A semiconductor is a material that possesses an electrical conductivity value positioned precisely between that of a conductor, like copper, and an insulator, such as glass. This intermediate ability to carry an electrical current is what makes them the fundamental building blocks of virtually all modern electronic devices. Without the unique properties of semiconductors, the transistors, diodes, and integrated circuits that power computers, smartphones, and solar panels would not be possible.
Intermediate Electrical Conductivity
The defining characteristic of a semiconductor is its controllable electrical conductivity, which is neither inherently high nor inherently low. In their purest state, materials like silicon or germanium are poor conductors because most electrons are locked into stable atomic bonds. This intrinsic material has a resistivity that falls between \(10^{-4}\) and \(10^{8}\) Ohm-centimeters, separating it from the low resistivity of metals and the high resistivity of insulators.
Unlike metals, which have a constant supply of free-moving electrons, a pure semiconductor must be energized to free up charge carriers. Its conductivity can be altered by many orders of magnitude through external factors. This capacity for precise modification allows engineers to design components that can either block or allow current flow on demand.
Unique Temperature Sensitivity
A property of semiconductors is the inverse relationship between temperature and resistivity, which contrasts sharply with the behavior of metals. In metals, increasing temperature causes atoms to vibrate, obstructing the flow of free electrons and increasing resistance. Semiconductors react in the opposite manner; their conductivity increases as temperature rises.
This effect occurs because thermal energy serves as an external energy source. When heated, the material absorbs this energy, freeing more electrons from their atomic bonds. These freed electrons become available to carry current, simultaneously leaving behind positively charged vacancies, known as holes, which also contribute to conduction. The resulting increase in charge carriers outweighs any increase in resistance from atomic vibrations, leading to a net rise in electrical conductivity.
Manipulating Conductivity Through Doping
The most powerful method for engineering the conductivity of a semiconductor is a process called doping, where trace amounts of impurity atoms, or dopants, are intentionally introduced into the crystal structure. This process converts an intrinsic, poorly conductive material into an extrinsic semiconductor with predictable electrical properties. Even small concentrations of dopants, sometimes as low as one impurity atom per 100 million host atoms, can drastically change the material’s conductive behavior.
Doping creates two fundamental types of semiconductors, defined by the charge carrier they introduce. N-type doping involves adding elements with five valence electrons, such as phosphorus or arsenic, to a material like silicon, which naturally has four. When the dopant atom integrates into the silicon lattice, four electrons form bonds, leaving the fifth electron free to act as a negative charge carrier. This process makes electrons the majority carrier, hence the designation “N-type”.
Conversely, P-type doping uses elements with only three valence electrons, such as boron or gallium. When a P-type dopant atom is added, it forms three bonds but creates a missing electron in the fourth bond, known as a “hole.” This hole behaves as a mobile, positive charge carrier, allowing an electron from a neighboring atom to jump into the vacancy. The movement of these holes makes them the majority charge carrier, defining the material as “P-type.” By controlling the type and concentration of these dopants, engineers precisely tune conductivity and create the foundational P-N junctions used in diodes and transistors.
The Underlying Physics of the Band Gap
The electrical behavior of a semiconductor is explained by the concept of energy bands within the material’s atomic structure. Electrons exist in distinct energy ranges, primarily the Valence Band and the Conduction Band. The Valence Band holds electrons tightly bound to their atoms, making them unable to conduct electricity. The Conduction Band represents a higher energy level where electrons are free from atomic bonds and can move through the material to create an electric current.
Separating these two bands is the Band Gap, an energy barrier that electrons must overcome to transition from the non-conducting Valence Band to the conducting Conduction Band. In a good insulator, this gap is very large, requiring substantial energy to promote an electron. In a conductor, the Valence and Conduction Bands overlap, meaning electrons are always ready to flow with minimal energy input.
Semiconductors are defined by having a Band Gap that is small and manageable, typically around 1 electron-volt for silicon, compared to 5 electron-volts or more found in insulators. This intermediate gap size permits the control of conductivity. Small amounts of external energy—from heat, light, or doping—can easily excite electrons across the gap. The size of this Band Gap governs the material’s sensitivity to temperature and its responsiveness to doping.