Table of Contents
Introduction
In the era of rapid technological advancement, fields such as Artificial Intelligence, software development, robotics, embedded systems and the internet of things(IOT) are transforming industries and daily life. At the core of these innovations are electronic components, which serve as the fundamental building blocks of modern technology.
understanding these components requires a deep knowledge of semiconductors the materials that have fundamentally shaped the digital world. Among the most critical of these are silicon (Si), Germanium (Ge) and Gallium arsenide (GaAs) each playing a vital role in the performance and efficiency of todays electronic systems.
In this blog, we will begin by exploring how these semiconductor materials have influenced the development of modern technology and continue to drive across multiple domains.
Silicon (Si), germanium (Ge) and gallium arsenide (GaAs)
Atoms are composed of three fundamental particles: protons, neutrons, and electrons. Protons and neutrons form the nucleus at the center of the atom, while electrons revolve around the nucleus in defined orbits or energy levels. To better understand the arrangement of atoms, here we will use lattice structure (a systematic representation that helps visualize the organization of matter).
From the periodic table, we observe that silicon (Si), germanium (Ge), gallium (Ga) and arsenic (As) have atomic numbers 14, 32, 31 and 33 respectively. This means they possess 14, 32, 31 and 33 electrons. Notably, both silicon and germanium have 4 electrons in their outermost orbit, which are known as valence electrons.
Valence electrons play a crucial role in determining the electrical properties of an element. Compared to inner electrons, valence electrons require relatively less ionization potential( minimum amount of energy required to remove an elctron from an atom ) to be detached, making them highly significant in the behavior of semiconductors.
Covalent Bonding and Intrinsic Carriers in Semiconductors
The valence electrons in the outermost orbit of silicon and germanium atoms form covalent bonds with four neighboring atoms, resulting in a stable, crystalline bonding structure.
In the case of Gallium Arsenide (GaAs), covalent bonding occurs between two different atoms—Gallium (Ga), which contributes three valence electrons, and Arsenic (As), which provides five valence electrons. Together, they form a complete covalent bond by sharing electrons to achieve stable outer shells.
These covalent bonds can be broken by external energy sources, causing electrons to become free and gain kinetic energy. Common sources of such energy include thermal energy at room temperature and light energy in the form of photons. When these electrons are liberated from their bonds, they become free charge carriers within the material.These carriers are referred to as intrinsic carriers, as they are generated within a pure semiconductor material—one that has been refined to reduce impurities to the lowest possible levels.
Intrinsic Carrier Concentration and Carrier Mobility
Among the commonly used semiconductor materials,
\[germanium (Ge) = 2.5 \times 10^{13}per cm^3\]
\[silicon (Si) = 1.5 \times 10^{10} per cm^3\]
\[gallium arsenide (GaAs) = 1.7 \times 10^6 per cm^3\]
This parameters reflect the number of free charge carriers available in a pure (intrinsic) material at room temperature.
Another critical property in evaluating semiconductor performance is carrier mobility \(\mu_n\), which measures how quickly free carriers (electrons or holes) can move through a material when an electric field is applied. This mobility directly influences the speed and responsiveness of semiconductor devices.
GaAs exhibits the highest carrier mobility, with values reaching \(8,500 cm^2/V\cdot s\), followed by Ge with \(3,900 cm^2/V\cdot s\), and Si with \(1,500 cm^2/V\cdot s\). The superior mobility of GaAs enables it to support much faster signal transmission, resulting in electronic devices with response times up to five times faster than those based on silicon.
Doping: Enhancing Electrical Conductivity
In their pure (intrinsic) form, silicon and germanium are poor conductors of electricity. This is because all of their valence electrons are tightly bound in covalent bonds, leaving no free electrons available to conduct electric current.
To improve their conductivity, a controlled introduction of impurities—known as doping—is performed. Through the doping process, specific foreign atoms are added to the semiconductor crystal. These atoms either contribute free electrons or create electron vacancies (holes), effectively increasing the number of charge carriers within the material.
Temperature Dependence of Semiconductor Conductivity
When the temperature of a pure semiconductor increases, the valence electrons gain sufficient kinetic energy to break free from their covalent bonds. As a result, these electrons become free charge carriers, contributing to an increase in electrical conductivity.
This behavior gives semiconductors a negative temperature coefficient, meaning their conductivity increases with rising temperature—in contrast to conductors like metals, where conductivity typically decreases due to increased atomic vibrations.
Energy Bands and the Role of Bandgap in Semiconductors
When discussing the energy levels of atoms within a material, it’s important to understand that each electron occupies a specific energy level associated with its shell. The farther an electron is from the nucleus, the higher its energy state.
In a solid, when atoms are brought close together, their electrons begin to interact, leading to slight variations in the energy levels of electrons. As a result, the discrete energy levels of individual atoms expand into energy bands.
The two most important bands in semiconductor physics are the valence band—which contains electrons bound to atoms—and the conduction band, which consists of free electrons that can move through the material and conduct electric current. The energy gap between the top of the valence band and the bottom of the conduction band is known as the bandgap, denoted by \(E_g\).
\[E_g of Si = 1.1 eV\]
\[E_g of Ge = 0.67 eV\]
\[E_g of GaAs = 1.43 eV\]
It is derived from the basic relation:
\[W=q\times V\]
W is the energy in joules,
q is the charge of a single electron (\(1.602\times10^{−19} coulombs\)),
V is the electric potential difference in volts.
An electron volt is defined as the amount of energy gained or lost by an electron when it moves across a potential difference of 1 volt. Therefore:
\(1 eV=1.602\times10^{−19} joules\)
A smaller bandgap, like that of germanium, means that less energy is required for valence electrons to transition into the conduction band. Therefore, even a modest increase in temperature or exposure to light can generate a significant number of free carriers. This makes germanium highly suitable for applications like thermal imaging sensors, where sensitivity to light and temperature is essential.
However, germanium’s low thermal stability due to its narrow bandgap limits its use in transistor, where high-temperature performance and stability are crucial. In contrast, materials like silicon and GaAs are preferred for such applications due to their larger bandgaps and better thermal characteristics.
Conclusion
Understanding the fundamental concepts of atomic structure, energy bands, and semiconductor properties is essential to grasp the science behind today’s technological advancements. From the role of valence electrons and covalent bonding to the significance of intrinsic carriers, bandgap energy, and carrier mobility, each concept forms a vital piece of the semiconductor puzzle.
Materials like silicon, germanium, and gallium arsenide showcase how variations in atomic structure and energy levels can dramatically influence electrical behavior and suitability for different applications. This knowledge not only empowers innovation but also bridges the gap between theoretical science and practical engineering in shaping the digital world around us. If you find it helpful please write to us.
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