What Are Charge Carriers?

Charge carriers are the mobile entities in a material that transport electric charge when an electric field is applied. In metals, electrons serve as the sole charge carriers, moving through the conduction band and constituting the drift current. In semiconductors, two types of carriers coexist: electrons in the conduction band and holes in the valence band, where a hole represents the absence of an electron in an otherwise filled energy level and behaves as a positively charged quasi-particle. The presence and balance of these two carrier types governs the electrical conductivity of a semiconductor and makes it possible to engineer devices with controlled current-carrying properties by introducing dopant impurities.

In intrinsic (undoped) semiconductor materials, electrons and holes are always generated and annihilated in equal numbers through thermal excitation across the bandgap. Doping with donor atoms, such as phosphorus in silicon, contributes additional conduction-band electrons without a corresponding hole, making electrons the majority carrier in n-type material. Doping with acceptor atoms, such as boron in silicon, creates additional holes, making holes the majority carrier in p-type material. Conductivity in a doped semiconductor is approximately proportional to the product of carrier density and carrier mobility for the majority species.

Electrons and Holes

The effective mass of an electron or hole in a semiconductor differs from the free-electron mass because of the periodic crystal potential. Electrons near the bottom of the conduction band and holes near the top of the valence band are described by parabolic band approximations in which the effective mass mₑ and mₕ replace the free mass. In silicon, the electron effective mass is approximately 0.26 times the free mass, and the hole effective mass is approximately 0.36 times the free mass, accounting for the higher electron mobility compared to hole mobility. In direct-bandgap III-V semiconductors such as gallium arsenide, the electron effective mass is much smaller (about 0.063 free masses), yielding the high electron mobilities that enable fast transistors and efficient optical devices. The ScienceDirect overview of charge carriers in semiconductor materials provides a systematic treatment of band structure and how effective mass governs carrier transport.

Impact Ionization

Impact ionization occurs when a carrier accelerated by a strong electric field gains sufficient kinetic energy to promote an electron from the valence band to the conduction band, generating a new electron-hole pair. This process is self-multiplying: the newly created carriers can themselves cause further ionization if the field is high enough, leading to avalanche multiplication. Controlled avalanche multiplication is the operating principle of avalanche photodiodes (APDs), which use internal gain to detect weak optical signals in fiber-optic receivers. In power semiconductor devices such as MOSFETs and IGBTs, uncontrolled avalanche breakdown defines the maximum operating voltage and is a critical design constraint. IEEE Xplore publications on impact ionization in semiconductors document how ionization coefficients for electrons and holes vary with electric field across silicon, GaAs, and wide-bandgap materials including GaN and SiC.

Semiconductivity and Carrier Control

The semiconductivity of a material, its ability to support both conductive and insulating states under external control, arises directly from the controllability of carrier type and density. In a p-n junction, the spatial separation of n-type and p-type regions creates a built-in electric field that controls carrier injection across the interface, enabling rectification and transistor action. Metal-oxide-semiconductor structures use a gate-induced electric field to invert the carrier type at the surface, forming the conductive channel in a MOSFET. The range of carrier concentrations achievable through doping, from near-intrinsic at 10¹⁰ cm⁻³ to degenerately doped at 10²⁰ cm⁻³, spans ten orders of magnitude and provides the design space for devices from high-voltage power switches to low-power logic transistors. The NIST database on semiconductor properties provides reference values for carrier-related parameters across a range of technologically important semiconductor materials.

Applications

Charge carriers have applications in a wide range of disciplines, including:

• p-n junction diodes and bipolar transistors for rectification and amplification
• MOSFET-based logic and memory in CMOS integrated circuits
• Avalanche photodiodes for optical communication receivers
• Power semiconductor switches for motor drives and power conversion systems
• Photovoltaic cells where photogenerated carriers produce an electrical current