What is the difference between knee voltage and breakdown voltage ?

When discussing electronic components, particularly diodes and transistors, terms like knee voltage and breakdown voltage often come up. While they may sound similar, they refer to distinct concepts that are crucial for understanding how these devices operate.

Knee Voltage Explained

The knee voltage, also known as the threshold voltage, is the point at which a diode begins to conduct significant current. For a silicon diode, this is typically around 0.7 volts. Below this voltage, the diode is in a non-conductive state, and only a very small leakage current flows. Once the applied voltage exceeds the knee voltage, the diode starts to turn on, allowing current to flow more freely.

Characteristics of Knee Voltage

• Current Flow: At knee voltage, the current increases rapidly with a small increase in voltage.
• Device Behavior: It marks the transition from the off state to the on state in diodes.
• Temperature Dependence: The knee voltage can vary with temperature, typically decreasing as temperature increases.

Understanding Breakdown Voltage

Breakdown voltage, on the other hand, is the maximum reverse voltage that a diode can withstand before it begins to conduct in the reverse direction. This is a critical parameter for ensuring that the diode does not get damaged. When the reverse voltage exceeds this threshold, the diode enters a breakdown region, which can lead to a large increase in reverse current.

Key Features of Breakdown Voltage

• Reverse Bias Condition: Breakdown voltage is relevant when the diode is reverse-biased, meaning the voltage is applied in the opposite direction.
• Types of Breakdown: There are two main types: Zener breakdown (occurs at lower voltages) and avalanche breakdown (occurs at higher voltages).
• Device Protection: Exceeding the breakdown voltage can damage the diode unless it is designed to operate in this region, like Zener diodes.

Comparative Summary

To summarize, the knee voltage is the point where a diode starts to conduct in the forward direction, while breakdown voltage is the threshold for reverse conduction. Understanding these two voltages is essential for designing circuits that utilize diodes effectively, ensuring they operate within safe limits and perform their intended functions.

Practical Example

Imagine a simple circuit with a silicon diode connected to a battery. When you apply a voltage of 0.6 volts, the diode remains off, and no significant current flows. As you increase the voltage to 0.7 volts, the diode reaches its knee voltage, and current begins to flow. If you were to reverse the polarity and apply a voltage of -50 volts, the diode would eventually reach its breakdown voltage, leading to a surge of current that could potentially damage the diode unless it is a Zener diode designed for such conditions.

In essence, both knee voltage and breakdown voltage are critical for understanding the behavior of diodes in electronic circuits, each serving a unique purpose in the operation and protection of electronic components.