Understanding Reverse-biased p-n Junctions

A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are joined. When a voltage is applied across the p-n junction, it is said to be biased. There are two main types of biasing: forward bias and reverse bias.

Characteristics of a Reverse-biased p-n Junction

In a reverse-biased p-n junction, the positive terminal of the external voltage source is connected to the n-type semiconductor, and the negative terminal is connected to the p-type semiconductor. Let's examine the effects of this biasing:

• Effect on Depletion Region: The applied reverse voltage pulls the free electrons in the n-type away from the junction and the holes in the p-type away from the junction. This action effectively widens the depletion region. The depletion region is the area around the junction where mobile charge carriers have diffused across, leaving behind immobile ions. A wider depletion region means a larger barrier potential.
• Effect on Current Flow: In a reverse-biased junction, the majority carriers (electrons in n-type, holes in p-type) are pulled away from the junction, making it very difficult for them to cross. However, minority carriers (holes in n-type, electrons in p-type) are pushed towards the junction by the reverse bias voltage. These minority carriers can cross the junction, creating a very small current known as the reverse saturation current or leakage current. This reverse current is typically very small, often in the microampere or nanoampere range for silicon diodes, and is almost negligible compared to the current flow in forward bias.
• Effect on Resistance: Since the reverse current is very small for a given applied voltage (except at very high voltages which can cause breakdown), the resistance offered by the p-n junction in reverse bias is very high. Using Ohm's law, Resistance = Voltage / Current. A large voltage divided by a very small current results in a very high resistance.

Analyzing the Options for Reverse Bias Characteristic

• Option 1: Very narrow depletion region. This is incorrect. As explained above, reverse bias causes the depletion region to widen, not narrow.
• Option 2: Large current flow. This is incorrect. Large current flow is characteristic of a forward-biased p-n junction, where majority carriers easily cross the junction. In reverse bias, the current is very small.
• Option 3: Almost no current. This is correct. The current flow in reverse bias is due to minority carriers and is called reverse saturation current, which is typically very small, approaching zero for an ideal diode. Therefore, it is accurate to say there is almost no current.
• Option 4: Very low resistance. This is incorrect. Since the current in reverse bias is very low, the resistance is very high. Very low resistance is a characteristic of a forward-biased p-n junction.

Based on the analysis, a key characteristic of a reverse-biased p-n junction is that there is almost no current flowing through it.