As with any productive industry, the electronics sector is constantly evolving and advancing. Currently, one of the biggest trends is miniaturization as demand for smaller and lighter components grows, especially in the PCB market.
Semiconductor technology has also shrunk in size, leading to miniaturization and circuit integration. Now, with the exception of inductors and some passive components, it is possible to manufacture most electronic devices on a single silicon chip.
Silicon is the most common semiconductor material used in integrated circuit (IC) manufacturing, although it is not the only one. Let's investigate the different semiconductor materials used in IC manufacturing and investigate why silicon still dominates the semiconductor industry.
Materials
Solid-state devices are built using a type of semiconductor material through which electricity can flow. Semiconductors have an electrical conductivity (10-7 to 10-13 mho/m) that is between a good conductor (> 10-7 mho/m) and an insulator (< 10-13 mho/m).
Semiconductor materials are either a single crystal or a compound.
Examples of crystal semiconductors:
- Silicon (Si)
- Germanium (Ge)
Examples of compound semiconductors:
- Gallium arsenide (GaAs)
- Gallium nitride (GaN)
- Gallium Arsenide Phosphide (GaAsP)
- Cadmium sulfide (CdS)
- Lead sulfide (PbS)
- Silicon carbide (SiC)
Originally, solid-state devices were manufactured using germanium, but its strong temperature sensitivity was a disadvantage. As manufacturing techniques improved, the use of silicon gradually became popular due to its thermal stability and availability.
As the electronics focus has advanced from switching and control to computing and communication, the demand for high-speed devices has increased. As a result, gallium arsenide was found to be ideal as it offers a transistor rate five times faster than silicon.
Silicon remains the most widely used semiconductor material, however, germanium arsenide is used exclusively for large-scale, high-speed integration (VLSI) projects. Germanium is also still used for certain applications.
This trio – silicon, germanium and gallium arsenide – are the most commonly used semiconductor materials. Still others can be used, but only for specific assemblies or purposes.
IC manufacturing
There are reasons why semiconductor materials are used in manufacturing electronic ICs. On the one hand, such materials (including silicon, germanium and gallium) offer a crystalline structure due to the covalent bond between their atoms.
These covalent bonds are formed through shared valence electrons (which are the outermost shell electrons) with adjacent atoms. Due to the abundance of ambient heat and light, many of these electrons gain enough kinetic energy to move freely throughout the material. In fact, it is often said that electrons move from the valence energy band to the conduction energy band.
Because the electrons within the material are technically “free,” they can be pointed in a certain direction by applying an electric field or external voltage.
To ensure optimal electrical and physical properties, semiconductor materials are typically refined to their purest form – and as such are labeled as intrinsic materials. Technology has made it possible to obtain just one impurity atom in 10 billion within an intrinsic, refined crystal.
What is interesting about these intrinsic semiconductor materials is that their conductivity and electrical properties can be easily changed and controlled by adding impurity atoms to their lattice structure. For example, adding just one impurity atom per 10 million atoms can drastically change its conductivity.
This process is called doping and subsequently a semiconductor material is called extrinsic material. There are two types of extrinsic materials: n-type and p-type. Silicon and germanium are tetravalent, meaning they have four valences. For gallium arsenide, gallium is trivalent and arsenide is pentavalent (a valence of five).
Regardless of the number of valence electrons, it is important to note that silicon, germanium and gallium arsenide have a crystalline structure due to covalent bonding.
Now, when a pentavalent impurity like arsenic, phosphorus or antimony is added to an intrinsic semiconductor, it becomes an extrinsic n-type material. Pentavalent atoms give an extra electron to the crystal, which acts as a free charge carrier.
Diffuse impurities with five valence electrons are called donor atoms. The diffusion of impurity atoms does not change the overall electrical charge of the material, as the number of electrons remains the same as the positive charge in the nuclei. Instead, the impurity atoms produce another discrete energy band between the valence band and the conduction band. This is called the donor level.
If just one atom of impurity is added to 10 million atoms of intrinsic material – which initially had just one atom of impurity to 10 billion atoms – the concentration of free charge carriers is changed by a factor of 1,000.
