Avalanche Transistors

Avalanche transistors are semiconducting devices that utilize the avalanche breakdown principle!

Avalanche breakdown is a unique phenomenon that occurs when a high voltage is applied to a p-n junction, to switch from an off state to an on state.

Avalanche transistors are broadly used in various high-voltage and high-frequency electronic circuits like:

• Fast (High-Speed) Pulse Generators
• Microwave Frequency Generators
• Laser Diode Drivers for LIDARs
• Fast Edge Switch Generators...

Avalanche transistors are similar in structure to conventional bipolar junction transistors (BJTs), consisting of three layers of semiconductor material that form two p-n junctions. These layers are known as the Emitter, Base, and Collector.

The working of an avalanche transistor is characterized by the breakdown of the collector-base junction. When the voltage across the collector-base junction exceeds the breakdown voltage, the transistor rapidly switches from off to on, allowing a large current to flow.

Nowadays, Avalanche Transistors specifically designed for low voltage Avalanche Mode operation is also available from various vendors. The FMMT411 NPN Avalanche Transistor, available in SOT23 SMD package, is a good example (click below image to enlarge).

Even for an electronics student or hobbyist, fast pulses are sometimes required when measuring slew rate or propagation delay, and for sampling. In such cases, an avalanche transistor circuit can be used to generate pulses with sub-nanosecond rise and fall times.

Avalanche transistor applications in the hobby electronics world was made popular following the application note -A Seven-Nanosecond Comparator for Single Supply Operation - by analog guru Jim Williams.

Many people have built such avalanche pulse generators for practical applications like oscilloscope bandwidth measurements.

It's worth noting that using a fast-rising pulse to measure rise-time leans to render too affirmative rise-time estimates because the pulse may not have reached its peak before falling off due to the short pulse width and the limited bandwidth of the oscilloscope under test.

If you'd like to build another Avalanche Pulse Generator yourself, you will get a simple build idea through an article scheduled to be posted here in a few weeks. Anyone else interested?