P-N Junction Diode in Reverse Bias

The behavior of a P-N junction diode under reverse bias condition is a fundamental concept in semiconductor physics. When a P-N junction diode is connected in reverse bias, the positive terminal of the battery is connected to the N-side (n-type semiconductor) and the negative terminal to the P-side (p-type semiconductor).

Reverse Bias Condition Explained

• In reverse bias, the external voltage applied across the P-N junction adds to the built-in potential barrier.
• This causes the depletion region, which is a region devoid of mobile charge carriers, to widen.
• The wider depletion region effectively increases the resistance of the diode, significantly reducing the flow of majority charge carriers (holes from p-side and electrons from n-side) across the junction.

Minority Charge Carriers and Leakage Current

Despite the high resistance to majority carrier flow, a very small current, typically in the order of $\text{10}^{-6}$ A (microamperes) or even nanoseconds for silicon diodes, flows through the circuit. This small current, known as the reverse saturation current or leakage current, is primarily due to the flow of minority charge carriers.

• Minority charge carriers are thermally generated electron-hole pairs within the semiconductor material. In the p-type material, electrons are minority carriers, and in the n-type material, holes are minority carriers.
• The strong electric field across the widened depletion region, established by the reverse bias voltage, acts on these minority charge carriers.
• Electrons from the p-side and holes from the n-side that are generated near or within the depletion region are swept across the junction by this electric field.
• This drift of minority charge carriers constitutes the small reverse current observed in the P-N junction diode. This current is largely independent of the applied reverse voltage until breakdown occurs.

Breakdown Voltage Analysis

The option 'breakdown voltage' refers to a specific reverse voltage at which the current through the diode increases very sharply and uncontrollably. This is due to phenomena like Zener breakdown or avalanche breakdown, where the diode's structure is altered, or carriers gain enough energy to cause further ionization. While breakdown voltage is related to reverse bias, the small current in the order of $\text{10}^{-6}$ A is observed before breakdown and is not *due* to breakdown voltage, but rather a normal operating characteristic of the diode in reverse bias. The breakdown voltage signifies a point of significant current increase, not the cause of the initial small leakage.

Threshold Current Misconception

The term 'threshold current' is not typically used to describe the small current flow in a reverse-biased P-N junction diode. 'Threshold voltage' or 'cut-in voltage' is relevant in forward bias, representing the voltage at which the diode begins to conduct significantly. Therefore, this option is incorrect in the context of reverse bias current.

Dynamic Resistance in Reverse Bias

'Low dynamic resistance' is also an incorrect reason for the small reverse current. In reverse bias (before breakdown), the P-N junction diode exhibits very high dynamic resistance because the depletion region is wide, and it opposes the flow of majority carriers. A low dynamic resistance would imply a large current flow for a small change in voltage, which is characteristic of the diode in forward bias or after reverse breakdown, not during the small leakage current phase.

Based on the analysis, the small current in the order of $\text{10}^{-6}$ A observed when a P-N junction diode is connected in reverse bias condition is attributed to the flow of minority charge carriers.