In electronics, materials are categorized by how easily they allow an electric current to pass through them. Conductors permit the flow of charge with minimal resistance, making them ideal for carrying electrical power over distances. Semiconductors have an intermediate electrical conductivity, falling between that of an efficient conductor and a non-conducting insulator. This controlled, tunable property is the foundation of modern digital technology, allowing for the manipulation and processing of electrical signals. The distinction dictates their function, from simple wiring to complex microprocessors.
Defining Electrical Conductivity
Conductors, like metals such as copper and silver, have extremely high conductivity, meaning they possess very low electrical resistivity, often on the order of \(10^{-8}\) ohm-meters (\(\Omega\text{m}\)). This high conductivity is due to a massive supply of free-moving electrons that are not tightly bound to individual atoms. These charge carriers are instantly available to form a current when a voltage is applied, resulting in efficient and stable electron flow.
Semiconductors, such as silicon and germanium, exhibit a much higher resistivity than conductors, typically ranging from \(10^{-5}\) to \(10^{5}\) ohm-meters (\(\Omega\text{m}\)). Their conductivity is moderate and, crucially, can be altered over a wide range by external conditions. Unlike conductors, the number of charge carriers in a pure semiconductor is relatively low at room temperature. Conduction involves both the movement of negative electrons and the apparent movement of positive “holes,” which are vacancies left behind by excited electrons.
The Mechanism of the Energy Band Gap
The root cause of the difference in conductivity lies in the material’s electronic band structure. In a solid material, electron energies are grouped into bands, the two most important being the valence band and the conduction band. The valence band contains the electrons that are bound to their atoms, while the conduction band contains the electrons that are free to move and conduct electricity. Between these two bands lies the band gap, or forbidden gap, which is an energy range where electrons cannot exist.
In conductors, the valence band and the conduction band physically overlap. This means there is effectively zero energy gap, allowing electrons to move freely into the conduction states. This overlap provides the material with an immediate and abundant supply of charge carriers for current flow.
Semiconductors, by contrast, possess a small but finite band gap between 0 eV and approximately 3 eV. At absolute zero temperature, this gap means the valence band is full and the conduction band is empty, causing the material to act as an insulator. However, a small amount of energy—like thermal energy from room temperature—is enough to excite some electrons across this narrow gap and into the conduction band. This conditional movement means a semiconductor’s conductivity is not constant but depends on the energy available to bridge the gap.
Response to External Factors
The response of these materials to changes in their environment further distinguishes their properties and utility. When a conductor’s temperature increases, its electrical resistance also increases. The increased thermal energy causes the atoms in the metal lattice to vibrate more intensely, which physically impedes the flow of the abundant free electrons. This increased scattering of electrons reduces the overall current flow.
Semiconductors exhibit the opposite behavior: an increase in temperature leads to a decrease in their electrical resistance. The added thermal energy excites more electrons across the small band gap from the valence band to the conduction band. The resulting dramatic increase in the number of charge carriers available for conduction outweighs the negative effect of increased atomic vibration.
Another significant external factor is the intentional introduction of impurities, a process known as doping. Doping has little beneficial effect on conductors and often degrades their performance, but it is the primary method of controlling semiconductor conductivity. By introducing specific impurity atoms, such as boron or phosphorus, the number of charge carriers can be precisely controlled. This process creates P-type (positive charge carrier) or N-type (negative charge carrier) semiconductors, allowing engineers to build devices with customized electrical properties.
Essential Functional Applications
The distinct electrical properties of conductors and semiconductors lead them to be used in fundamentally different functional roles. Conductors are utilized in applications where the goal is the maximum, most efficient transfer of electrical energy or heat. This includes power transmission lines, electrical wiring within homes and devices, and heat sinks, where their stable, high conductivity is paramount. Their function is to move energy reliably from one point to another.
Semiconductors, conversely, are the basis of logic, switching, and control functions within modern electronics. Their unique ability to switch between being a conductor and an insulator, or to have their conductivity varied, allows them to be used as electronic switches and amplifiers. Silicon-based devices like transistors, diodes, and integrated circuits form the core of computers, smartphones, and every other digital device. They are used for processing and manipulating information through conditional current flow, which is a capability pure conductors simply do not possess.