A semiconductor material has an electrical conductivity that sits precisely between that of a highly conductive metal, like copper, and an insulating material, like glass. This intermediate conductivity is not fixed but can be precisely controlled, making these materials the foundation of modern electronics. Semiconductors can be made to act like an insulator, blocking current, or a conductor, allowing current to flow freely, simply by applying a small external stimulus such as voltage, light, or heat. The ability to switch between these states allows for the creation of components like transistors and diodes, which are the building blocks of every microchip and digital device.
Defining the Unique Electrical Behavior
The ability of a semiconductor to switch its conductivity is explained by the concept of the band gap, an energy difference that separates electrons within the material. Electrons are normally bound in the valence band, where they are tied to their parent atoms and cannot move freely to conduct electricity. For current to flow, an electron must gain enough energy to jump across the band gap and enter the conduction band, where it is free to travel. In an insulator, this band gap is so large that electrons almost never make the jump.
In a conductor, the valence and conduction bands overlap, meaning electrons are always free to move and conduct current without any energy input. A semiconductor possesses a relatively small band gap, which is neither too large nor zero. This narrow energy barrier means that a small amount of external energy, such as heat, can excite some electrons into the conduction band.
When an electron jumps into the conduction band, it leaves behind an empty spot in the valence band called a “hole.” This hole behaves like a positive charge carrier, as other valence electrons can move to fill it, causing the hole itself to migrate across the material. In its pure, or intrinsic, state, a semiconductor is not a good conductor because the number of free electrons and holes is low. However, the presence of both negative (electrons) and positive (holes) charge carriers is the foundation for controlled electrical flow.
Controlling Conductivity Through Doping
The controlled conductivity of semiconductors is achieved through doping, which involves intentionally introducing specific impurities into the pure material. This process creates extrinsic semiconductors, where the concentration of one type of charge carrier is greatly increased. The impurities, known as dopants, are added in extremely small amounts, yet they fundamentally alter the material’s electrical characteristics.
One type of doping uses elements, such as Phosphorus or Arsenic, that have five valence electrons—one more than the four required to bond with the host material like Silicon. When these dopants are incorporated into the crystal structure, the fifth electron is left free to move, creating an abundance of negative charge carriers. This results in an N-type semiconductor, where the majority of the current is carried by these free electrons.
P-type doping uses elements like Boron or Gallium that possess only three valence electrons. When these atoms bond with the host material, they are one electron short of completing the required four bonds, which creates a mobile “hole.” This process introduces a high concentration of positive charge carriers, or holes, which become the majority carriers for electrical current in the P-type material.
Key Semiconductor Substances
While the principles of band gaps and doping apply to many materials, the industry relies on a few specific substances chosen for their stability and performance. Silicon is the most widely used semiconductor, forming the basis of over 90% of all electronic devices due to its abundance, low cost, and ability to form a stable oxide layer for insulation. It is the standard material for manufacturing integrated circuits and computer processors.
Germanium was historically the first material used to create transistors, but it was largely supplanted by Silicon due to its lower operating temperature limits. Germanium still finds application in specialized devices requiring high electron mobility, such as high-speed electronic components and fiber-optic systems. Compound semiconductors, such as Gallium Arsenide, are formed from two or more elements and are used where superior speed or light interaction is demanded. Gallium Arsenide is commonly used in high-frequency devices, including mobile communications and satellite systems, and in optoelectronics, such as LEDs and laser diodes.
Why Semiconductors Power Everything
The semiconductor material is realized when N-type and P-type materials are brought together to form a P-N junction. This junction creates a boundary that acts as a one-way valve for electrical current, which is the fundamental operation of a diode. Electrons and holes recombine at the boundary, forming a depletion region that prevents current flow until a specific voltage is applied.
By arranging these P-N junctions in various configurations, engineers create transistors, which are tiny, controllable electronic switches. A transistor uses a small electrical signal to rapidly turn a larger current on or off, functioning as the primary mechanism for all digital logic and memory. The ability to fabricate billions of these microscopic, reliable switches on a single chip enables the complexity of modern computing. P-N junctions are also the mechanism behind solar cells, which convert light into electricity, and light-emitting diodes (LEDs), which convert electricity into light.