Similarly, p-type material is produced by adding trivalent impurities such as gallium, boron, or indium to an intrinsic material. The trivalent impurities have not enough electron to completely complete the covalent bond in the new crystal structure. This vacancy acts as a positive charge and is called a hole. The free electrons in the extrinsic p-type material will continually fill one hole or another, always leaving a gap.
When an external electric field is applied to an n-type or p-type material, electrons and holes present as free charge carriers are moved by the direction of the field and resulting in the conduction of electric current.
This electric current is the result of the movement of electrons and holes. In n-type materials, electrons are carriers with a majority charge, while holes are carriers with a minority charge. In p-type material, it is inverted. Holes are the carriers with the majority charge, while electrons are the carriers with the minority charge.
Solid-state electronic devices are constructed with N-type PE-type materials. Much like a diode, an n-type and a p-type of material are sandwiched together to form a current-conducting junction between the two terminals. In a transistor, two p-types and one n-type, or one p-type and two n-types are sandwiched to form two junctions between them – and current conduction flows through three terminals.
This is similar to how all other solid-state devices are manufactured.
Material Comparisons
As mentioned, silicon, germanium and gallium arsenide are currently the most widely used intrinsic semiconductors for manufacturing ICs.
The free electrons in the intrinsic material are called intrinsic carriers and are as follows for these materials (per cubic centimeter):
- Silicon: 1.5*1010
- Germanium: 2.5*1013
- Gallium Arsenide: 1.7*106
Another important factor is the relative mobility of the intrinsic transporters, since the ability of free transporters to move through the material is determined by it. By definition, relative mobility is the average speed of the charge carrier when subjected to an electric field. It is measured in meters per second divided by volts per meter.
The relative mobility for these materials is as follows:
- Silicon: 1500 cm2/Vs
- Germanium: 3900 cm2/Vs
- Gallium arsenide: 8500 cm2/Vs
You will notice that gallium arsenide has the fewest intrinsic carriers but the highest relative mobility, which is why devices made with GaAs offer the highest response speed.
The electrical properties of semiconductor materials depend on the number of free carriers and their relative mobility. Furthermore, the thermal and optical behavior of semiconductor materials largely depends on the gap between their valence and conduction bands.
Semiconductors have a negative temperature coefficient of resistance while conductors have a positive temperature coefficient of resistance. The smaller the gap, the lower the thermal stability of the material.
The band gap is measured in electron volts as follows:
- Silicon: 1.14 eV
- Germanium: 0.67 eV
- Gallium arsenide: 1.43 eV
Germanium is not very thermally stable due to its smaller band gap. This is why it is typically selected for use in heat and light sensitive devices.
The larger the gap, the more thermally stable the material is – meaning it is also more likely to emit energy in the form of light rather than heat. For this reason, GaAs is often used in the design of light-emitting diodes.
Thermally stable materials are also better suited for computing and communications applications.
Silicon Options
Silicon wafers are still the most commonly used material in IC manufacturing. There are three main reasons for this.
1. Silicon is abundant and easily obtained. What's more, the silicon refining process has improved dramatically in recent decades, so that it is possible to obtain intrinsic silicon with extremely high purity levels compared to other semiconductor materials.
2. Modern electronic applications are based on computing and communication rather than switching and control. These applications require circuits to be thermally stable, which means that silicon (with a band gap of 1.14 eV) is an ideal match, especially compared to other compound semiconductors.
3. Silicon has a history. The first silicon transistor was designed in 1954, so chip designers are familiar with it and have developed highly efficient silicon chip and network designs over the years. This is also why it is more economical to design an integrated circuit on a silicon substrate compared to any other semiconductor material.
Eventually, gallium arsenide could completely replace silicon and currently serves as an alternative for VLSI and ULSI designs. GaAs offer speeds five times faster than silicon circuits, so as demand for high-speed circuits intensifies, it could become more attractive.
This is particularly true as technology and network designs evolve – which, if the past is any indication of the future, is only a matter of time.