Gallium is a soft, silvery-blue metal located just below aluminum on the periodic table. As a true metal, it exhibits electrical conductivity, allowing an electric current to pass through it. This ability, shared by all metals, stems from the presence of delocalized electrons. These electrons are not tethered to a single atom but form a shared “sea” that moves freely throughout the material when voltage is applied. Gallium’s unique physical properties, however, make its conductive behavior distinct from common metals like copper or gold.
The Electrical Properties of Gallium
Gallium conducts electricity through metallic bonding, where its valence electrons are shared among the atoms to create the mobile charge carriers necessary for current flow. However, the efficiency of this electron movement is highly dependent on its physical arrangement. Unlike common metals that form highly symmetrical crystal lattices, solid gallium crystallizes in a complex orthorhombic structure. This low symmetry means the atoms are spaced differently along the three perpendicular axes of the crystal.
This structural complexity results in anisotropy, meaning the electrical conductivity varies significantly depending on the direction the current travels through the crystal. In single-crystal solid gallium, conductivity along its three main axes can vary by a ratio as wide as 1:3.2:7. For example, a current traveling in one direction may face seven times the resistance compared to the most favorable direction. The resistivity of pure solid gallium is approximately \(1.4 \times 10^{-7}\) ohm-meters at room temperature, making it a good conductor, though less efficient than silver or copper.
Conductivity in Solid vs. Liquid Gallium
Gallium is known for its extremely low melting point, approximately 29.76 °C (85.6 °F), meaning it melts easily just above room temperature. This low melting point allows observation of an unusual change in electrical behavior during the phase transition. Most metals experience a slight increase in conductivity when they melt, as increased atomic movement can sometimes reduce electron scattering.
Gallium, however, is one of the few exceptions, as its electrical conductivity decreases when it changes from a solid to a liquid. At its melting point of 303 Kelvin, the solid state has an electrical conductivity of about 3560 Siemens per centimeter, which drops to approximately 3230 Siemens per centimeter upon liquefaction. This reduction is caused by a unique structural rearrangement. The complex, low-symmetry crystal structure of solid gallium is replaced by a less-ordered, but surprisingly denser, liquid structure.
The greater density in the liquid state, combined with the loss of crystalline order, causes the free-moving electrons to scatter more frequently off the disordered atoms. This increased scattering impedes the flow of current, leading to a measurable decrease in conductivity. This counterintuitive behavior, where the liquid phase is a less efficient conductor than the solid phase, distinguishes gallium from most other metallic elements.
Gallium in Modern Electronics
While pure gallium is not used for electrical wiring due to its low melting point and anisotropic conductivity, its compounds are essential for high-performance electronics. Gallium is primarily used to create compound semiconductors, such as Gallium Arsenide (GaAs) and Gallium Nitride (GaN). These compounds are superior to traditional silicon in applications requiring high speed and power.
Gallium Arsenide (GaAs)
Gallium Arsenide is widely used in radio frequency (RF) circuits for devices like cell phones, Wi-Fi routers, and 5G technology. The electrons in GaAs move much faster than those in silicon, enabling the rapid signal processing required for high-frequency communication at gigahertz speeds. This fast electron mobility makes GaAs ideal for power amplifiers and integrated circuits in wireless devices.
Gallium Nitride (GaN)
Gallium Nitride has become indispensable in high-power applications, including smaller, more efficient mobile device chargers and 5G network infrastructure. GaN devices can handle higher voltages and temperatures than silicon. This allows them to operate with higher efficiency and significantly reduced power loss, enabling the creation of smaller, more robust power conversion components.
Thermal Management
Liquid gallium alloys, often mixed with indium and tin, are employed in thermal management systems. Their high thermal and electrical conductivity in the liquid state makes them excellent, non-toxic replacements for traditional heat sinks. These alloys efficiently draw heat away from hot semiconductor chips.