How Do Semiconductors Work?

Semiconductors are materials with properties that fall between a good conductor (like metals) and a good insulator (like rubber). Depending on the conditions, semiconductors can be conductive or insulating. This ability to control the flow of electrical current in modern electrical devices, such as microchips and photovoltaics.

How Does a Semiconductor Work?

For a semiconductor to manipulate electricity in a device, two requirements must be met:

There should be an imbalance of electrons across the semiconductor (areas of high and low electron density)
Under certain conditions e.g., applied heat or light, electrons should be able to move freely throughout the semiconductor.

There are different ways charge can be conducted through a semiconductor. The mechanism is dependent on the material itself. Typical semiconducting materials, such as silicon, are solid state lattices of one element that can be modified to incorporate different atoms. Charge flows through the crystalline lattice of that material with different atoms incorporating more of a particular type of charge carrier (electron or hole).

Other materials such as small organic molecule do not form lattices. Charge flows within the molecule and also hops between molecules. Whilst this might limit conductivity to some extent these materials offer more opportunities within an electronic device that traditional inorganic semiconductor materials can not.

Properties of Semiconductors

To understand how semiconductors work, you must understand the fundamental properties of solids.

Electrons that are tied to an atom in a solid are confined to energy bands. The valence band is occupied by electrons when the temperature of the solid is absolute zero. In this band, electrons are tightly bound to parent atoms through strong covalent or ionic bonds, making it difficult for them to move freely. The conduction band, on the other hand, contains electrons that gain sufficient energy to break free from the bonds to move more freely. The energy gap between the valence and the conduction band is known as the band gap. The band gap of a material determines many of the optical and electronic properties.

Conductors

Conductors have overlapping valence and conduction bands. The band gap is almost non-existent so electrons can easily break free of the atom and move throughout the material. These materials can conduct electricity and heat easily.

Insulators

Insulators have a very large band gap. A large amount of energy is needed for electrons to move from the valence band to the conduction band. This is unlikely to happen at room temperature, so these materials will not conduct heat or electricity.

Semiconductors

The band gap of semiconductors lies in-between that of conductors and insulators. It is large enough so that electrons do not ordinarily flow freely through the material. However, when excited by external factors like heat or light, some of the valence electrons gain enough energy to jump to the conduction band, creating ‘holes’ in the valence band. The creation of free electrons and ‘holes’ is very important for many applications, including photovoltaics, LEDs, and transistors.

For semiconducting molecules and polymers the energy levels are referred to as the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). They are comparable to the conduction band and the valence band. Semiconducting organic molecules are made up of atomic orbitals that come together to form molecular orbitals. This is where the electrons of the molecule exist or are likely to be found.

In the same way as the conduction and valence band, electrons that exist in the HOMO can be excited to the LUMO. The gap between the HOMO and LUMO is also referred to as the band gap. The advantage of organic molecules over tradition solid state semiconductors is that the HOMO and LUMO can be easily modified to change its electronic and therefore semiconducting properties. This makes them exciting materials to add to electronic devices.

Examples of Organic Semiconducting Materials

PCBM

From $338

Bathocuproine (BCP)

From $324

PNDI(2OD)2T

From $540

PTB7

From $540

ITIC

From $311

What is a p-n Junction?

A p-n junction is the most basic kind of semiconductor device. N-type and P-type semiconductors are joined to create a p-n junction diode. One side is more negative and the other is more positive, creating imbalance across the device. The mid-point where the semiconductors meet is the junction. Electrons can move across the junction to the positive side of the diode. This creates a neutrally charged area at the junction called the depletion zone.

The size of the depletion zone affects how easy or difficult it is for electrons to move through the semiconductor. When a voltage is applied, the depletion region will decrease enabling electrons to flow in a single direction. If the potential difference is reversed, the depletion zone becomes wider, making it more difficult for electrons to cross the junction. This is known as reverse bias. It acts as a one-way switch, which is useful in devices such as transistors.

Transistors

Transistors, essential in modern technology, consist of three semiconductors joined together. They are used in switching and amplification devices, allowing current to be turned off or amplified through the device. Transistors are used in various electronic devices, including telecommunications systems, computer memory chips, and multimedia storage devices.

Traditional Semiconductor: Silicon

Silicon is commonly used in semiconductor devices. However, naturally occurring silicon is electrically inert. Sitting between carbon and germanium on the periodic table, these elements have much in common. Each have four electrons in their outer orbital shells which can form covalent bonds with the outer shell electrons of surrounding atoms. As a result, they can form a crystalline structure.

Despite the metallic appearance of silicon crystals, they are not metals. Like their carbonic counterparts, they are very stable. In fact, the lack of free moving electrons within this material renders it more like an insulator than a metal.

Other materials can be added to increase the electrical potentiality of silicon. In a process called doping, impurities are introduced to the material to alter its physicochemical properties. These impurities are often atoms with either 3 or 5 electrons in their valence band. The imbalance created allows for the free movement of electrons within the material. There are two ways of doping silicon: N-type doping, and P-type doping.

N-Type Doping

Impurities with five electrons (pentavalent) in their outer shell are added to the silicon crystal structure. This results in an unbound ‘free’ electron.

N-type stands for ‘negative’ due to the extra electrons.

P-Type Doping

Impurities with three electrons (trivalent) in their outer shell are added to the silicon crystal structure. Three of the electrons can bond with silicon outer shell electrons leaving a ‘hole’. These spare holes enable electrical conductivity in the materials as they are able to accept free moving electrons once a current is flowing.

P-type stands for ‘positive’ to remind us that this material has a lesser number of electrons.