P-N Junction Diode Characteristics Explained

A p-n junction diode is a fundamental semiconductor device formed by joining p-type and n-type semiconductor materials. It allows current to flow easily in one direction (forward bias) and restricts current flow significantly in the opposite direction (reverse bias).

P-N Junction Diode: Analysis of Statement (a)

Statement (a) discusses the threshold voltage, also known as the cut-in voltage, for a germanium diode.

• The threshold voltage (or cut-in voltage) is the minimum forward-bias voltage required across a p-n junction diode for it to start conducting significant current. Below this voltage, the current is very small.
• For a germanium diode, the typical threshold voltage is approximately \(0.2 \text{ V}\) to \(0.3 \text{ V}\).
• For a silicon diode, which is more commonly used, the typical threshold voltage is approximately \(0.6 \text{ V}\) to \(0.7 \text{ V}\).

Since statement (a) claims the threshold voltage for a germanium diode is about \(0.7 \text{ V}\), which is characteristic of a silicon diode, statement (a) is incorrect.

P-N Junction Diode: Analysis of Statement (b)

Statement (b) describes the behavior of current under reverse bias in a p-n junction diode.

• When a p-n junction diode is reverse biased, the positive terminal of the voltage source is connected to the n-type material, and the negative terminal is connected to the p-type material. This increases the width of the depletion region.
• Under reverse bias, only a very small current flows. This current is known as the reverse saturation current or leakage current. It is primarily due to the flow of minority carriers (electrons in the p-type region and holes in the n-type region) that are swept across the junction by the electric field.
• This reverse saturation current is remarkably stable and remains almost constant, essentially voltage independent, as the reverse bias voltage increases. This holds true until a specific voltage known as the breakdown voltage is reached.
• Once the reverse bias voltage reaches the breakdown voltage, the current increases very sharply due to avalanche or Zener breakdown phenomena, which can damage the diode if not limited.

Therefore, statement (b), which says the current under reverse bias is essentially voltage independent up to breakdown voltage, is correct.

P-N Junction Diode: Conclusion

Based on the detailed analysis:

• Statement (a) is incorrect because the threshold voltage for a germanium diode is typically around \(0.3 \text{ V}\), not \(0.7 \text{ V}\).
• Statement (b) is correct because the reverse bias current in a p-n junction diode is indeed largely independent of the applied reverse voltage until the breakdown voltage is reached.

Thus, only statement (b) is correct about the p-n junction diode.