A diode is a fundamental electronic component built entirely from semiconductor material. This two-terminal device acts as an electronic one-way street, allowing electric current to flow in a single direction while blocking the opposite way. The construction and function of the diode depend directly on the unique electrical properties of the materials used in its core structure.
Defining Semiconductors
Semiconductors are a class of materials with an electrical conductivity that falls between that of highly conductive metals and highly insulating materials like glass. Common examples include Silicon and Germanium, which are the foundational substances for modern electronics. Pure semiconductor materials have limited practical use because their conductivity is too low at room temperature.
To make them useful, a process called doping is necessary, which involves intentionally introducing specific impurities into the pure crystal structure. This doping creates an extrinsic semiconductor whose electrical properties are carefully controlled by the added impurity atoms. The goal is to create two distinct types of material: N-type and P-type.
N-type semiconductors are created by adding impurities that contribute extra electrons, which carry a negative charge. P-type semiconductors are formed by introducing impurities that create “holes,” which are vacancies in the electron structure that behave like positive charge carriers.
The Diode as a P-N Junction Structure
A diode is created when a piece of P-type semiconductor material is brought into direct contact with a piece of N-type semiconductor material, forming what is known as a P-N junction. This junction is the operational heart of the diode device. The N-type material has an excess of electrons, while the P-type material has an abundance of holes.
Once the two materials are joined, the excess electrons from the N-side diffuse across the junction into the P-side, where they combine with the holes. This movement of charge carriers immediately forms a region around the junction that is depleted of mobile charge carriers, called the depletion region. The electrons leave behind positively charged atoms on the N-side, and the holes leave behind negatively charged atoms on the P-side.
These fixed, charged atoms create an internal electric field that spans the depletion region, acting as a natural barrier to further charge movement. This electric field establishes a potential difference, known as the barrier potential, which must be overcome by an external voltage for current to flow.
Operational Function and Current Control
The primary function of the diode is its ability to control the direction of current flow by manipulating the internal barrier potential. When an external voltage is applied across the diode in the direction that pushes current through, this is called forward bias. Connecting the positive terminal of a voltage source to the P-type material and the negative terminal to the N-type material reduces the width of the depletion region.
Once the external voltage overcomes the barrier potential, which is about 0.7 volts for a silicon diode, the depletion region shrinks significantly, and the diode permits a large current flow with minimal resistance. Applying voltage in the opposite direction is known as reverse bias. In this setup, the external voltage source attracts the free charge carriers away from the junction, causing the depletion region to widen substantially.
This expanded, non-conductive region acts as an insulator, effectively blocking the flow of current. The diode’s ability to conduct easily in forward bias and block current in reverse bias is called rectification.
Common Uses of Diodes
The unique unidirectional current control of the diode makes it an indispensable component across all forms of electronics. The most widespread application is in AC/DC power conversion, where rectifier diodes are used in power supplies to change the AC electricity from a wall outlet into the DC power required by nearly all electronic devices.
Another highly visible application involves Light-Emitting Diodes (LEDs), which are specialized P-N junctions that release energy in the form of light when current flows through them in the forward direction. Specialty diodes, such as Zener diodes, are used for voltage regulation; they are designed to conduct current in the reverse direction once a specific voltage threshold is reached, protecting sensitive circuitry from excessive voltage. Diodes are also incorporated into circuits for surge protection, where they quickly divert damaging voltage spikes away from delicate components.