 Charge carrier

A charge carrier transports electric charge by moving: Electrons are one example, and ions are another. For species $s$ with number density $n_s$, electric charge $q_s$ and mean velocity ${\mathbf{v}}_s$, the transport contributions add as $\mathbf{J}={\sum }_s{n}_s{q}_s{\mathbf{v}}_s$. A stationary charged background contributes to charge density but not to this current density. ^[ref].

A charge carrier is a particle or entity that carries an electric charge and is responsible for electrical conduction in a material. In the context of solid materials, charge carriers can be classified primarily into two types: 1. **Electrons**: Negative charge carriers that are typically found in conductive materials like metals and semiconductors. They move through the material to conduct electricity. 2. **Holes**: Positive charge carriers that can be considered as the absence of an electron in a semiconductor.