Question

Explain how a p–n junction makes an excellent rectifier

Short answer

A p-n junction diode acts as an excellent rectifier because it allows current flow during the forward bias condition when a positive voltage is applied to the p-side and a negative voltage to the n-side, and blocks current flow during the reverse bias condition when a positive voltage is applied to the n-side and a negative voltage to the p-side. In this way, when an alternating current is applied to the p-n junction diode, the positive half cycle is conducted while the negative half cycle is blocked, effectively converting alternating current to direct current.

Step-by-step solution

Understand n-type and p-type semiconductors

An n-type semiconductor has an excess of electrons, also known as majority carriers, while a p-type semiconductor has an excess of holes, which are the majority carriers. The p-n junction diode can be visualized by joining the p-type and n-type semiconductors together. When these two types of semiconductors are brought into contact, electrons near the junction in the n-region recombine with holes near the junction in the p-region, forming a region known as the depletion region.

Depletion region and built-in potential

The depletion region is an insulating layer formed at the junction due to this recombination process. It consists of immobile positive and negative ions near the junction. These immobile ions create an electric field that opposes the flow of majority charge carriers, i.e., electrons from the n-region to the p-region and holes from the p-region to the n-region. The potential difference created by this electric field is called the built-in potential.

Forward bias condition

When a positive voltage is applied to the p-side of the diode and a negative voltage to the n-side, the diode is said to be in forward bias. The applied voltage reduces the potential barrier height across the depletion region, and an electric field is created in the same direction as the built-in potential field. Due to this, the majority charge carriers easily flow across the junction, and the diode conducts current in the forward direction.

Reverse bias condition

When a positive voltage is applied to the n-side and a negative voltage to the p-side, the diode is in reverse bias. In this case, the applied voltage increases the potential barrier across the depletion region, widening the insulating layer and further opposing the flow of majority carriers across the junction. As a result, the diode does not conduct current in the reverse direction (due to negligible minority carrier flow).

Rectification process

A p-n junction diode under forward bias condition allows the current to flow, and under reverse bias condition, it blocks the current flow. So, when an alternating current is applied, during the positive half cycle, the diode is in forward bias and conducts current, whereas in the negative half cycle, it is in reverse bias and does not conduct current. Therefore, the p-n junction diode allows current flow mostly in one direction while blocking it in the other, essentially converting alternating current to direct current, and acting as an excellent rectifier.

Key concepts

Rectifier

A rectifier is a device that converts alternating current (AC) to direct current (DC). This conversion is essential for many electronic devices that require a steady, unidirectional flow of electricity. The p-n junction diode is a key component used as a rectifier because of its ability to allow current flow in only one direction. When AC voltage is applied, the diode conducts electricity during the positive half-cycle under forward bias. However, it blocks the current during the negative half-cycle under reverse bias.

In this way, the p-n junction diode efficiently converts AC to DC, making it a highly effective rectifier. Its simple operation, reliability, and cost-effectiveness make it indispensable in power supply systems.

Semiconductors

Semiconductors are materials with electrical conductivity between that of conductors and insulators. This unique property allows them to control electrical current precisely. The most common semiconductor materials used in electronics are silicon and germanium. By doping these materials with certain impurities, we can create n-type and p-type semiconductors.

• N-type semiconductor: Contains extra electrons, which are the majority charge carriers.
• P-type semiconductor: Contains extra holes, considered as positive charge carriers.

Combining n-type and p-type semiconductors leads to the formation of a p-n junction, which is essential in the operation of various electronic components, including rectifiers and transistors.

Depletion Region

The depletion region is a critical aspect of a p-n junction. It forms at the interface where the n-type and p-type semiconductors meet. In this region, the free electrons from the n-type side recombine with holes from the p-type side, creating an area devoid of mobile charge carriers.

This process results in a layer of immobile ions, creating an internal electric field, known as the built-in potential. The depletion region acts as an insulator, preventing the free flow of majority carriers from one side to the other. The width of this region is sensitive to external voltages and changes based on whether the junction is forward or reverse biased.

Forward Bias

In forward bias, a voltage is applied across a p-n junction by connecting the positive terminal to the p-type side and the negative terminal to the n-type side. This external voltage reduces the potential barrier created by the depletion region. As a result, the majority carriers—electrons from the n-side and holes from the p-side—have enough energy to overcome the barrier and flow across the junction.

The reduction in barrier height facilitates a significant increase in current through the diode. This behavior is essential for allowing current flow in one direction, playing a crucial role in the rectification process.

Reverse Bias

When the voltage applied to a p-n junction is such that the positive terminal is connected to the n-type and the negative terminal to the p-type, the diode is in reverse bias. In this scenario, the external voltage increases the potential barrier of the depletion region, causing it to widen.

This wider barrier prevents the flow of majority charge carriers across the junction, effectively blocking current flow. The diode conducts a very small leakage current due to minority carriers, but this is typically negligible. This behavior under reverse bias is what enables the diode to act as a one-way street for current, crucial for its function as a rectifier, allowing it to block current flow during the negative half of an AC cycle.