Drift current is the net electric current that arises in a material when charged particles experience an external electric field and begin to move with an average velocity in the direction of that field. Even in the absence of any applied voltage, electrons in a conductor or charge carriers in a semiconductor are constantly in motion due to thermal energy, but these random motions produce no overall transport of charge. Only when an electric field is applied do the carriers acquire a small but directed component of velocity, called the drift velocity, and this directed motion generates drift current.
In a conductor such as a metal, the charge carriers are primarily electrons. Metals contain a dense “sea” of free electrons that are only loosely bound to atoms and can move throughout the lattice. When an electric field is applied, each electron experiences an electric force opposite the direction of the field and accelerates between collisions with atoms in the lattice. Because electrons collide frequently, they do not accelerate indefinitely; instead they settle into an average drift velocity. This drift velocity is typically quite small—on the order of millimeters per second—but because the number of charge carriers is extremely large, the resulting current can be substantial. The relationship between drift velocity and current can be expressed as ( I = nqAv_d ), where ( n ) is the number density of charge carriers, ( q ) is the charge of each carrier, ( A ) is the cross-sectional area of the conductor, and ( v_d ) is the drift velocity.
In semiconductors, drift current works slightly differently because there are two types of mobile charge carriers: electrons and holes. Electrons drift in a direction opposite the electric field, while holes drift in the direction of the field, since holes behave as positively charged particles. Both types of carriers can contribute to the total current. The magnitude of drift current in a semiconductor depends on carrier concentration, electric field strength, and mobility. Mobility, denoted ( \mu ), measures how easily a carrier accelerates in response to an electric field; materials with higher mobility produce more drift current for the same field strength.
Drift current plays a central role in understanding Ohm’s law, which relates current density ( J ) to electric field ( E ) through conductivity ( \sigma ): ( J = \sigma E ). This relation emerges because drift velocity is proportional to the electric field: ( v_d = \mu E ). Therefore, the drift current density becomes ( J = nq\mu E ), showing that conductivity depends on both the density and mobility of charge carriers. In metals, carrier density is fixed and mobility is limited by lattice scattering, so conductivity is primarily determined by atomic structure and temperature. In semiconductors, carrier density can vary dramatically with doping and temperature, allowing precise control over drift current in devices such as diodes and transistors.
Temperature has an important influence on drift current. In metals, increased temperature leads to more vigorous lattice vibrations, which increase scattering and reduce mobility; consequently, the drift current for a given electric field decreases. In intrinsic semiconductors, however, higher temperature increases the generation of electron–hole pairs, raising carrier density and thereby increasing drift current despite reductions in mobility. This contrasting behavior is crucial to semiconductor device functionality.
Drift current must also be distinguished from diffusion current. While drift current is driven by an electric field, diffusion current arises from concentration gradients of charge carriers. In many semiconductor devices, both drift and diffusion currents coexist, interacting in complex ways. For example, in a p–n junction, the electric field created by the junction’s depletion region produces drift current, while gradients in carrier concentration produce diffusion current. The balance between these two phenomena determines junction behavior in equilibrium and under bias.
Overall, drift current is a fundamental concept in solid-state physics and electrical engineering. It explains how charged particles move in materials in response to applied electric fields, governs the operation of conductors and semiconductors, and forms the basis for understanding electrical transport, device behavior, and the microscopic origins of Ohm’s law.