Avalanche Breakdown
When a p-type and n-type semiconductor comes in contact, a depletion region is formed around the p-n junction. The width of the depletion region decreases with the increase in voltage of forward bias, while the depletion region increases in reverse bias condition. Below figure shows the I-V characteristics of a p-n junction in forward bias and reverse bias condition.
At the initial stage reverse saturation current Is is independent of the applied voltage, but on reaching a particular point the junction breaks down leading to the heavy flow of reverse current through the device. This is because as the reverse voltage increases the kinetic energy of minority charge carrier also increases. These fast moving electrons collide with the other atoms to knock off some more electrons from them.The electrons so released further release much more electrons from the atoms by breaking the covalent bond. This process is known as carrier multiplication and this leads to a considerable increase in the flow of current through the p-n junction. This phenomenon is called Avalanche breakdown and the voltage is called Avalanche breakdown voltage (VBR).Avalanche breakdown occurs in the lightly doped p-n junction when the reverse voltage increases beyond 5V. Further, it is difficult to control this phenomenon as the number of charge carriers generated cannot be directly controlled. Moreover, the avalanche breakdown voltage has a positive temperature coefficient which means the avalanche breakdown voltage increases with the increase in the junction temperature.

Transistors in Digital Circuits

When transistors are used in digital circuits, they typically operate in the breakdown region. Let's explore the reasons behind this choice.

1. Introduction to Transistors

Transistors are semiconductor devices that are fundamental building blocks of modern electronic circuits. They can amplify or switch electronic signals and are commonly used in digital circuits for signal processing and control.

2. Different Regions of Operation

Transistors have different regions of operation, namely the linear region, active region, saturation region, and cutoff region. Each region has specific characteristics and is suitable for different applications.

- Linear Region: In this region, the transistor operates as an amplifier, and the output current is directly proportional to the input voltage. It is commonly used in analog circuits.
- Active Region: The transistor operates as a switch, allowing a controlled amount of current to flow between the collector and emitter terminals. It is commonly used in amplifier circuits.
- Saturation Region: The transistor is fully turned on, and maximum current flows between the collector and emitter terminals. It is commonly used in switching applications.
- Cutoff Region: The transistor is fully turned off, and no current flows between the collector and emitter terminals. It is commonly used to completely stop the flow of current.

3. Operation in Digital Circuits

In digital circuits, transistors are primarily used as switches to represent logic states. They are either in an "on" state (conducting) or an "off" state (non-conducting). The breakdown region, also known as the reverse-active region, is chosen for transistor operation in digital circuits due to the following reasons:

- Fast Switching: The breakdown region offers fast switching times, allowing the transistor to quickly transition between the on and off states. This is crucial for digital circuits, where high-speed operations are required.
- No Intermediate States: In the breakdown region, the transistor operates in a binary manner, ensuring that there are no intermediate states between the on and off states. This helps in maintaining the integrity of digital signals.
- Reduced Power Consumption: The breakdown region provides a low leakage current when the transistor is in the off state. This helps in reducing power consumption in digital circuits, making them more energy-efficient.

4. Conclusion

In summary, transistors in digital circuits operate in the breakdown region. This choice is made to achieve fast switching times, maintain binary operation, and reduce power consumption. Understanding the different regions of transistor operation is essential for designing and analyzing digital circuits effectively.