A semiconductor diode is a two-terminal electronic component consisting of a P–N junction that conducts electricity primarily in one direction. It has a high resistance at one end and low resistance at the other end. It is also called a rectifier.
Diodes are used to limit the voltage and protect the circuit by converting AC to DC. Semiconductors such as silicon and germanium are used to make the most of diodes. Diodes transmit current in only one direction, and the way they transmit it varies. There are different types of diodes and each type has its own applications.
Symbol of Diode
The anode, which is the positive terminal of a diode, is denoted by A and the cathode, which is the negative terminal, is denoted by K. The anode represents the direction of conventional current flow in the forward-biased position.
P-type and N-type materials are the only semiconductors, such as silicon (Si) or germanium (Ge), with atomic impurities; The type of impurity present determines the type of semiconductor.
Related tutorial: P-N type Semiconductors
Construction of Diode
Diodes can be made of either of two semiconductor materials, silicon, and germanium. When the anode voltage is more positive than the cathode voltage, the diode is said to be forward-biased, and it operates easily with a relatively low voltage drop. Similarly, when the cathode voltage is more positive than the anode, the diode is said to be reverse-biased. The arrow in the diode symbol represents the direction of conventional current flow during the diode’s operation.
Formation of diodes
If P-type and N-type materials are brought close to each other, the two form a junction called a P–N junction.
A P-type material has holes as the majority carrier and an N-type material has electrons as the majority carrier. As opposite charges attract, some holes in the P-type move to the N-side, while some of the electrons in the N-type move to the P-side.
As both travels towards the junction, the hole and electron recombine with each other and neutralize the ions. Now, in this junction, there exists a region where positive and negative ions are formed, which is called the P-N junction or junctional barrier.
Negative ions on the P-side and positive ions on the N-side result in the formation of a narrow-charge region on either side of the P-N junction. This area is now free from mobile charge carriers. The ions present here are stable and maintain a space of space between them without any charge carriers.
Since this region acts as a barrier between P-type and N-type material, it is also called a barrier junction. It has another name called the depletion region which means it eliminates both regions. There is a potential difference in the VD due to the formation of ions across the junction, which is called a potential barrier because it prevents the further movement of holes and electrons through the junction.
Biasing of a Diode
When a diode or any two-terminal component is connected in a circuit, it has two biased conditions with a given supply. They are forward-biased conditions and reverse-biased conditions.
Forward Biased Condition
When a diode is connected in a circuit, with its anode connected to the positive terminal of the supply and the cathode to the negative terminal of the supply, such a connection is said to be in forward-biased condition. This type of connection makes the circuit more forward-biased and helps in more conduction. A diode conducts well in forward-biased conditions.
Reverse Biased Condition
When a diode is connected to the circuit, its anode is connected to the negative terminal of the supply and cathode to the positive terminal of the supply, such a connection is said to be in reverse biased condition. This type of connection makes the circuit more and more reverse biased and helps reduce and prevent conduction. A diode cannot conduct in reverse biased conditions.
Working under Forward Biased
When an external voltage is applied to the diode in such a way that it cancels the potential barrier and allows current to flow it is called forward bias. When the anode and cathode are connected to the supply positive and negative terminals respectively, holes in the P-type and electrons in the N-type move across the junction breaking the barrier. With it begins a free flow of current, almost eliminating the obstacle.
With the repulsive force provided by the positive terminal to the holes and the negative terminal to the electrons, recombination occurs at the junction. The supply voltage must be high enough that it forces the movement of electrons and holes through the barrier and across it to provide further current.
Forward current is the current that is generated by the diode when operating in the forward-biased condition and is represented by If.
Working under Reverse Biased
When an external voltage is applied to the diode in such a way that it increases the potential barrier and restricts the flow of current it is called reverse bias. When the anode and cathode are connected to the negative and positive terminals of the supply, respectively, electrons are attracted to the positive terminal, and holes are attracted to the negative terminal. Hence both the junctions will be away from the potential barrier increasing the resistance and preventing any electrons from crossing the junction.
The graph of conductance is also drawn when no field is applied and when no external field is applied.
With increasing reverse bias, the junction has some minority carriers cross the junction. This current is normally negligible. This reverse current remains almost constant when the temperature is constant. But when this reverse voltage rises further, a point called reverse breakdown occurs, where an avalanche of current flows through the junction. This high reverse current damages the device.
The current produced by the diode when operating in the reverse biased condition is the reverse current and is indicated by Ir. Hence a diode provides a high resistance path in reverse biased conditions and does not conduct, whereas it provides a low resistance path in forward biased condition and conducts. Thus, we can conclude that the diode is a one-way device that operates in forward bias and acts as an insulator in reverse bias. This behavior makes it work as a rectifier, which converts AC to DC.
Peak Inverse Voltage
The peak inverse voltage is shortly referred to as PIV. It tells the maximum voltage applied in reverse bias. Peak inverse voltage can be defined as “the maximum reverse voltage that a diode can withstand without destroying“. Therefore, this voltage is assumed during reverse biased conditions. It shows how a diode can be operated safely in reverse bias.
V – I Characteristics of a Diode
A practical circuit arrangement for a p-n junction diode is shown in the following figure. An ammeter is connected in series and a voltmeter in parallel, while the supply is controlled through a variable resistor.
During operation, when the diode is in the forward-biased state, at a particular voltage, the potential barrier is dissipated. Such voltage is called cut-off voltage or knee voltage. If the forward voltage exceeds the limit, the forward current rises rapidly and if this is carried forward, the equipment is damaged due to overheating.
The following graph shows the condition of diode conduction in forward and backward biased positions.
During reverse bias, the current produced through the minority carriers is known as “reverse current”. As the reverse voltage increases, the reverse current increases and it suddenly breaks down at one point, resulting in the permanent destruction of the junction.
The purpose of a Diode
Diodes are used to block electric current in one direction, that is, in the forward direction, and in the opposite direction. This principle of diode makes it act as a rectifier.
For a circuit to allow current to flow in one direction but stop in the other, a rectifier diode is the best choice. Thus, the output will be DC which will remove the AC components. Circuits like half-wave and full-wave rectifiers are made using diodes.
A diode is also used as a switch. It helps to switch on and off faster for the output that should be at an accelerated rate.