The term “semiconductor” describes a material’s electrical behavior, not a category of elements on the periodic table. This functional classification is distinct from the elemental groupings of metals, nonmetals, and metalloids, which categorize elements based on broader chemical and physical properties. To understand this distinction, it is necessary to first establish the traditional definitions of elemental classes.
Understanding Metals, Nonmetals, and Metalloids
The three primary classifications for elements are largely defined by their ability to conduct electricity. Metals are positioned on the left side of the periodic table and are known for their high electrical conductivity. This excellent current flow results from a “sea” of freely moving valence electrons that are not tightly bound to individual atoms.
Nonmetals occupy the right side of the table and act as electrical insulators, exhibiting extremely low conductivity. In these materials, valence electrons are tightly bound, requiring substantial energy to free them for current flow.
Metalloids form a narrow, diagonal line separating the metals from the nonmetals. These elements possess intermediate properties, displaying a mixture of metallic and nonmetallic characteristics. Their electrical conductivity is neither high like metals nor negligible like nonmetals, placing them in an intermediate zone.
The Defining Characteristic of a Semiconductor
A semiconductor exhibits electrical conductivity that falls between that of a conductor and an insulator. This intermediate state is governed by the material’s electronic band structure, specifically the presence of a small energy gap, known as the band gap. This gap separates the valence band, where electrons are bound, from the conduction band, where electrons can move freely.
At extremely low temperatures, a pure semiconductor acts like an insulator because its electrons lack the energy to bridge the band gap. However, a small increase in energy, such as a rise in ambient temperature, can excite some electrons across the gap. Once in the conduction band, these electrons can carry a current, making the material slightly conductive.
The true utility of a semiconductor lies in its capacity for precisely controlled conductivity, a property not shared by simple metals or nonmetals. This control is achieved through a process called doping, where trace amounts of specific impurity atoms are intentionally introduced into the crystal lattice.
Adding atoms with extra valence electrons, such as phosphorus, creates an n-type semiconductor, increasing the number of negative charge carriers. Conversely, adding atoms with fewer valence electrons, like boron, creates a p-type semiconductor, which increases positive charge carriers, known as “holes.” This manipulation allows engineers to create regions of different conductivity, which is the foundational principle for nearly all modern electronic devices.
Placing Semiconductors in the Material Classification System
The elements most commonly associated with the term “semiconductor” are Silicon (Si) and Germanium (Ge), both classified as metalloids on the periodic table. Their inherent intermediate properties make them naturally suited for the controlled conductivity required in electronics. The metalloid classification, signifying a blend of metallic and nonmetallic traits, aligns perfectly with the functional definition of a semiconductor material.
The term “semiconductor” is not restricted to single elements. Many modern electronic devices rely on compound semiconductors, materials composed of two or more elements. Examples include Gallium Arsenide (GaAs) and Indium Phosphide (InP), formed by combining a metal from Group III with a nonmetal from Group V.
These compound materials also exhibit controlled conductivity, despite being formed from elements that are not themselves metalloids. Therefore, the term “semiconductor” is fundamentally a functional classification, describing a material’s operational behavior in an electronic circuit. While elemental semiconductors fall squarely within the metalloid category, the overall classification system for semiconductors is based on performance.