not in equilibrium, and the bands would have to deform until the system returned to equilibrium.

If the n-type region is now connected to the positive terminal of a d.c. source and the p-type to the negative side, the bands will be further deformed at the interface, creating larger potential barriers for both electrons and holes to move across the junction and a wider depletion layer (i.e., a wider space charge region). In this situation of reverse bias, the only current is the very small contribution from the drift current arising from the minority carriers on both sides of the junction.

When the n-type region is connected to the negative terminal of a d.c. source and the p-type to the positive side, the depletion layer becomes narrower and the potential barriers are decreased in size. For this forward biasing, there will be a large net flow of electrons from the n-type semiconductor into the p-type semiconductor, and there will also be a net flow of holes moving into the n-type semiconductor from the p-type semiconductor.

In a p-n junction rectifier, an increase in the strength of the reverse biasing will eventually lead to an increase in the current that flows. This is because for a sufficiently high field electric field, dielectric breakdown of the semiconductor occurs. The bias at which this occurs is called the breakdown voltage. The overall current – voltage characteristic of the p-n junction is shown in the diagram below.

Current-Voltage characteristic of the p-n junction