Explain the VI characteristics of a PN junction diode and show that PN diode works as a rectifier.

VI Characteristics of a PN Junction Diode:

A PN junction diode is a fundamental electronic device formed by joining p-type and n-type semiconductor materials, creating a depletion region at their interface. This unique structure gives rise to distinct electrical characteristics, which are often visually represented by its VI (voltage-current) characteristics.

Forward Bias Region:

In forward bias, a positive voltage is applied to the p-side of the diode relative to the n-side. This causes the depletion region to narrow, allowing majority carriers (holes from the p-side and electrons from the n-side) to cross the junction more easily. As the forward bias voltage increases, the current through the diode also increases exponentially, following the Shockley diode equation:

$$I_D = I_S \left(e^{\frac{eV_D}{k_BT}}-1\right)$$

where:

• $$I_D$$ is the diode current
• $$I_S$$ is the reverse saturation current
• $$e$$ is the elementary charge
• $$V_D$$ is the voltage across the diode
• $$k_B$$ is Boltzmann's constant
• $$T$$ is the absolute temperature

Reverse Bias Region:

When a negative voltage is applied to the p-side of the diode relative to the n-side, the diode is said to be reverse-biased. In this case, the depletion region widens, creating a potential barrier that impedes the flow of majority carriers across the junction. Consequently, the reverse current through the diode is minimal and remains relatively constant even as the reverse bias voltage increases.

Breakdown Region:

At sufficiently high reverse bias voltages, the electric field across the depletion region becomes strong enough to cause avalanche breakdown or Zener breakdown. In avalanche breakdown, minority carriers (electrons in the p-side and holes in the n-side) gain enough energy from the electric field to generate new electron-hole pairs through impact ionization. These newly generated carriers further contribute to the current, leading to a rapid increase in reverse current. In Zener breakdown, the intense electric field causes valence electrons from the p-side to tunnel directly into the conduction band of the n-side, resulting in a sharp increase in reverse current.

PN Diode as a Rectifier:

The ability of a PN junction diode to conduct current in one direction (forward bias) and block current in the opposite direction (reverse bias) makes it a useful component for rectification. In a rectifier circuit, the diode is used to convert alternating current (AC) into direct current (DC).

During the positive half-cycle of the AC input, the diode is forward-biased, allowing current to flow through it. During the negative half-cycle, the diode is reverse-biased, blocking the flow of current. As a result, the output of the rectifier is a pulsating DC signal that can be further smoothed by a filter circuit to obtain a pure DC output.

The efficiency of a diode as a rectifier is determined by its forward voltage drop and reverse leakage current. A low forward voltage drop minimizes the power loss during conduction, while a low reverse leakage current prevents unwanted current flow during the reverse bias cycle.

In conclusion, the VI characteristics of a PN junction diode are pivotal in understanding its behavior and applications. The ability of the diode to rectify AC into DC makes it a fundamental component in power electronics and various electronic circuits.