Drift current

In condensed matter physics and electrochemistry, drift current is the electric current, or movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance. When an electric field is applied across a semiconductor material, a current is produced due to the flow of charge carriers. In condensed matter physics and electrochemistry, drift current is the electric current, or movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance. When an electric field is applied across a semiconductor material, a current is produced due to the flow of charge carriers. The drift velocity is the average velocity of the charge carriers in the drift current. The drift velocity, and resulting current, is characterized by the mobility; for details, see electron mobility (for solids) or electrical mobility (for a more general discussion). See drift–diffusion equation for the way that the drift current, diffusion current, and carrier generation and recombination are combined into a single equation. Drift current is the electric current caused by particles getting pulled by an electric field. The term is most commonly used in the context of electrons and holes in semiconductors, although the same concept also applies to metals, electrolytes, and so on. Drift current is caused by the electric force: Charged particles get pushed by an electric field. Electrons, being negatively charged, get pushed in the opposite direction to the electric field, while holes get pushed in the same direction as the electric field, but the resulting conventional current points in the same direction as the electric field in both cases. If an electric field is applied to an electron in a vacuum, the electron will accelerate faster and faster, in approximately a straight line. A drift current looks very different than that up close. Typically, electrons are moving randomly in all directions (Brownian motion), frequently changing direction when they collide with grain boundaries or other disturbances. Between collisions, the electric field subtly accelerates them in one direction. So over time, they move at the drift velocity on average, but at any instant the electrons are moving at the (typically much faster) thermal velocity. The amount of drift current depends on the concentration of charge carriers and their mobility in the material or medium. Drift current frequently occurs at the same time as diffusion current; the following table compares the two forms of current: In a p-n junction diode, electrons and holes are the minority charge carriers in the p-region and the n-region, respectively. In an unbiased junction, due to the diffusion of charge carriers, the diffusion current, which flows from the p to n region, is exactly balanced by the equal and opposite drift current. In a biased p-n junction, the drift current is independent of the biasing, as the number of minority carriers is independent of the biasing voltages. But as minority charge carriers can be thermally generated, drift current is temperature dependent.