At What Voltage Does a Diode Begin to Conduct Current?

Understanding the threshold voltage at which a diode begins to conduct current is crucial for designing and analyzing electronic circuits. A diode starts to conduct when the voltage across it exceeds a certain threshold, known as the forward voltage drop. This threshold varies depending on the type of diode. For silicon diodes, it is typically around 0.6 to 0.7 volts, while for germanium diodes, it is lower, usually around 0.3 volts.

Understanding the Forward Voltage Drop

To start conducting, a silicon diode requires an applied voltage of 0.7 volts or more. In the case of a germanium diode, the threshold is much lower, around 0.3 volts. For special materials like selenium, the threshold can be significantly higher, sometimes reaching 1 volt.

Conduction Behavior

Once the applied voltage surpasses the forward voltage drop, the diode allows current to flow in the forward direction. This behavior is consistent for both silicon and germanium diodes. The key is to recognize that the diode will only conduct when the voltage across it meets or exceeds the threshold voltage.

Current Behavior Over Time

It is important to note that once a silicon diode begins to conduct, the current will not remain constant. Instead, it will rapidly increase over time. For example, if a silicon diode starts to conduct and the current initially rises to 0.1 amperes, it will continue to rise exponentially. Within seconds, it might increase to 0.5 amperes, then 1.0 amperes, and potentially even higher, such as 3 amperes, before finally saturating.

Understanding the Negative Temperature Coefficient

A silicon diode has a negative temperature coefficient. This means that as the diode heats up, its resistance decreases. As a result, the current through the diode increases rapidly, leading to a phenomenon called thermal runaway. This can be dangerous as it can cause the diode to fail. To avoid this, it is essential to ensure that a diode is operated with a constant current rather than a constant voltage.

Practical Application

The key takeaway is to think of a diode not as a device that has a constant voltage across it, but as one that has a constant current. When analyzing a circuit, you should consider the current through the diode first, and the voltage across it will naturally adjust to around the forward voltage drop. This approach can help in designing circuits that are safer and more reliable.

Always remember, for silicon diodes, the threshold voltage is approximately 0.6 to 0.7 volts, and for germanium diodes, it is around 0.3 volts. Understanding these principles can significantly enhance your ability to design and troubleshoot electronic circuits effectively.