Electric current is defined as the movement or flow of electric charge, which can be composed of electrons, ions, or other charged particles, through a material. This flow powers every electronic device and system. The amount of current is determined by the number of charged particles passing a specific point within a given timeframe. The specific mechanism driving this movement can vary depending on the force acting upon the charge carriers.
The Role of the Electric Field
Drift current is the specific type of electrical flow that occurs when an external electric field is applied to a material. This field, often generated by applying a voltage, exerts a force on the free charge carriers within the material, causing them to move in a directed manner. For example, in a semiconductor, negative electrons are pulled opposite the electric field, while positive charge carriers, called holes, move in the same direction.
The application of this force results in a net average movement known as the drift velocity. This movement is not a smooth acceleration, but rather a chaotic path where the charge carriers frequently collide with the atoms of the material. Between these collisions, the electric field accelerates the carriers, ensuring that over time, they maintain a slight, directed velocity superimposed on their rapid, random thermal motion. The resulting current is a direct consequence of the electric field dictating the overall direction of the charged particles.
Factors Determining Current Strength
The magnitude of the drift current is determined by two primary characteristics of the material itself.
Carrier Concentration
The first factor is the carrier concentration, which is the number of free charge carriers available to move. A material with a higher concentration of electrons and holes will naturally carry a greater current for the same applied electric field.
Carrier Mobility
The second factor is the carrier mobility, which describes how easily the charge carriers can move through the material under the influence of the electric field. Mobility is a measure of the internal resistance to movement, reflecting how often the carriers collide with the crystal lattice atoms or impurities. A material with high mobility allows carriers to achieve a greater average drift velocity, thus generating a larger current.
The relationship between the drift current, the electric field strength, the carrier concentration, and the carrier mobility is generally linear. If the electric field is doubled, the drift velocity and the resulting current will also double, provided the material properties remain constant. This proportional relationship provides a predictable way to control the flow of charge.
Comparing Drift and Diffusion Current
While drift current is driven by an external electric field, diffusion current is driven by a completely different mechanism. Diffusion current is the movement of charge carriers from a region of high concentration to a region of lower concentration. This process does not require any external voltage or electric field, relying instead on the natural tendency of particles to spread out and achieve a uniform distribution.
A simple example of diffusion is the way an odor spreads throughout a room until the concentration of scent particles is equal everywhere. In a semiconductor, if a region has a much higher density of electrons than an adjacent region, those electrons will randomly move toward the lower-concentration area, creating a flow of charge. This movement is entirely dependent on the concentration gradient.
The two currents are fundamentally different in their driving force and their direction. Drift current always flows in response to an electric field, while diffusion current flows in response to a concentration gradient. In many electronic devices, both phenomena occur simultaneously, and the total current is the sum of the drift component and the diffusion component.
Where Drift Current Matters Most
Drift current is a fundamental mechanism of charge transport in all conductive materials, including metals and electrolytes. Its behavior is studied most intently in the field of semiconductors, such as silicon, which are used to construct modern electronic components. Semiconductors are engineered to control both the number and the movement of charge carriers.
In devices like diodes and transistors, the ability to precisely control the drift current through the application of an electric field enables their function. The movement of charge across the p-n junction within these devices is governed by the interplay between a built-in electric field that causes drift, and concentration differences that cause diffusion. Understanding how the electric field manipulates charge carriers to create drift current is foundational to the design and operation of modern integrated circuits.