A semiconductor is a material with electrical conductivity positioned between that of a highly conductive metal and an insulating nonmetal. This intermediate nature allows the material to act as a controlled switch for electricity, forming the foundation of all modern electronics. The term “semiconductor” describes an electrical behavior rather than a strict chemical classification. This functional definition makes these materials indispensable, as their conductivity is not fixed but can be precisely manipulated for technological use.
How Elements are Classified by Electrical Properties
The periodic table divides pure elements into three broad categories based on physical and chemical characteristics, with electrical conductivity being a defining trait. Metals occupy the left side and center of the table and are known for their high electrical conductivity and low resistivity. This is due to delocalized valence electrons, which form a “sea of electrons” that move freely and efficiently transfer charge. Metals are typically lustrous, malleable, and ductile, existing as solids at room temperature, with mercury being the only exception.
Nonmetals are situated on the right side of the periodic table and stand in sharp contrast to metals. These elements are generally poor conductors of electricity, functioning as electrical insulators. Their electrons are tightly bound within covalent bonds or molecular structures. Solid nonmetals tend to be brittle, lack a metallic luster, and have low melting points, often existing as gases, liquids, or volatile solids.
Between these two extremes lies the category of metalloids, which form a diagonal “stair-step” boundary on the table. Metalloids exhibit properties intermediate between metals and nonmetals, often possessing a metallic sheen but being brittle. Electrically, they are poor conductors compared to metals but far better than nonmetals. They display a moderate conductivity that is highly sensitive to external conditions like temperature or impurities. Elements in this group, such as Silicon and Germanium, are important because their intermediate conductivity forms the foundation for most modern semiconductors.
Semiconductor Classification: A Class Beyond the Periodic Table
Whether a semiconductor is a metal, nonmetal, or metalloid depends on referring to its elemental composition or its functional behavior. Most common elemental semiconductors, namely silicon (Si) and germanium (Ge), are chemically classified as metalloids. These are Group 14 elements whose intermediate position on the periodic table corresponds to their intermediate electrical conductivity.
The term “semiconductor” is fundamentally a functional classification describing a material’s ability to have its conductivity controlled, not a rigid chemical one. This is evidenced by compound semiconductors, which are made up of two or more elements. A prominent example is Gallium Arsenide (GaAs), a binary compound formed from Group III (Gallium) and Group V (Arsenic) elements, which is widely used in high-speed and optoelectronic devices.
Semiconductor behavior is also found in materials that are not metalloids, such as specialized allotropes of carbon, which is a nonmetal. Organic semiconductors, which are carbon-based materials, are increasingly used in flexible electronics and organic light-emitting diodes (OLEDs). These examples demonstrate that semiconductor function depends on the material’s crystal structure and electronic behavior, particularly its manageable energy gap.
The Physics of Controlled Conductivity
The unique nature of a semiconductor lies in its ability to have its electrical conductivity precisely manipulated, a capability not shared by metals or insulators. This control is explained by the material’s electronic band structure and the presence of a band gap. Electrons exist in two energy bands: the valence band, where electrons are bound, and the conduction band, where electrons are free to move and conduct current. The band gap is the energy barrier between these two bands.
Metals have virtually no band gap, as the valence and conduction bands overlap, allowing electrons to move freely and resulting in high conductivity. Insulators, conversely, have a very large band gap, which prevents almost all electrons from jumping into the conduction band to carry a current. Semiconductors possess a relatively small band gap, typically around 1 eV. This gap is small enough that a manageable amount of energy, like heat or light, can excite electrons to cross the gap and allow a small current to flow.
The most powerful method for controlling conductivity is doping, which involves intentionally introducing minute amounts of impurity atoms into the semiconductor crystal. Doping creates two types of charge carriers to enhance conductivity.
N-Type Doping
In N-type (negative) doping, impurity atoms from Group 15, such as Phosphorus or Arsenic, are added. These donor atoms have one extra valence electron, which is easily excited into the conduction band, increasing the number of free, negatively charged electrons available to carry current.
P-Type Doping
The second type is P-type (positive) doping, which uses impurity atoms from Group 13, like Boron or Gallium. These atoms have one less valence electron than the host material and create a “hole,” or a missing electron, in the valence band. Electrons from neighboring atoms can jump into this hole, causing the positively charged hole to move through the crystal lattice and act as a mobile charge carrier. Combining N-type and P-type materials forms a P-N junction, the fundamental building block for transistors, diodes, and integrated circuits, allowing for the precise control of current flow.