Question

In a P-N junction, there is no appreciable current if (A) a potential difference is applied across the junction (B) it is impossible (C) P-section is a made positive and N-section negative (D) a potential difference is applied across junction making P section negative and N-section positive.

Short answer

In a P-N junction, there is no appreciable current when a potential difference is applied across the junction, making the P-section negative and the N-section positive. This condition is known as reverse bias, where the applied voltage increases the barrier created by the depletion region, preventing the current flow. Therefore, the correct answer is (D).

Step-by-step solution

Understanding the P-N junction

In a P-N junction, the p-section (p-type material) has an excess of positively charged holes, while the n-section (n-type material) has an excess of negatively charged electrons. When the junction is formed, the electrons from the n-section recombine with the holes in the p-section, creating a depletion region near the junction with no charge carriers. The electric field from this region opposes the further movement of electrons and holes across the junction.

Analyzing Option (A)

Option (A) states that there is no appreciable current if a potential difference is applied across the junction. However, this statement is too general. The direction and magnitude of the applied potential difference, or voltage, determine the behavior of the P-N junction, as we will see in the other options.

Analyzing Option (B)

Option (B) claims that having no appreciable current in a P-N junction is impossible. This statement is incorrect because there are specific conditions under which the P-N junction will not have an appreciable current, as we will discuss in the following options.

Analyzing Option (C)

Option (C) suggests that having no appreciable current occurs when the P-section is made positive and the N-section negative. This condition is known as forward bias because the applied voltage helps to overcome the barrier created by the depletion region and allows the current to flow. When the P-N junction is forward-biased, the electrons from the n-side can move to the p-side and recombine with holes, creating a current flow. Thus, option (C) is incorrect.

Analyzing Option (D)

Option (D) states that having no appreciable current occurs when a potential difference is applied across the junction, making the P-section negative and the N-section positive. This condition is known as reverse bias because the applied voltage increases the barrier created by the depletion region, preventing the current flow. When the P-N junction is reverse-biased, the electrons in the n-side and the holes in the p-side are both moved away from the junction, and the depletion region widens. Consequently, there is no appreciable current in the P-N junction under this condition. Solution: Based on our analysis, the correct answer is (D) a potential difference is applied across the junction, making the P-section negative and the N-section positive.

Key concepts

Depletion Region

In a P-N junction, the depletion region is a key area where charge carriers such as electrons and holes are absent. This zone forms when electrons from the N-type region recombine with holes from the P-type region, neutralizing each other's charges. As a result, a band at the interface of the P and N sections is left without any mobile charge carriers.
This region is crucial because it behaves like an insulator and creates a barrier that controls the movement of charges across the junction.

• In the depletion region, an electric field is established due to the ions left behind after electron-hole recombination.
• This field opposes the movement of new charge carriers, essentially preventing the natural flow of current across the junction without an external influence.

Understanding the depletion region is vital for explaining how biases affect the P-N junction's behavior.

Forward Bias

When a P-N junction is in a state of forward bias, the external voltage applied reduces the potential barrier created by the depletion region. This is achieved by connecting the positive terminal of a power source to the P-type side and the negative to the N-type side.
Forward bias encourages charge carrier movement which counteracts the electric field from the depletion region. This results in a steady flow of current.

• Electrons in the N-side move towards the P-side as the depletion region narrows.
• Holes in the P-side move towards the N-side, enabling recombination and current flow.

This application of voltage allows electrons to move into areas of lower potential, overcoming the previous charge barrier.

Reverse Bias

Reverse bias in a P-N junction occurs when the applied potential causes an expansion of the depletion region's barrier. This scenario arises when the positive end of an external voltage source connects to the N-type side, and the negative end to the P-type side.
Under reverse bias:

• The electric field inside the depletion region becomes stronger, inhibiting the movement of charge carriers across the junction.
• The depletion region widens as electrons and holes are pushed away from the junction, adding resistance to any current flow.

The increased potential barrier effectively prevents significant current from flowing across, thus achieving a state where the junction blocks current almost entirely.

Potential Difference

Potential difference, often known as voltage, is crucial in determining the behavior of a P-N junction. It influences how charges move across the junction, thus controlling the flow of current.
Potential difference is the driving force applied over the P-N junction and can modify its state between forward and reverse bias. Here are some key points:

• A positive potential difference aligned in a direction to reduce the depletion region leads to forward bias and allows current flow.
• A reverse applied potential difference enhances the barrier, setting the junction to reverse bias, thus reducing the current flow significantly.

Understanding potential difference helps in predicting and manipulating the behavior of semiconductor devices like diodes.

Electric Field

The electric field in a P-N junction is generated due to ionized donor and acceptor atoms in the depletion region. This field establishes an internal voltage across the depletion layer.
It plays an essential role in regulating electron and hole movements between the P and N regions.

• In the default state, it prevents immediate recombination by creating a barrier to more charge carrier movement.
• Alterations in the electric field occur due to external biases, influencing how carriers respond.

The inner electric field's strength and direction are crucial for device operation, especially as it changes based on the applied potential difference. By understanding these dynamics, one can effectively manage device functions like rectification and switching.