A diode functions like a one-way valve, allowing current to flow in one direction while blocking it in the opposite direction. The operation and construction of a diode are rooted in the properties of semiconductors. To understand how a diode works, it is first necessary to grasp what semiconductors are.
Types of Materials Based on Conductivity
Materials are categorized based on their electrical conductivity into three main groups: conductors, insulators, and semiconductors. Conductors, such as copper, exhibit high electrical conductivity and low electrical resistivity. Insulators, like plastic, have low electrical conductivity and high electrical resistivity. Semiconductors fall in between, offering moderate electrical conductivity. Silicon and germanium are among the most commonly used semiconductors.
The Conducting Behavior of Semiconductors
To understand the electrical behavior of semiconductors, consider silicon as an example. Silicon is the second most abundant element in the Earth’s crust. Each silicon atom contains four valence electrons and forms covalent bonds by sharing these electrons with four neighboring silicon atoms. As a result, pure silicon lacks free electrons and is a poor conductor of electricity at normal temperatures. However, the conductivity of silicon can be enhanced through a process known as doping.
Doping and Semiconductor Types
Doping is the process of adding specific impurities to a pure semiconductor to alter its electrical conductivity. These impurities are classified into two types: trivalent and pentavalent. A trivalent impurity, such as boron, has three electrons in its valence shell, while a pentavalent impurity, like phosphorus, possesses five electrons. When a trivalent impurity is introduced into pure silicon, it creates a vacancy known as a hole. This type of semiconductor is referred to as a P-type semiconductor. Conversely, when a pure semiconductor is doped with a pentavalent impurity, free electrons are introduced, forming an N-type semiconductor. Due to the presence of free electrons and holes, doped semiconductors exhibit enhanced electrical conductivity.
Formation of a PN Junction Diode
A diode is created by combining P-type and N-type semiconductors. Typically, one half of the semiconductor is doped with a trivalent impurity to form the P-type, while the other half is doped with a pentavalent impurity to form the N-type. At the atomic level, free electrons from the N-side migrate naturally to fill the holes on the P-side. Consequently, the P-type boundary becomes slightly negatively charged, and the N-type boundary becomes slightly positively charged. This results in an induced potential known as the barrier potential, forming a depletion region. The barrier potential prevents further migration of holes and electrons, thereby creating a diode.
Forward and Reverse Biasing of a Diode
A diode can be connected in two ways within a circuit: forward biasing and reverse biasing. In reverse biasing, the positive terminal of the battery is connected to the N-side, while the negative terminal is connected to the P-side. This arrangement causes holes to move toward the negative terminal and electrons toward the positive terminal, thus preventing the flow of current. Therefore, the diode does not permit current flow in reverse bias.
In forward biasing, the battery’s polarity is reversed so that the P-side is connected to the positive terminal, and the N-side is connected to the negative terminal. In this configuration, current flows freely through the diode.
VI Characteristics of a Diode
The relationship between voltage and current in a diode is described by its voltage-current (VI) characteristics. In forward bias, the diode begins conducting when the applied voltage exceeds the potential barrier. In reverse bias, a small current may flow, but it is independent of the applied voltage. However, if the reverse voltage continues to increase, the diode reaches a breakdown point, causing the current to rise sharply.
