Bench Talk

It is true, that behind any commercial, high-tech, or major scientific application, there is an underlying chemical mechanism. This is also true for many areas of electronics, especially when most areas of chemistry are governed by the movement of electrons. Whilst the fundamental principles of chemistry are ubiquitous, perhaps one of the most important areas of electronics that utilizes chemical principles to the full, is semiconducting junctions.

A semiconducting junction, otherwise known as a p-n junction, is the interface between two different types of semiconducting regions—p-type and n-type. In its most basic sense, it is a junction that enables a current to be passed in one direction by utilizing the properties at the point where two chemically doped regions meet.

There is a lot of chemistry within these junctions, and this ranges from doping the materials that make up the p-type and n-type materials, to how the junction passes a current. To understand why these junctions work like they do, we must first look at the semiconducting materials involved in the junction.

The Effects of Chemical Doping

The semiconducting materials used in these junctions are both extrinsic semiconductors, which means that they are chemically doped; and this alters their electronic properties. P-type semiconductors are materials doped with an element that has a lower valence state than the original material, whereas an n-type material is one which has been doped with an element of a higher valence state.

For reference, we will consider silicon, as that is one of the most widely used materials in these junctions. Silicon has four valence electrons (as it is a group IV element), which means that it can form 4 covalent bonds with the other silicon atoms in the crystal lattice. If a silicon atom is replaced with a group (III) element, such as gallium, then the doped atom will only be able to form three bonds with the surrounding lattice instead of four. This leaves an absence of a chemical bond in the lattice (i.e. an atomic vacancy), which is known as a hole. In practice, these holes act as positively charge particles. The large number of holes also causes the valence band electrons to be excited into the conduction band, and this leaves the holes within the valence band. The Fermi level of these p-type semiconductors can be found at the bottom of the band gap, just above the valence band.

By comparison, when silicon is doped with a group (V) element, such as arsenide, this dopant has the ability to form five bonds within the lattice. However, because the lattice geometry is only designed to accommodate four bonds per atom (a dopant won’t alter the lattice to the extent where it rearranges), the arsenide atoms form the four bonds required to fit within the lattice, but an extra electron is left over, which then becomes delocalized. In an n-type semiconductor, the Fermi level of the extra electrons is at the top of the band gap, just below the conduction band, which means that the delocalized electrons can be easily excited into the conduction band.

It should be noted, that even though the semiconductor has localized changes to its electronic structure, both types of semiconductor material remain electronically neutral. This is because they still have the same number of protons as they do electrons (even though the overall numbers will change through doping).

How the Junction Works

Once both p-type and n-type regions have been fabricated, the interface between these differing semiconducting regions acts as the junction. Hence the name—p-n junction. It should be noted that a single crystal material is usually employed with one half of the material being p-doped and the other half being n-doped, rather than joining two materials together—as the fusion process will often create grain boundaries (a type of atomic defect) that can suppress the electrical current.

So, on one side of the junction is a series of negatively charged electrons, with a series of positively charged holes on the other. This charge separation on either side of the junction creates an electrical field and this becomes the built-in field of the p-n junction. This electrical field is created by the oppositely charged particles coming together and recombining (also known as annihilating), which then repels both the electrons on the n-side of the junction and the holes on the p-side. The interfacial region where the electrical field is active is known as the depletion region. This leaves the positively charged and negatively charged particles separated on their respective sides. To maintain the neutral charge around the junction, the total number of charges on each side of the junction must be the same. In the middle of the junction, the Fermi level is halfway between the valence and conduction bands.

In an equilibrium state, the flux of charge carriers is zero. The depletion region also acts a potential energy barrier that the charge carriers need to overcome before they can recombine, and the size of this potential barrier is determined by the thickness of the depletion layer. When a forward electrical bias is applied across the junction, extra energy is given to the free holes and electrons which enables them to move into the depletion zone. This reduces the width of the depletion zone to the point where the electrical field can no longer counteract the motion of the charge carriers. Once this happens, the electrons penetrate to the other side of the junction where they recombine with the holes. This causes the depletion zone to increase again, although it doesn’t fully return to the equilibrium state until the bias is removed. The total current remains constant because the holes also migrate to the n-side of the junction, and this means that the current can flow uninterrupted, even though the electrons don’t move that far within the p-side of the material.

By comparison, when a reverse biased is applied to the junction, no current will flow. Therefore, the current will only flow in one direction. The current will not flow under a reverse bias because the applied field is in the same direction as the built-in electric field of the depletion layer (although there are exceptions). The addition of an extra electrical field in the same direction causes the depletion layer to increase, which in turn causes the electrical resistance to increase.

Even though the chemistry of the “standard” semiconducting junction can be explained from a general perspective, the exact mechanism of producing an electrical current can vary depending on the electronic component and/or application it is employed in. Here, we discuss a couple of common examples that differ from the standard model.

One of the biggest applications of p-n junctions is in photovoltaic systems (solar cells). In a photovoltaic junction, the current will be induced through photons of light. The photon is absorbed into the depletion zone of the junction, and this causes some of the covalent bonds within the depletion zone (commonly silicon) to break and release electrons (and holes). Because the junction is connected to an electrical circuit, the electrons move to the p-side and generate a current, whereas the holes move to the n-side where they recombine with an electron and restore the electrical neutrality.

Diodes is another area where semiconducting junctions are used effectively. Diodes come in many forms, some of which use different mechanisms to generate a current. Most diodes will operate as above and only allow the current to be passed through in one direction—through the forward bias mode. However, there is one type, known as Zener diodes, that use a reverse bias. Once the positive terminal has a much lower potential than the negative terminal, a reverse breakdown occurs, and the electrons break free from the covalent bonds within the p-doped material and a reverse (avalanche) current flows (which is used for regulating the voltage in a circuit).

Overall, there are many reasons why chemistry is important for semiconductor junctions; and semiconductor materials (and junctions) would not be possible nowadays without advancements across the various disciplines of chemistry. Semiconductor junctions employ the principles of inorganic, physical, quantum and materials chemistry (and in some cases organic); and the combination of utilizing many areas of chemistry is responsible for the ability to change the electronic and chemical properties of a material by using different dopant elements, as well as the fundamental electron (and hole) migration mechanisms that occur between the doped regions, and the more specialist phenomena, such as the ability for photon particles to generate an electric current via a semiconducting junction.