Problem 35 - Master Resource Book in JEE Main Physics

Question

The readings corresponding to zener diode are given below in the table. From given table, determine the reverse breakdown voltage of the zener diode. $$ \begin{array}{c|c|c|c|} \hline {\text { Forward bias }} & & {\text { Reverse bias }} \\ \hline V \text { (volt) } & I(\mathrm{~mA}) & V \text { (volt) } & I(\mu \mathrm{A}) \\ \hline 0.5 & 5 & 0.5 & 2.0 \\ 0.7 & 20 & 1.0 & 2.0 \\ 0.8 & 40 & 3.0 & 2.0 \\ 1.0 & 250 & 5.0 & 2.0 \\ & & 5.5 & 100.0 \\ & & 5.5 & 120.0 \\ \hline \end{array} $$ (a) It is lying between \(1.0 \mathrm{~V}\) to \(5.0 \mathrm{~V}\) (b) \(1.0 \mathrm{~V}\) (c) Approx. \(5.3 \mathrm{~V}\) (d) None of the above

Step-by-Step Solution

Answer

The reverse breakdown voltage is approximately 5.3 V (Option c).

Step 1: Understanding the Question

We have a table with forward and reverse bias voltages and currents for a Zener diode. Our task is to determine the reverse breakdown voltage of the Zener diode.

Step 2: Analyzing Forward Bias

Look at the forward bias column of the table. This information is generally not needed to determine the reverse breakdown voltage, but it's important to note that the currents are relatively high compared to the reverse bias side.

Step 3: Identifying Key Reverse Bias Data

Focus on the reverse bias voltage and current. Notice that for 0.5V to 5.0V, the reverse current remains constant at 2µA, indicating no breakdown has occurred yet.

Step 4: Finding Breakdown Voltage

Observe that at 5.5V, the reverse current spikes sharply from 2µA to 100µA and then 120µA, indicating the breakdown voltage is reached and the diode conducts significantly.

Step 5: Determination of Reverse Breakdown Voltage

The breakdown voltage is the point where a steep increase in current is observed. So, the breakdown voltage can be approximated as around 5.3V, since significant current change starts at 5.5V.

Step 6: Choosing the Correct Option

Compare the estimated breakdown voltage with the options given. Option (c) "Approx. 5.3 V" matches our finding, correctly identifying the reverse breakdown voltage.

Key Concepts

Reverse Breakdown Voltage

The reverse breakdown voltage of a Zener diode is a critical parameter. It is the voltage at which the diode starts to conduct a significant amount of current in the reverse direction. You'll notice this when there's a sudden spike in current, even though the voltage remains constant. This characteristic is what defines a Zener diode and makes it useful for voltage regulation.

In practical terms, a Zener diode will remain primarily non-conductive until the reverse voltage approaches this breakdown point. For example, in the given exercise, as the reverse voltage increases from 0.5V to 5.0V, the current stays almost constant at 2µA. It’s only once we hit the 5.5V mark that we observe a sharp increase in current to 100µA, indicating that the breakdown point has been reached.

• The breakdown starts when current increases sharply.
• Important for voltage regulation applications.
• Allows precise control of the voltage at the load.

This sudden increase, around 5.3V as concluded from the exercise, confirms the Zener diode's reverse breakdown voltage, which stabilizes the output voltage for circuits even with varying input conditions.

Reverse Bias

Zener diodes operate under reverse bias in applications where they stabilize voltage. Reverse bias means applying a voltage across the diode in the opposite direction of the forward current flow. Usually, this keeps the diode at a high resistance until the reverse breakdown voltage is reached.

In the exercise, the data shows how the diode behaves under reverse bias. From 0.5V to 5.0V, only small amounts of current (2µA) are observed, indicating minimal conduction. This is typical of reverse bias operation, where the diode should ideally block current.

• Zener diodes use reverse bias to control voltage.
• Initially, little to no current flows, preventing circuit damage.
• Useful in protecting circuits from voltage spikes.

When the reverse breakdown voltage is exceeded, the diode transitions from blocking to conducting, ensuring the voltage doesn’t exceed a predetermined level. This makes Zener diodes invaluable in circuit protection and voltage regulation tasks.

Forward Bias

Forward bias in a diode refers to the condition where the external voltage applied reduces the barrier potential of the diode, allowing current to flow easily across it. In the context of Zener diodes, forward bias is not typically where their unique properties shine, but it’s still a fundamental concept.

During forward bias, as seen in the exercise, the diode allows current to flow at relatively low voltages. For instance, in the forward bias portion of the data, as the voltage increases from 0.5V to 1.0V, the current jumps from 5mA to 250mA. This is a typical behavior of diodes, where a small increase in voltage results in a large increase in current.

• Forward bias improves current flow in diodes.
• Low resistance in this state allows for signal or power passage.
• Zener diodes behave similarly to regular diodes under forward conditions.

However, when using a Zener diode for voltage regulation or over-voltage protection, it is usually the reverse bias operation that is most relevant, keeping the forward bias function as a standard diode property.